Chapter XII: Impact and Reaction of Water (4)

§ 207. _Centrifugal Pump._--For large volumes of water on lifts not exceeding about 60 ft. the most convenient pump is the centrifugal pump. Recent improvements have made it available also for very high lifts. It consists of a wheel or fan with curved vanes enclosed in an annular chamber. Water flows in at the centre and is discharged at the periphery. The fan may rotate in a vertical or horizontal plane and the water may enter on one or both sides of the fan. In the latter case there is no axial unbalanced pressure. The fan and its casing must be filled with water before it can start, so that if not drowned there must be a foot valve on the suction pipe. When no special attention needs to be paid to efficiency the water may have a velocity of 6 to 7 ft. in the suction and delivery pipes. The fan often has 6 to 12 vanes. For a double-inlet fan of diameter D, the diameter of the inlets is D/2. If Q is the discharge in cub. ft. per second D = about 0.6 [root]Q in average cases. The peripheral speed is a little greater than the velocity due to the lift. Ordinary centrifugal pumps will have an efficiency of 40 to 60%.

The first pump of this kind which attracted notice was one exhibited by J. G. Appold in 1851, and the special features of his pump have been retained in the best pumps since constructed. Appold's pump raised continuously a volume of water equal to 1400 times its own capacity per minute. It had no valves, and it permitted the passage of solid bodies, such as walnuts and oranges, without obstruction to its working. Its efficiency was also found to be good.

Fig. 209 shows the ordinary form of a centrifugal pump. The pump disk and vanes B are cast in one, usually of bronze,

and the disk is keyed on the driving shaft C. The casing A has a spirally enlarging discharge passage into the discharge pipe K. A cover L gives access to the pump. S is the suction pipe which opens into the pump disk on both sides at D.

Fig. 210 shows a centrifugal pump differing from ordinary centrifugal pumps in one feature only. The water rises through a suction pipe S, which divides so as to enter the pump wheel W at the centre on each side. The pump disk or wheel is very similar to a turbine wheel. It is keyed on a shaft driven by a belt on a fast and loose pulley arrangement at P. The water rotating in the pump disk presses outwards, and if the speed is sufficient a continuous flow is maintained through the pump and into the discharge pipe D. The special feature in this pump is that the water, discharged by the pump disk with a whirling velocity of not inconsiderable magnitude, is allowed to continue rotation in a chamber somewhat larger than the pump. The use of this whirlpool chamber was first suggested by Professor James Thomson. It utilizes the energy due to the whirling velocity of the water which in most pumps is wasted in eddies in the discharge pipe. In the pump shown guide-blades are also added which have the direction of the stream lines in a free vortex. They do not therefore interfere with the action of the water when pumping the normal quantity, but only prevent irregular motion. At A is a plug by which the pump case is filled before starting. If the pump is above the water to be pumped, a foot valve is required to permit the pump to be filled. Sometimes instead of the foot valve a delivery valve is used, an air-pump or steam jet pump being employed to exhaust the air from the pump case.

§ 208. _Design and Proportions of a Centrifugal Pump._--The design of
the pump disk is very simple. Let r_i, r_o be the radii of the inlet
and outlet surfaces of the pump disk, d_i, d_o the clear axial width
at those radii. The velocity of flow through the pump may be taken
the same as for a turbine. If Q is the quantity pumped, and H the
lift,

u_i = 0.25 [root](2gH). (1)

2[pi]r_i d_i = Q/u_i.

Also in practice

d_i = 1.2 r_i ....

Hence,

r_i = .2571 [root](Q/[root]H). (2)

Usually

r_o = 2r_i,

and

d_o = d_i or ½d_i

according as the disk is parallel-sided or coned. The water enters the
wheel radially with the velocity u_i, and

u_o = Q/2[pi]r_o d_o. (3)

Fig. 211 shows the notation adopted for the velocities. Suppose the
water enters the wheel with the velocity v_i, while the velocity of
the wheel is V_i. Completing the parallelogram, v_(ri) is the relative
velocity of the water and wheel, and is the proper direction of the
wheel vanes. Also, by resolving, u_i and w_i are the component
velocities of flow and velocities of whir of the velocity v_i of the
water. At the outlet surface, v_o is the final velocity of discharge,
and the rest of the notation is similar to that for the inlet surface.

Usually the water flows equally in all directions in the eye of the
wheel, in that case v_i is radial. Then, in normal conditions of
working, at the inlet surface,

v_i = u_i \
w_i = 0 > (4)
tan[theta] = u_i/V_i |
v_(ri) = u_i cosec [theta] = [root](u_i² + V_i²) /

If the pump is raising less or more than its proper quantity, [theta]
will not satisfy the last condition, and there is then some loss of
head in shock.

