Chapter XVIII: Silicon and the Other Elements of the Fourth Group (3)

Tin decomposes the vapour of water when heated with it, liberating the hydrogen and forming stannic oxide. Sulphuric acid, diluted with a considerable quantity of water, does not act, or at all events acts very slightly, on tin, but tin reduces hot strong sulphuric acid, when not only sulphurous anhydride but also sulphuretted hydrogen is evolved. Hydrochloric acid acts very easily on tin, with evolution of hydrogen and formation of stannous chloride, SnCl_{2}, in solution, which, with an excess of hydrochloric acid and access of air, is converted into stannic chloride: SnCl_{2} + 2HCl + O = SnCl_{4} + H_{2}O.[36 bis] Nitric acid diluted with a considerable quantity of water dissolves tin at the ordinary temperature, whilst the nitric acid itself is reduced, forming, amongst other products, ammonia and hydroxylamine. Here the tin passes into solution in the form of stannous nitrate. Stronger nitric acid (also more dilute, when heated) transforms the tin into its highest grade of oxidation, SnO_{2}, but the latter then appears as the so-called metastannic acid, which does not dissolve in nitric acid, and therefore the tin does not pass into solution. Feeble acids--for instance, carbonic and organic acids--do not act on tin even in the presence of oxygen, because tin does not form any powerful bases.

[36 bis] The action of a mixture of hydrochloric acid and tin forms an
excellent means of reducing, wherein both the hydrogen liberated
by the mixture (at the moment of separation) and the stannous
chloride act as powerful reducing and deoxidising agents. Thus,
for instance, by this mixture nitro-compounds are transformed into
amido-compounds--that is, the elements of the group NO_{2} are
reduced to NH_{2}.

It is important to remark as a characteristic of tin that it is reduced from its solutions by many metals which are more easily oxidised, as, for instance, by zinc.

_In combination_, _tin_ appears in the two types, SnX_{4} and SnX_{2},[37] compounds of the intermediate type, Sn_{2}X_{6}, being also known, but these latter pass with remarkable facility in most cases into compounds of the higher and lower types, and therefore the form SnX_{3} cannot be considered as independent.

[37] Many volatile compounds of tin are known, whose molecular weights
can therefore be established from their vapour densities. Among
these may be mentioned stannic chloride, SnCl_{4}, and stannic
ethide, Sn(C_{2}H_{5})_{4} (the latter boils at about 150°). But
V. Meyer found the vapour density of stannous chloride, SnCl_{2},
to be variable between its boiling point (606°) and 1100°, owing,
it would seem, to the fact that the molecule then varies from
Sn_{2}Cl_{4} to SnCl_{2}, but the vapour density proved to be less
than that indicated by the first and greater than that shown by
the second formula, although it approaches to the latter as the
temperature rises--that is, it presents a similar phenomenon to
that observed in the passage of N_{2}O_{4} into NO_{2}.

_Stannous oxide_, SnO, in an anhydrous condition is obtained by boiling solutions of stannous salts with alkalis, the first action of the alkali being to precipitate a white hydrate of stannous oxide, Sn(OH)_{2}SnO. The latter when heated parts with water as easily as the hydrate of copper oxide. In this form stannous oxide is a black crystalline powder (specific gravity 6·7) capable of further oxidation when heated. The hydrate is freely soluble in acids, and also in potassium and sodium hydroxides, but not in aqueous ammonia.[38] This property indicates the feeble basic properties of this lower oxide, which acts in many cases as a reducing agent.[39] Among the compounds corresponding with stannous oxide the most remarkable and the one most frequently used is stannous chloride or _chloride of tin_, SnCl_{2}, also called proto-chloride of tin (because it is the lowest chloride, containing half as much Cl as SnCl_{4}). It is a transparent, colourless, crystalline substance, melting at 250° and boiling at 606°. Water dissolves it, without visible change (in reality partial decomposition occurs, as we shall see presently). It is also soluble in alcohol. It is obtained by heating tin in dry hydrochloric acid gas, the hydrogen being then liberated, or by dissolving metallic tin in hot strong hydrochloric acid and then evaporating quickly. On cooling, crystals of the monoclinic system are obtained having the composition SnCl_{2},2H_{2}O. An aqueous solution of this substance absorbs oxygen from the atmosphere, and gives a precipitate containing stannic oxide. From this it follows that a solution of stannous chloride will act as a reducing agent, a fact frequently made use of in chemical investigations--for example, for reducing metals from their solutions--since even mercury may be reduced to a metallic state from its salts by means of stannous chloride. This reducing property is also employed in the arts, especially in the dyeing industry, where this substance in the form of a crystalline salt finds an extensive application, and is known as _tin salt_ or tin crystals.

[38] When rapidly boiled, an alkaline solution of stannous oxide
deposits tin and forms stannic oxide, 2SnO = Sn + SnO_{4}, which
remains in the alkaline solution.

[39] Weber (1882) by precipitating a solution of stannous chloride with
sodium sulphite (this salt as a reducing agent prevents the
oxidation of the stannous compound) and dissolving the washed
precipitate in nitric acid, obtained crystals of _stannous
nitrate_, Sn(NO_{3})_{2},20H_{2}O, on refrigerating the solution.
This crystallo-hydrate easily melts, and is deliquescent. Besides
this, a more stable anhydrous basic salt, Sn(NO_{3})_{2},SnO, is
easily formed. In general, stannous oxide as a feeble base easily
forms basic salts, just as cupric and lead oxides do. For the same
reason SnX_{2} easily forms double salts. Thus a potassium salt,
SnK_{2}Cl_{4},H_{2}O, and especially an ammonium salt,
Sn(NH_{4})_{2}Cl_{4},H_{2}O, called _pink salt_, are known. Some
of these salts are used in the arts, owing to their being more
stable than tin salts alone. Stannous bromide and iodide, SnBr_{2}
and SnI_{2}, resemble the chloride in many respects.

Among other stannous salts a sulphate, SnSO_{4}, is known. It is
formed as a crystalline powder when a solution of stannous oxide
in sulphuric acid is evaporated under the receiver of an air-pump.
The feeble basic character of the stannous oxide is clearly seen
in this salt. It decomposes with extreme facility, when heated,
into stannic oxide and sulphurous anhydride, but it easily forms
double salts with the salts of the alkali metals.

In gaseous hydrochloric acid, stannous chloride, SnCl_{2},2H_{2}O,
forms a liquid having the composition SnCl_{2},HCl,3H_{2}O (sp gr.
2·2, freezes at -27°), and a solid salt, SnCl_{2},H_{2}O (Engel).

_Stannic oxide_, SnO_{2}, occurring in nature as _tinstone_, or _cassiterite_, is formed during the oxidation or combustion of heated tin in air as a white or yellowish powder which fuses with difficulty. It is prepared in large quantities, being used as a white vitreous mixture for coating ordinary tiles and similar earthenware objects with a layer of easily fusible glass or enamel. Acid solutions of stannic oxide treated with alkalis, and alkaline solutions treated with acids, give a precipitate of stannic hydroxide, Sn(OH)_{4}, also known as stannic acid, which, when heated, gives up water and leaves the anhydride, SnO_{2}, which is insoluble in acids, clearly showing the feebleness of its basic character. When fused with alkali hydroxides (not with their carbonates or acid sulphates), an alkaline compound is obtained which is soluble in water. Stannic hydroxide, like the hydrates of silica, is a colloidal substance, and presents several different modifications, depending on the method of preparation, but having an identical composition; the various hydroxides have also a different appearance, and act differently with reagents. For instance, a distinction is made between ordinary stannic acid and metastannic acid. _Stannic acid_ is produced by precipitation by soda or ammonia from a freshly-prepared solution of stannic chloride, SnCl_{4}, in water; on drying the precipitate thus obtained, a non-crystalline mass is formed, which is freely soluble in strong hydrochloric or nitric acids, and also in potassium and sodium hydroxides. This ordinary stannic acid may be still better obtained from sodium stannate by the action of acids. _Metastannic acid_ is insoluble in sulphuric and nitric acids. It is obtained in the form of a heavy white powder by treating tin with nitric acid; hydrochloric acid does not dissolve it immediately, but changes it to such an extent that, after pouring off the acid, water extracts the stannic chloride, SnCl_{4}, already formed. Dilute alkalis not only dissolve metastannic acid, but also transform it into salts, which, slowly, yet completely, dissolve in _pure water_, but are insoluble even in dilute alkali hydroxides. Dilute hydrochloric acid, especially when boiling, changes the ordinary hydrate into metastannic acid. On this depends, by the way, the formation of a white precipitate, stannic hydroxide, from solutions of stannous and stannic chlorides diluted with water. The stannic oxide first dissolved changes under the influence of hydrochloric acid into metastannic acid, which is insoluble in water in the presence of hydrochloric acid. Solutions of metastannic acid differ from solutions of ordinary stannic acid, and in the presence of alkali they change into solutions of ordinary acid, so that metastannic acid corresponds principally with the acid compounds of stannic oxide, and ordinary stannic acid with the alkaline compounds.[40] Graham obtained a soluble colloidal hydroxide; it is subject to the same transformations that are in general peculiar to colloids.

[40] Frémy supposes the cause of the difference to consist in a
difference of polymerisation, and considers that the ordinary acid
corresponds with the oxide SnO_{2}, and the meta-acid with the
oxide Sn_{5}O_{10}, but it is more probable that both are
polymeric but in a different degree. Stannic acid with sodium
carbonate gives a salt of the composition Na_{2}SnO_{3}. The same
salt is also obtained by fusing metastannic acid with sodium
hydroxide, whilst metastannic acid gives a salt,
Na_{2}SnO_{3},4SnO_{2} (Frémy), when treated with a dilute
solution of alkali; moreover, stannic acid is also soluble in the
ordinary stannate, Na_{2}SnO_{3} (Weber), so that both stannic
acids (like both forms of silica) are capable of polymerisation,
and probably only differ in its degree. In general, there is here
a great resemblance to silica, and Graham obtained a solution of
stannic acid by the direct dialysis of its alkaline solution. The
main difference between these acids is that the meta-acid is
soluble in hydrochloric acid, and gives a precipitate with
sulphuric acid and stannous chloride, which do not precipitate the
ordinary acid. Vignon (1889) found that more heat is evolved in
dissolving stannic acid in KHO than metastannic.

Stannic oxide shows the properties of a slightly energetic and intermediate oxide (like water, silica, &c.); that is to say, it forms saline compounds both with bases and with acids, but both are easily decomposed, and are but slightly stable. But still the acid character is more clearly developed than the basic, as in silica, germanic oxide, and lead dioxide. This determines the character of the compounds SnX_{4}, corresponding to stannic chloride, SnCl_{4} (also called tetrachloride of tin). It is obtained in an anhydrous condition by the direct action of chlorine on tin, and is then easily purified, because it is a liquid boiling at 114°, and therefore can be easily distilled. Its specific gravity is 2·28 (at 0°), and it fumes in the open air (spiritus fumans libavii), reacting on the moisture of the air, thus showing the properties of a chloranhydride. Water however does not at first decompose it, but dissolves it, and on evaporation gives the crystallo-hydrate SnCl_{4},5H_{2}O. If but little water be taken, crystals containing SnCl_{4},3H_{2}O are formed, which part with one-third of the water when placed under the receiver of the air-pump. A large quantity of water however, especially on heating, causes a precipitate of metastannic acid[41] and formation of HCl.

[41] The formation of the compound SnCl_{4},3H_{2}O is accompanied by
so great a contraction that these crystals, although they contain
water, are heavier than the anhydrous chloride SnCl_{4}. The
penta-hydrated crystallo-hydrate absorbs dry hydrochloric acid,
and gives a liquid of specific gravity 1·971, which at 0° yields
crystals of the compound SnCl_{4},2HCl,6H_{2}O (it corresponds
with the similar platinum compound), which melt at 20° into a
liquid of specific gravity 1·925 (Engel).

Stannic chloride combines with ammonia (SnCl_{4},4NH_{3}),
hydrocyanic acid, phosphoretted hydrogen, phosphorus pentachloride
(SnCl_{4},PCl_{5}), nitrous anhydride and its chloranhydride
(SnCl_{4},N_{2}O_{3} and SnCl_{4},2NOCl), and with metallic
chlorides (for example, K_{2}SnCl_{6}, (NH_{4})_{2}SnCl_{6}, &c.)
In general, a highly-developed faculty for combination is observed
in it.