At the outer circumference of the wheel or outlet surface,

v_(ro) = u_o cosec [phi] \
w_o = V_o - u_o cot [phi] > (5)
v_o = [root]{u_o² + (V - _o - u_o cot [phi])²} /

_Variation of Pressure in the Pump Disk._--Precisely as in the case of
turbines, it can be shown that the variation of pressure between the
inlet and outlet surfaces of the pump is

h_o - h_i = (V_o² - V_i²)/2g - (v_(ro)² - v_(ri)²)/2g.

Inserting the values of v_(ro), v_(ri) in (4) and (5), we get for
normal conditions of working

h_o -h_i = (V_o² - V_i²)/2g - u_o² cosec² [phi]/2g + (u_i² + V_i²)/2g
= V_o²/2g - u_o² cosec² [phi]/2g + u_i²/2g. (6)

_Hydraulic Efficiency of the Pump._--Neglecting disk friction, journal
friction, and leakage, the efficiency of the pump can be found in the
same way as that of turbines (§ 186). Let M be the moment of the
couple rotating the pump, and [alpha] its angular velocity; w_o, r_o
the tangential velocity of the water and radius at the outlet surface;
w_i, r_i the same quantities at the inlet surface. Q being the
discharge per second, the change of angular momentum per second is

(GQ/g)(w_o r_o - w_i r_i).

Hence

M = (GQ/g)(w_o r_o - w_i r_i).

In normal working, w_i = 0. Also, multiplying by the angular velocity,
the work done per second is

M[alpha] = (GQ/g)w_o r_o[alpha].

But the useful work done in pumping is GQH. Therefore the efficiency
is

[eta] = GQH/M[alpha] = gH/w_o r_o[alpha] = gH/w_o V_o. (7)

§ 209. Case 1. _Centrifugal Pump with no Whirlpool Chamber._--When no
special provision is made to utilize the energy of motion of the water
leaving the wheel, and the pump discharges directly into a chamber in
which the water is flowing to the discharge pipe, nearly the whole of
the energy of the water leaving the disk is wasted. The water leaves
the disk with the more or less considerable velocity v_o, and impinges
on a mass flowing to the discharge pipe at the much slower velocity
v_s. The radial component of v_o is almost necessarily wasted. From
the tangential component there is a gain of pressure

(w_o² - v_s²)/2g - (w_o - v_s)²/2g
= v_s(w_o - v_s)g,

which will be small, if v_s is small compared with w_o. Its greatest
value, if v_s = ½w_o, is ½w_o²/2g, which will always be a small part
of the whole head. Suppose this neglected. The whole variation of
pressure in the pump disk then balances the lift and the head u_i²/2g
necessary to give the initial velocity of flow in the eye of the
wheel.

u_i²/2g + H = V_o²/2g - u_o² cosec² [phi]/2g + u_i²/2g,

H = V_o²/2g - u_o² cosec² [phi]/2g

or

V_o = [root](2gH + u_o² cosec² [phi]). (8)

and the efficiency of the pump is, from (7),

[eta] = gH/V_o w_o = gH/{V (V_o - n_o cot [phi])},

= (V_o² - u_o² cosec² [phi])/{2V_o (V_o - u_o cot [phi]) }, (9).

For [phi] = 90°,

[eta] = (V_o² - u_o²)/2V_o²,

which is necessarily less than ½. That is, half the work expended in
driving the pump is wasted. By recurving the vanes, a plan introduced
by Appold, the efficiency is increased, because the velocity v_o of
discharge from the pump is diminished. If [phi] is very small,

cosec [phi] = cot [phi];

and then

[eta] = (V_o, + u_o cosec [phi])/2V_o,

which may approach the value 1, as [phi] tends towards 0. Equation (8)
shows that u_o cosec [phi] cannot be greater than V_o. Putting u_o =
0.25 [root](2gH) we get the following numerical values of the
efficiency and the circumferential velocity of the pump:--

[phi] [eta] V_o

90° 0.47 1.03 [root](2gH)
45° 0.56 1.06 "
30° 0.65 1.12 "
20° 0.73 1.24 "
10° 0.84 1.75 "

[phi] cannot practically be made less than 20°; and, allowing for the
frictional losses neglected, the efficiency of a pump in which [phi] =
20° is found to be about .60.

§ 210. Case 2. _Pump with a Whirlpool Chamber_, as in fig.
210.--Professor James Thomson first suggested that the energy of the
water after leaving the pump disk might be utilized, if a space were
left in which a free vortex could be formed. In such a free vortex the
velocity varies inversely as the radius. The gain of pressure in the
vortex chamber is, putting r_o, r_w for the radii to the outlet
surface of wheel and to outside of free vortex,

v_o² / r_o² \ v_o² / \
---- ( 1 - ---- ) = ---- ( 1 - k² ),
2g \ r_w² / 2g \ /

if

k = r_o/r_w.

The lift is then, adding this to the lift in the last case,

H = {V_o² - u_o² cosec² [phi] + v_o²(1 - k²)}/2g.