Tin does not combine directly with iodine, but if its filings be
heated in a closed tube with a solution of iodine in carbon
bisulphide, it forms stannic iodide, SnI_{4}, in the form of red
octahedra which fuse at 142° and volatilise at 295°. The fluorine
compounds of tin have a special interest in the history of
chemistry, because they give a series of double salts which are
isomorphous with the salts of hydrofluosilicic acid, SiR_{2}F_{6},
and this fact served to confirm the formula SiO_{2} for silica, as
the formula SnO_{2} was indubitable. Although _stannic fluoride_,
SnF_{4}, is almost unknown in the free state, its corresponding
double salts are very easily formed by the action of hydrofluoric
acid on alkaline solutions of stannic oxide; thus, for example, a
crystalline salt of the composition SnK_{2}F_{6},H_{2}O is
obtained by dissolving stannic oxide in potassium hydroxide and
then adding hydrofluoric acid to the solution. The barium salt,
SnBaF_{6},3H_{2}O, is sparingly soluble like its corresponding
silicofluoride. The more soluble salt of strontium,
SnSrF_{6},2H_{2}O, crystallises very well, and is therefore more
important for the purposes of research; it is isomorphous with the
corresponding salt of silicon (and titanium); the magnesium salt
contains 6H_{2}O.

Stannic sulphide, SnS_{2}, is formed, as a yellow precipitate, by
the action of sulphuretted hydrogen on acid solutions of stannic
salts; it is easily soluble in ammonium and potassium sulphides,
because it has an acid character, and then forms thiostannates
(see Chapter XX.). In an anhydrous state it has the form of
brilliant golden yellow plates, which may be obtained by heating a
mixture of finely-divided tin, sulphur, and sal-ammoniac for a
considerable time. It is sometimes used in this form under the
name of mosaic gold, as a cheap substitute for gold-leaf in
gilding wood articles. On ignition it parts with a portion of its
sulphur, and is converted into stannous sulphide SnS. It is
soluble in caustic alkalis. Hydrochloric acid does not dissolve
the anhydrous crystalline compound, but the precipitated powdery
sulphide is soluble in boiling strong hydrochloric acid, with the
evolution of hydrogen sulphide.

_The alkali compounds of stannic oxide_--that is, the compounds in which it plays the part of an acid, corresponding in this respect to the compounds of silica and other anhydrides of the composition RO_{2}--are very easily formed and are used in the arts. Their composition in most cases corresponds with the formula SnM_{2}O_{3}--that is, SnO(MO)_{2}, similar to CO(MO)_{2}, where M = K, Na. Acids, even feeble acids like carbonic, decompose the salts, like the corresponding compounds of alumina or silica. In order to obtain _potassium stannate_, which crystallises in rhombohedra, and has the composition SnK_{2}O_{3},3H_{2}O, potassium hydroxide (8 parts) is fused, and metastannic acid (3 parts) gradually added. _Sodium stannate_ is prepared in practice in large quantities by heating a solution of caustic soda with lead oxide and metallic tin. In this last case an alkaline solution of lead oxide is formed, and the tin acts on the solution in such a way as to reduce the lead to the metallic state, and itself passes into solution. It is very remarkable that lead displaces tin when in combination with acids, whilst tin, on the contrary, displaces lead from its alkali compounds. By dissolving the mass obtained in water, and adding alcohol, sodium stannate is precipitated, which may then be dissolved in water and purified by re-crystallisation. In this case it has the composition SnNa_{2}O_{3},3H_{2}O if separated from strong solutions, and SnNa_{2}O_{3},10H_{2}O when crystallised at a low temperature from dilute solutions. In the arts this salt is used as a mordant in dyeing operations. With a cold solution of sodium hydroxide metastannic acid forms a salt of the composition (NaHO)_{2},5SnO_{2},3H_{2}O, from which Frémy drew his conclusions concerning the polymerism of metastannic acid. Tin, like other metals and many metalloids, gives a peroxide form of combination or _perstannic oxide_. This substance was obtained by Spring (1889) in the form of a hydrate, H_{2}Sn_{2}O_{7} = 2(SnO_{3})H_{2}O, by mixing a solution of SnCl_{2}, containing an excess of HCl, with freshly prepared peroxide of barium. A cloudy liquid is then obtained, and this after being subjected to dialysis leaves a gelatinous mass which on drying is found to have the composition Sn_{2}H_{2}O_{7}. Above 100° this substance gives off oxygen and leaves SnO_{2}. It is evident that SnO_{3} bears the same relation to SnO_{2} as H_{2}O_{2} to H_{2}O or ZnO_{2} to ZnO, &c.

Tin occupies the same position amongst the analogues of silicon as cadmium and indium amongst the analogues of magnesium and aluminium respectively, and as in each of these cases the heavier analogues with a high atomic weight and a special combination of properties--namely, mercury and thallium--are known, so also for silicon we have _lead_ as the heaviest analogue (Pb = 206), with a series of both kindred and special properties. The higher type, PbX_{4}--for instance, PbO_{2}--is in a chemical sense far less stable than the lower type, PbX. The ordinary compounds of lead correspond with the latter, and in addition to this, PbO, although not particularly energetic, is still a decided base easily forming basic salts, PbX_{2}(PbO)_{n}. Although the compounds PbX_{4}, are unstable they offer many points of analogy with the corresponding compounds of tin SnO_{2}; this is seen, for instance, in the fact that PbO_{2} is a feeble acid, giving the salt PbK_{2}O_{3}, that PbCl_{4} is a liquid like SnCl_{4} which is not affected by sulphuric acid, and that PbF_{4} gives double salts, like SnF_{4} or SiF_{4} (Brauner 1894. See Chapter II., Note 49 bis); Pb(C_{2}H_{5})_{4} also resembles Sn(C_{2}H_{5})_{4} &c. All this shows that lead is a true analogue of tin, as Hg is of cadmium.[41 bis]

[41 bis] Although this has long been generally recognised from the
resemblance between the two metals, still from a chemical point of
view it has only been demonstrated by means of the periodic law.

_Lead_ is found in nature in considerable masses, in the form of galena, _lead sulphide_, PbS.[42] The specific gravity of galena is 7·58, colour grey; it crystallises in the regular system, and has a fine metallic lustre. Both the native and artificial sulphides are insoluble in acids (hydrogen sulphide gives a black precipitate with the salts PbX_{2}).[42 bis] When heated, lead melts, and in the open air is either totally or partially transformed into white lead sulphate, PbSO_{4}, as it also is by many oxidising agents (hydrogen peroxide, potassium nitrate). Lead sulphate is also insoluble in water,[43] and lead is but rarely met with in this form in nature. The chromates, vanadates, phosphates, and similar salts of lead are also somewhat rare. The carbonate, PbCO_{2}, is sometimes found in large masses, especially in the Altai region. Lead sulphide is often worked for extracting the silver which it contains; and as the lead itself also finds manifold industrial applications, this work is carried out on an exceedingly large scale. Many methods are employed. Sometimes the lead sulphide is decomposed by heating it with cast iron. The iron takes up the sulphur from the lead and forms easily-fusible iron sulphide, which does not mix with the heavier reduced lead. But another process is more frequently used: the lead ore (it must be clean; that is, free from earthy matter, which may be easily removed by washing) is heated in a reverberatory furnace to a moderate temperature with a free access of air. During this operation part of the lead sulphide oxidises and forms lead sulphate, PbSO_{4}, and lead oxide. When the oxidation of part of the lead has been attained, it is necessary to shut off the air supply and increase the temperature, then the oxidised compounds of the lead enter into reaction with the remaining lead sulphide, with formation of sulphurous anhydride and metallic lead. At first from PbS + O_{3}, PbO + SO_{2} are formed, and also from PbS + O_{4} lead sulphate PbSO_{4}, and then PbO and PbSO_{4} react with the remaining PbS, according to the equations 2PbO + PbS = 3Pb + SO_{2} and also PbSO_{4} + PbS = 2Pb + 2SO_{2}.[44]

[42] Mixed ores of copper compounds together with PbS and ZnS are
frequently found in the most ancient primary rocks. As the
separation of the metals themselves is difficult, the ores are
separated by a method of selection or mechanical sorting. Such
mixed ores occur in Russia, in many parts of the Caucasus, and in
the Donetz district (at Nagolchik).

[42 bis] Lead sulphide in the presence of zinc and hydrochloric acid is
completely reduced to metallic lead, all the sulphur being given
off as hydrogen sulphide.

[43] Lead sulphate, PbSO_{4}, occurs in nature (_anglesite_) in
transparent brilliant crystals which are isomorphous with barium
sulphate, and have a specific gravity of 6·3. The same salt is
formed on mixing sulphuric acid or its soluble salts with
solutions of lead salts, as a heavy white precipitate, which is
insoluble in water and acids, but dissolves in a solution of
ammonium tartrate in the presence of an excess of ammonia. This
test serves to distinguish this salt from the similar salts of
strontium and barium.

[44] According to J. B. Hannay (1894) the last named decomposition
(PbS + PbSO_{4} = 2Pb + 2SO_{2}) is really much more complicated,
and in fact a portion of the PbS is dissolved in the Pb, forming a
slag containing PbO, PbS and PbSO_{4}, whilst a portion of the
lead volatilises with the SO_{2} in the form of a compound
PbS_{2}O_{2}, which is also formed in other cases, but has not yet
been thoroughly studied.

Besides these methods for extracting lead from PBS in its ores,
roasting (the removal of the S in the form of SO_{2}) and smelting
with charcoal with a blast in the same manner as in the
manufacture of pig iron (Chapter XXII.) are also employed.

We may add that PbS in contact with Zn and hydrochloric acid
(which has no action upon PbS alone) entirely decomposes, forming
H_{2}S and metallic lead: PbS + Zn + 2HCl = Pb + ZnCl_{2} +
H_{2}S.

As lead is easily reduced from its ores, and the ore itself has a
metallic appearance, it is not surprising that it was known to the
ancients, and that its properties were familiar to the alchemists,
who called it 'Saturn.' Hence metallic lead, reduced from its
salts in solution by zinc, having the appearance of a tree-like
mass of crystals, is called 'arbor saturni,' &c.

The appearance of lead is well known; its specific gravity is 11·3; the bluish colour and well-marked metallic lustre of freshly-cut lead quickly disappear when exposed to the air, because it becomes coated with a layer--although a very thin layer--of oxide and salts formed by the moisture and acids in the atmosphere. It melts at 320°, and crystallises in octahedra on cooling. Its softness is apparent from the flexibility of lead pipes and sheets, and also from the fact that it may be cut with a knife, and also that it leaves a grey streak when rubbed on paper. On account of its being so soft, lead naturally cannot be applied in many cases where most metals may be used; but on the other hand it is a metal which is not easily changed by chemical reagents, and as it is capable of being soldered and drawn into sheets, &c., lead is most valuable for many technical uses. Lead pipes are used for conveying water[45] and many other liquids, and sheet lead is used for lining all kinds of vessels containing liquids--(acids, for instance) which act on other metals. This particularly refers to sulphuric and hydrochloric acids, because at a low temperature they do not act on lead, and if they form lead sulphate, PbSO_{4}, and chloride, PbCl_{2}, these salts being insoluble in water and in acids, cover the lead and protect it from further corrosion.[46] All soluble preparations of lead are poisonous. At a white heat lead may be partially distilled; the vapours oxidise and burn. Lead may also be easily oxidised at low temperatures. Lead only decomposes water at a white heat, and does not liberate hydrogen from acids, with the exception only of very strong hydrochloric acid and then only when boiling. Sulphuric acid diluted with water does not act on it, or only acts very feebly at the surface; but strong sulphuric acid, when heated, is decomposed by it, with the evolution of sulphurous anhydride. The best solvent for lead is nitric acid, which transforms it into a soluble salt, Pb(NO_{3})_{2}.

[45] Freshly laid new lead pipes contaminate the water with a certain
amount of lead salts, arising from the presence of oxygen,
carbonic acid, &c., in the water. But the lead pipes under the
action of running water soon become coated with a film of
salts--lead sulphate, carbonate, chloride, &c.--which are
insoluble in water, and the water pipes then become harmless.

[46] Lead is used in the arts, and owing to its considerable density,
it is cast, mixed with small quantities of other metals, into
shot. A considerable amount is employed (together with mercury) in
extracting gold and silver from poor ores, and in the manufacture
of chemical reagents, and especially of lead chromate. _Lead
chromate_, PbCrO_{4}, is distinguished for its brilliant yellow
colour, owing to which it is employed in considerable quantities
as a dye, mainly for dyeing cotton tissues yellow. It is formed on
the tissue itself, by causing a soluble salt of lead to react on
potassium chromate. Lead chromate is met with in nature as 'red
lead ore.' It is insoluble in water and acetic acid, hut it
dissolves in aqueous potash. The so-called pewter vessels often
consist of an alloy of 5 parts of tin and 1 part of lead, and
solder is composed of 1 to 2 parts of tin with 1/2 part of lead.
Amongst the alloys of lead and tin, Rudberg states that the alloy
PbSn_{3} stands out from the rest, since, according to his
observations, the temperature of solidification of the alloy is
187°.

Although acids thus have directly but little effect on lead, and this is one of its most important practical properties, _yet when air has free access, lead (like copper) very easily reacts with many acids_, even with those which are comparatively feeble. The action of acetic acid on lead is particularly striking and often applied in practice. If lead be plunged into acetic acid it does not change at all and does not pass into solution, but if part of the lead be immersed in the acid, and the other part remain in contact with the air, or if lead be merely covered with a thin layer of acetic acid in such a way that the air is practically in contact with the metal, then it unites with the oxygen of the air to form oxide, which combines with the acetic acid and forms lead acetate, soluble in water. The formation of lead oxide is especially marked from the fact that with a sufficient quantity of air not only is the normal lead acetate formed but also the basic salts.[47]

[47] The normal lead acetate, known in trade as _sugar of lead_, owing
to its having a sweetish taste, has the formula
Pb(C_{2}H_{3}O_{2})_{2},3H_{2}O. This salt only crystallises from
acid solutions. It is capable of dissolving a further quantity of
lead oxide or of metallic lead in the presence of air. A basic
salt of the composition Pb(C_{2}H_{3}O_{2})_{2},PbH_{2}O_{2} is
then formed which is soluble in water and alcohol. As in this salt
the number of atoms is even and the same as in the hydrate of
acetic acid, C_{2}H_{4}O_{2},H_{2}O = C_{2}H_{3}(OH)_{3}, it may
be represented as this hydrate in which two of hydrogen are
replaced by lead--that is, as C_{2}H_{3}(OH)(O_{2}Pb). This basic
salt is used in medicine as a remedy for inflammation, for
bandaging wounds, &c., and also in the manufacture of white lead.
Other basic acetates of lead, containing a still greater amount of
lead oxide, are known. According to the above representation of
the composition of the preceding lead acetate, a basic salt of the
composition (C_{2}H_{3})_{2}(O_{2}Pb)_{3} would be also possible,
but what appear to be still more basic salts are known. As the
character of a salt also depends on the property of the base from
which it is formed, it would seem that lead forms a hydroxide of
the composition HOPbOH, containing two water residues, one or both
of which may be replaced by the acid residues. If both water
residues are replaced, a normal salt, XPbX, is obtained, whilst if
only one is replaced a basic salt, XPbOH, is formed. But lead does
not only give this normal hydroxide, but also polyhydroxides,
Pb(OH),_n_PbO, and if we may imagine that in these polyhydroxides
there is a substitution of both the water residues by acid
residues, then the power of lead for forming basic salts is
explained by the properties of the base which enters into their
composition.

When oxidising in the presence of air,[48] when heated or in the presence of an acid at the ordinary temperature, lead forms compounds of the type PbX_{2}. _Lead oxide_, PbO, known in industry as _litharge_, silberglätte (this name is due to the fact that silver is extracted from the lead ores of this kind) and massicot. If the lead is oxidised in air at a high temperature, the oxide which is formed fuses, and on cooling is easily obtained in fused masses which split up into scales of a yellowish colour, having a specific gravity of 9·3; in this form it bears the name of litharge. Litharge is principally used for making lead salts, for the extraction of metallic lead, and also for the preparation of drying oils--for instance, from linseed oil.[49] When oxidised carefully and slightly heated, lead forms a powdery (not fused) oxide known under the name of _massicot_. It is best prepared in the laboratory by heating lead nitrate, or lead hydroxide. It has a yellow colour, and differs from litharge in the greater difficulty with which it forms lead salts with acids. Thus, for instance, when massicot is moistened with water it does not attract the carbonic acid of the air so easily as litharge does. It may, however, be imagined that the cause of the difference depends only on the formation of dioxide on the surface of the lead oxide, on which the acids do not act. In any case lead oxide is comparatively easily soluble in nitric and acetic acids. It is but slightly soluble in water, but communicates an alkaline reaction to it, since it forms the hydroxide. This hydroxide is obtained in the shape of a white precipitate by the action of a small quantity of an alkali hydroxide on a solution of a lead salt. An excess of alkali dissolves the hydroxide separated, which fact demonstrates the comparatively indistinct basic properties of lead oxide. The normal lead hydroxide, which should have the composition Pb(OH)_{2}, is unknown in a separate state, but it is known in combination with lead oxide as Pb(OH)_{2},2PbO or Pb_{3}O_{2}(OH)_{2}. The latter is obtained in the form of brilliant, white, octahedral crystals when basic lead acetate is mixed with ammonia and gently heated. The basic qualities of this hydroxide are shown distinctly by its absorbing the carbonic anhydride of the air. When an alkaline solution of the hydroxide is boiled, it deposits lead oxide in the form of a crystalline powder.

[48] Few compounds are known of the lower type PbX, and still fewer of
the intermediate type PbX_{3}. To the first type belongs the
so-called lead suboxide, Pb_{2}O, obtained by the ignition of lead
oxalate, C_{2}PbO_{4}, without access of air. It is a black
powder, which easily breaks up under the action of acids, and even
by the simple action of heat, into metallic lead and lead oxide.
This is the character of all suboxides. They cannot be regarded as
independent salt-forming oxides, neither can those forms of
oxidation of lead which contain more oxygen than the oxide of
lead, PbO, and less than the dioxide, PbO_{2}. As we shall see, at
least two such compounds are formed. Thus, for example, an oxide
having the composition Pb_{2}O_{3} is known, but it is decomposed
by the action of acids into lead oxide, which passes into
solution, and lead dioxide, which remains behind. Such is red
lead. (See further on.)

[49] In the boiling of drying oils, the lead oxide partially passes
into solution, forming a saponified compound capable of attracting
oxygen and solidifying into a tar-like mass, which forms the oil
paint. Perhaps, however, glycerine partially acts in the process.

Ossovetsky by saturating drying oil with the salts of certain
metals obtained oil colours of great durability.

A mixture of very finely-divided litharge with glycerine (50 parts
of litharge to 5 c.c. of anhydrous glycerine) forms a very quick
(two minutes) setting cement, which is insoluble in water and
oils, and is very useful in setting up chemical apparatus. The
hardening is based on the reaction of the lead oxide with
glycerine (Moraffsky).

Lead oxide forms but few soluble salts--for instance, the nitrate and the acetate. The majority of its salts (sulphate, PbSO_{4}; carbonate, PbCO_{3}; iodide, PbI_{2}, &c.) are insoluble in water. These salts are colourless or light yellow if the acid be colourless. In lead oxide _the faculty of forming basic salts_, PbX_{2}_n_PbO or PbX_{2}_n_PbH_{2}O_{2}, is strongly developed. A similar property was observed in magnesium and also in the salts of mercury, but lead oxide forms basic salts with still greater facility, although double salts are in this case more rarely formed.[50]

[50] It is very instructive to observe that lead not only easily forms
basic salts, but also salts containing several acid groups. Thus,
for example, lead carbonate occurs in nature and forms compounds
with lead chloride and sulphate. The first compound, known as
_corneous lead_, _phosgenite_, has the composition
PbCO_{3},PbCl_{2}; it occurs in nature in bright cubical crystals,
and is prepared artificially by simply boiling lead chloride with
lead carbonate. A similar compound of normal salts,
PbSO_{4},PbCO_{3}, occurs in nature as _lanarkite_ in monoclinic
crystals. _Leadhillite_ contains PbSO_{4},3PbCO_{3}, and also
occurs in yellowish, monoclinic, tabular crystals. We will turn
our attention to these salts of lead, because it is very probable
that their formation is allied to the formation of the basic
salts, and the following considerations may lead to the
explanation of the existence of both. In describing silica we
carefully developed the conception of polymerisation, which it is
_also indispensable to recognise in the composition of many other
oxides_. Thus it may be supposed that PbO_{2} is a similar
polymerised compound to SiO_{2}--_i.e._ that the composition of
lead peroxide will be Pb_{_n_}O_{2_n_}, because lead methyl,
PbMe_{4}, and lead ethyl, PbEt_{4}, are volatile compounds, whilst
PbO_{2} is non-volatile, and is very like silica in this respect,
and not in the least like carbonic anhydride. Still more should a
polymeric structure, Pb_{_n_}O_{_n_}, be ascribed to lead oxide,
since it differs as little from lead dioxide in its physical
properties as carbonic oxide does from carbonic anhydride, and
being an unsaturated compound is more likely to be capable of
intercombination (polymerisation) than lead dioxide. These
considerations respecting the complexity of lead oxide could have
no real significance, and could not be accepted, were it not for
the existence of the above-mentioned basic and mixed salts. The
oxide apparently corresponds with the composition
Pb_{_n_}X_{2_n_}, and since, according to this representation, the
number of X's in the salts of lead is considerable, it is obvious
that they may be diverse. When a part of these X's is replaced by
the water residue (OH) or by oxygen, X_{2} = O, and the other
parts by an _acid residue_, X, then basic salts are obtained, but
if a part of the X's is replaced by acid residues of one kind, and
the other part by acid residues of another kind, then those mixed
salts about which we are now speaking are formed. Thus, for
example, we may suppose, for a comparison of the composition of
the majority of the salts of lead, that _n_ = 12, and then the
above-mentioned compounds will present themselves in the following
form:--Lead oxide, Pb_{12}O_{12}, its crystalline hydrate,
Pb_{12}O_{8}(OH)_{8}, lead chloride, Pb_{12}Cl_{24}, lead
oxychloride, Pb_{12}Cl_{12}O_{6}, the other oxychloride,
Pb_{12}(OH)_{8}Cl_{6}O_{6}, mendipite (_see_ Note 51),
Pb_{12}Cl_{8}O_{8}, normal lead carbonate, Pb_{12}(CO_{3})_{12},
crystalline basic salt, Pb_{12}(OH)_{6}(CO_{3})_{6}, white lead,
Pb_{12}(CO_{3})_{8}(HO)_{8}, corneous lead,
Pb_{12}Cl_{12}(CO_{3})_{6}, lanarkite,
Pb_{12}(CO_{3})_{6}(SO_{4})_{6}, leadhillite,
Pb_{12}(CO_{3})_{9}(SO_{4})_{3}, &c. The number 12 is only taken
to avoid fractional quantities. Possibly the polymerisation is
much higher than this. The theory of the polymerisation of oxides
introduced by me in the first edition of this work (1869) is now
beginning to be generally accepted.

Amongst the soluble lead salts, that best known and most often applied in practical chemistry is _lead nitrate_, obtained directly by dissolving lead or its oxide in nitric acid. The normal salt, Pb(NO_{3})_{2}, crystallises in octahedra, dissolves in water, and has a specific gravity of 4·5. When a solution of this salt acts on white lead or is boiled with litharge, the basic salt, having a composition Pb(OH)(NO_{3}), is formed in crystalline needles, sparingly soluble in cold water but easily dissolved in hot water, and therefore in many respects resembling lead chloride. When the nitrate is heated, either lead oxide is obtained or else the oxide in combination with peroxide.

_Lead chloride_, PbCl_{2}, is precipitated from the soluble salts of lead when a strong solution is treated with hydrochloric acid or a metallic chloride. It is soluble in considerable quantities in hot water, and therefore if the solutions be dilute or hot, the precipitation of lead chloride does not occur, and if a hot solution be cooled, the salt separates in brilliant prismatic crystals. It fuses when heated (like silver chloride), but is insoluble in ammonia. This salt is sometimes met with in nature, and when heated in air is capable of exchanging half its chlorine for oxygen, forming the basic salt or lead oxychloride, PbCl_{2}PbO, which may also be obtained by fusing PbCl_{2} and PbO together. The reaction of lead chloride with water vapour leads to the same conclusion, showing the feeble basic character of lead 2PbCl_{2} + H_{2}O = PbCl_{2},PbO + 2HCl. When ammonia is added to an aqueous solution of lead chloride a white precipitate is formed, which parts with water on being heated, and has the composition Pb(OH)Cl,PbO. This compound is also formed by the action of metallic chlorides on other soluble basic salts of lead.[51]

[51] A similar basic salt having a white colour, and therefore used as
a substitute for white lead, is also obtained by mixing a solution
of basic lead acetate with a solution of lead chloride. Its
formation is expressed by the equation: 2PbX(OH),PbO + PbCl_{2} =
2Pb(OH)Cl,PbO + PbX_{2}. Similar basic compounds of lead are met
with in nature--for instance, _mendipite_, PbCl,2PbO, which
appears in brilliant yellowish-white masses. The ignition of red
lead with sal-ammoniac results in similar polybasic compounds of
lead chloride, forming the _Cassel's_, or _mineral yellow_ of the
composition PbCl_{2}_n_PbO. _Lead iodide_, PbI_{2}, is still less
soluble than the chloride, and is therefore obtained by mixing
potassium iodide with a solution of a lead salt. It separates as a
yellow powder, which may be dissolved in boiling water, and on
cooling separates in very brilliant crystalline scales of a golden
yellow colour. The salts PbBr_{2}, PbF_{2}, Pb(CN)_{2},
Pb_{2}Fe(CN)_{6} are also insoluble in water, and form white
precipitates.

Lead carbonate, or _white lead_, is the most extensively used basic lead salt. It has the valuable property of 'covering,' which only to a certain extent appertains to lead sulphate and other white powdery substances used as pigments. This faculty of 'covering' consists in the fact that a small quantity of white lead mixed with oil spreads uniformly, and if such a mixture be spread over a surface (for instance, of wood or metal) the surface is quickly covered--that is, light does not penetrate through even a very thin layer of superposed white lead; thus, for example, the grain of the wood remains invisible.[52] White lead, or _basic lead carbonate_, after being dried at 120°, has a composition Pb(OH)_{2},2PbCO_{3}.[53] It may be obtained by adding a solution of sodium carbonate to a solution of one of the basic salts of lead--for instance, the basic acetate--and likewise by treating this latter with carbonic acid. For this purpose the solution of basic acetate is poured into the vessel _f_; it is prepared in the vat A, containing litharge, into which the pump P delivers the solution of the acetate, which remains after the action of carbonic anhydride on the basic salt. In A a basic salt is formed having a composition approaching to Pb_{4}(OH)_{6}(C_{2}H_{3}O_{2})_{2}; carbonic anhydride, 2CO_{2}, is passed through this solution and precipitates white lead, Pb_{2}(OH)_{2}(CO_{3})_{2}, and normal lead acetate, Pb(C_{2}H_{3}O_{2})_{2}, remains in the solution, and is pumped back into the vat A containing lead oxide, where the normal salt is again (on being agitated) converted into the basic salt. This is run into the vessel E, and thence into _f_. Into the latter carbonic anhydride is delivered from the generator D, and forms a precipitate of white lead.[53 bis]

[52] It is remarkable that a peculiar kind of attraction exists between
boiled linseed oil and white lead, as is seen from the following
experiments. White lead is triturated in water. Although it is
heavier than water, it remains in suspension in it for some time
and is thoroughly moistened by it, so that the trituration may be
made perfect; boiled linseed oil is then added, and shaken up with
it. A mixture of the oil and white lead is then found to settle at
the bottom of the vessel. Although the oil is much lighter than
the water it does not float on the top, but is retained by the
white lead and sinks under the water together with it. There is
not, however, any more perfect combination nor even any solution.
If the resultant mass be then treated with ether or any other
liquid capable of dissolving the oil, the latter passes into
solution and leaves the white lead unaltered.

[53] It may be regarded as a salt corresponding with the normal hydrate
of carbonic acid, C(OH)_{4}, in which three-quarters of the
hydrogen is replaced by lead. A salt is also known in which all
the hydrogen of this hydrate of carbonic acid is replaced by
lead--namely, the salt containing CO_{4}Pb_{2}. This salt is
obtained as a white crystalline substance by the action of water
and carbonic acid on lead. The normal salt, PbCO_{3}, occurs in
nature under the name of white lead ore (sp. gr. 6·47), in
crystals, isomorphous with aragonite, and is formed by the double
decomposition of lead nitrate with sodium carbonate, as a heavy
white precipitate. Thus both these salts resemble white lead, but
the first-named salt is exclusively used in practice, owing to its
being very conveniently prepared, and being characterised by its
great covering capacity, or 'body,' due to its fine state of
division.

[53 bis] One of the many methods by which white lead is prepared
consists in mixing massicot with acetic acid or sugar of lead, and
leaving the mixture exposed to air (and re-mixing from time to
time), containing carbonic acid, which is absorbed from the
surface by the basic salt formed. After repeated mixings (with the
addition of water), the entire mass is converted into white lead,
which is thus obtained very finely divided.

In order to mark the transition from lead oxide, PbO, into lead dioxide PbO_{2} (plumbic anhydride), it is necessary to direct our attention to the intermediate oxide, or _red lead_, Pb_{3}O_{4}.[54] In the arts it is used in considerable quantities, because it forms a very durable yellowish-red paint used for colouring the resins (shellac, colophony, &c.) composing sealing wax. It also forms a very good cheap oil paint, used especially for painting metals, more particularly because drying oils--for instance, hemp seed, linseed oils--very quickly dry with red lead and with lead salts. Red lead is prepared by slightly heating massicot, for which purpose two-storied stoves are used. In the lower story the lead is turned into massicot, and in the higher one, having the lower temperature (about 300°), the massicot is transformed into red lead. Frémy and others showed the instability of red lead prepared by various methods, and its decomposition by acids, with formation of lead dioxide, which is insoluble in acids, and a solution of the salts of lead oxide. The artificial production (synthesis) of red lead by double decomposition was most important. For this purpose Frémy mixed an alkaline solution of potassium plumbate, K_{2}PbO_{3} (prepared by dissolving the dioxide in fused potash),[54 bis] with an alkaline solution of lead oxide. In this way a yellow precipitate of minium hydrate is formed, which, when slightly heated, loses water and turns into bright red anhydrous minium Pb_{3}O_{4}.

[54] If lead hydroxide be dissolved in potash and sodium hypochlorite
be added to the solution, the oxygen of the latter acts on the
dissolved lead oxide, and partially converts it into dioxide, so
that the so-called lead sesquioxide is obtained; its empirical
formula is Pb_{2}O_{3}. Probably it is nothing but a lead
salt--_i.e._ is referable to the type of dioxide of lead, or its
hydroxide, PbO(OH)_{2}, in which two atoms of hydrogen are
replaced by lead, PbO(O_{2}Pb). The brown compound precipitated by
the action of dilute acids--for example, nitric--splits up, even
at the ordinary temperature, into insoluble lead dioxide and a
solution of a lead salt. This compound evolves oxygen when it is
heated. It dissolves in hydrochloric acid, forming a yellow
liquid, which probably contains compounds of the composition
PbCl_{2} and PbCl_{4}, but even at the ordinary temperature the
latter soon loses the excess of chlorine, and then only lead
chloride, PbCl_{2}, remains. In order to see the relation between
red lead and lead sesquioxide, it must be observed that they only
differ by an extra quantity of lead oxide--that is, red lead is a
basic salt of the preceding compound, and if the compound
Pb_{2}O_{3} may be regarded as PbO_{3}Pb, then red lead should be
looked on as PbO_{3}Pb,PbO--that is, as basic lead plumbate.

[54 bis] Frémy obtained potassium plumbate in the following manner.
Pure lead dioxide is placed in a silver crucible, and a strong
solution of pure caustic potash is poured over it. The mixture is
heated and small quantities are removed from time to time for
testing, which consists in dissolving in a small quantity of water
and decomposing the resultant solution with nitric acid. There is
a certain moment during the heating when a considerable amount of
insoluble lead dioxide is precipitated on the addition of the
nitric acid; the solution then contains the salt in question, and
the heating must be stopped, and a small amount of water added to
dissolve the potassium plumbate formed. On cooling the salt
separates in somewhat large crystals, which have the same
composition as the stannate--that is, PbO(KO)_{2},3H_{2}O.

Minium is the first and most ordinary means of producing _lead dioxide_, or plumbic anhydride, PbO_{2},[55] because when red lead is treated with dilute nitric acid it gives up lead oxide, and PbO_{2} remains, on which dilute nitric acid does not act. The composition of minium is Pb_{3}O_{4}, and therefore the action of nitric acid on it is expressed by the equation: Pb_{3}O_{4} + 4HNO_{3} = PbO_{2} + 2Pb(NO_{3})_{2} + 2H_{2}O. The dioxide may also be obtained by treating lead hydroxide suspended in water with a stream of chlorine. Under these conditions the chlorine takes up the hydrogen from the water, and the oxygen passes over to the lead oxide.[56] When a strong solution of lead nitrate is decomposed by the electric current, the appearance of crystalline lead dioxide is also observed upon the positive pole; it is also found in nature in the form of a black crystalline substance having a specific gravity of 9·4. When artificially produced it is a fine dark powder, resisting the action of acids, but nevertheless when treated with strong sulphuric acid it evolves oxygen and forms lead sulphate, and with hydrochloric acid it evolves chlorine. The oxidising property of lead dioxide depends of course on the facility of its transition into the more stable lead oxide, which is easily understood from the whole history of lead compounds. In the presence of alkalis it transforms chromium oxide into chromic acid, whilst lead chromate, PbCrO_{4}, is formed, remaining, however, in solution, on account of its being soluble in caustic alkalis. The oxidising action of lead dioxide on sulphurous anhydride is most striking, as it immediately absorbs it, with formation of lead sulphate. This is accompanied by a change of colour and development of heat, PbO_{2} + SO_{2} = PbSO_{4}. When triturated with sulphur the mixture explodes, the sulphur burning at the expense of the oxygen of the lead dioxide. _Tetrachloride of lead_, PbCl_{4}, belongs to the same class of lead compounds as PbO_{2}. This chloride is formed by the action of strong hydrochloric acid upon PbO_{2}, or, in the cold, by passing a stream of chlorine through water containing PbCl_{2} in suspension. The resultant yellow solution gives off chlorine when heated. With a solution of sal ammoniac (Nicolukin, 1885) it gives a precipitate of a double salt, (NH_{4})_{2}PbCl_{6} (very slightly soluble in a solution of sal ammoniac), which when treated with strong sulphuric acid (Friedrich, 1890) gives PbCl_{4} as a yellow liquid sp. gr. 3·18, which solidifies at -18°, and when heated gives PbCl_{2} + Cl_{2}. It is not acted upon by H_{2}SO_{4} like SnCl_{4}. Tetrafluoride of lead (Brauner) belongs to the same class of compounds, it easily forms double salts and decomposes with the evolution of fluorine (Chapter II., Note 49 bis).[56 bis]

[55] Lead dioxide is often called lead peroxide, but this name leads to
error, because PbO_{2} does not show the properties of true
peroxides, like hydrogen or barium peroxides, but is endowed with
acid properties--that is, it is able to form true salts with
bases, which is not the case with true peroxides. Lead dioxide is
a normal salt-forming compound of lead, as Bi_{2}O_{5} is for
bismuth, CeO_{2} for cerium, and TeO_{3} for tellurium, &c. They
all evolve chlorine when treated with hydrochloric acid, whilst
true peroxides form hydrogen peroxide. The true lead peroxide, if
it were obtained, would probably have the composition Pb_{2}O_{5},
or, in combination with peroxide of hydrogen, H_{2}Pb_{2}O_{7} =
H_{2}O_{2} + Pb_{2}O_{5}, judging from the peroxides corresponding
with sulphuric, chromic, and other acids, which we shall
afterwards consider.

As a proof of the fact, that the form PbO_{2}, or PbX_{4}, is the
highest normal form of any combination of lead, it is most
important to remark that it might be expected that the action of
lead chloride, PbCl_{2}, on zinc-ethyl, ZnEt_{2}, would result in
the formation of zinc chloride, ZnCl_{2}, and lead-ethyl,
PbEt_{2}, but that in reality the reaction proceeds otherwise.
Half of the lead is set free, and lead tetrethyl, PbEt_{4}, is
formed as a colourless liquid, boiling at about 200° (Butleroff,
Frankland, Buckton, Cahours, and others). The type PbX_{4} is not
only expressed in PbEt_{4} and PbO_{2}, but also in PbF_{4},
obtained by Brauner.

[56] According to Carnelley and Walker, the hydrate
(PbO_{2})_{3},H_{2}O is then formed; it loses water at 230°. The
anhydrous dioxide remains unchanged up to 280°, and is then
converted into the sesquioxide, Pb_{2}O_{3}, which again loses
oxygen at about 400°, and forms red lead, Pb_{3}O_{4}. Red lead
also loses oxygen at about 550°, forming lead oxide, PbO, which
fuses without change at about 600°, and remains constant as far as
the limit of the observations made (about 800°).

The best method for preparing pure lead dioxide consists in mixing
a hot solution of lead chloride with a solution of bleaching
powder (Fehrman).

[56 bis] The plumbates of Ca and other similar metals, mentioned in