But

v_o² = V_o² - 2V_o u_o cot [phi] + u_o² cosec² [phi];

.: H = {(2 - k²)V_o² - 2kV_o u_o cot [phi] - k²u_o² cosec² [phi]}/2g. (10)

Putting this in the expression for the efficiency, we find a
considerable increase of efficiency. Thus with

[phi] = 90° and k = ½, [eta] = 7/8 nearly,

[phi] a small angle and k = ½, [eta] = 1 nearly.

With this arrangement of pump, therefore, the angle at the outer ends
of the vanes is of comparatively little importance. A moderate angle
of 30° or 40° may very well be adopted. The following numerical values
of the velocity of the circumference of the pump have been obtained by
taking k = ½, and u_o = 0.25 [root](2gH).

[phi] V_o

90° .762 [root](2gH)
45° .842 "
30° .911 "
20° 1.023 "

The quantity of water to be pumped by a centrifugal pump necessarily
varies, and an adjustment for different quantities of water cannot
easily be introduced. Hence it is that the average efficiency of pumps
of this kind is in practice less than the efficiencies given above.
The advantage of a vortex chamber is also generally neglected. The
velocity in the supply and discharge pipes is also often made greater
than is consistent with a high degree of efficiency. Velocities of 6
or 7 ft. per second in the discharge and suction pipes, when the lift
is small, cause a very sensible waste of energy; 3 to 6 ft. would be
much better. Centrifugal pumps of very large size have been
constructed. Easton and Anderson made pumps for the North Sea canal in
Holland to deliver each 670 tons of water per minute on a lift of 5
ft. The pump disks are 8 ft. diameter. J. and H. Gwynne constructed
some pumps for draining the Ferrarese Marshes, which together deliver
2000 tons per minute. A pump made under Professor J. Thomson's
direction for drainage works in Barbados had a pump disk 16 ft. in
diameter and a whirlpool chamber 32 ft. in diameter. The efficiency of
centrifugal pumps when delivering less or more than the normal
quantity of water is discussed in a paper in the _Proc. Inst. Civ.
Eng._ vol. 53.

§ 211. _High Lift Centrifugal Pumps._--It has long been known that centrifugal pumps could be worked in series, each pump overcoming a part of the lift. This method has been perfected, and centrifugal pumps for very high lifts with great efficiency have been used by Sulzer and others. C. W. Darley (_Proc. Inst. Civ. Eng._, supplement to vol. 154, p. 156) has described some pumps of this new type driven by Parsons steam turbines for the water supply of Sydney, N.S.W. Each pump was designed to deliver 1½ million gallons per twenty-four hours against a head of 240 ft. at 3300 revs. per minute. Three pumps in series give therefore a lift of 720 ft. The pump consists of a central double-sided impeller 12 in. diameter. The water entering at the bottom divides and enters the runner at each side through a bell-mouthed passage. The shaft is provided with ring and groove glands which on the suction side keep the air out and on the pressure side prevent leakage. Some water from the pressure side leaks through the glands, but beyond the first grooves it passes into a pocket and is returned to the suction side of the pump. For the glands on the suction side water is supplied from a low-pressure service. No packing is used in the glands. During the trials no water was seen at the glands. The following are the results of tests made at Newcastle:--

+-------------------------------------+-------+-------+-------+-------+
| | I. | II. | III. | IV. |
+-------------------------------------+-------+-------+-------+-------+
| Duration of test hours | 2 | 1.54 | 1.2 | 1.55 |
| Steam pressure lb. per sq. in. | 57 | 57 | 84 | 55 |
| Weight of steam per water | | | | |
| h.p. hour lb. | 27.93 | 30.67 | 28.83 | 27.89 |
| Speed in revs, per min. | 3300 | 3330 | 3710 | 3340 |
| Height of suction ft. | 11 | 11 | 11 | 11 |
| Total lift ft. | 762 | 744 | 917 | 756 |
| Million galls. per day pumped-- | | | | |
| By Ventun meter | 1.573 | 1.499 | 1.689 | 1.503 |
| By orifice | 1.623 | 1.513 | 1.723 | 1.555 |
| Water h.p. | 252 | 235 | 326 | 239 |
+-------------------------------------+-------+-------+-------+-------+

In trial IV. the steam was superheated 95° F. From other trials under the same conditions as trial I. the Parsons turbine uses 15.6 lb. of steam per brake h.p. hour, so that the combined efficiency of turbine and pumps is about 56%, a remarkably good result.

§ 212. _Air-Lift Pumps._--An interesting and simple method of pumping by compressed air, invented by Dr J. Pohlé of Arizona, is likely to be very useful in certain cases. Suppose a rising main placed in a deep bore hole in which there is a considerable depth of water. Air compressed to a sufficient pressure is conveyed by an air pipe and introduced at the lower end of the rising main. The air rising In the main diminishes the average density of the contents of the main, and their aggregate weight no longer balances the pressure at the lower end of the main due to its submersion. An upward flow is set up, and if the air supply is sufficient the water in the rising main is lifted to any required height. The higher the lift above the level in the bore hole the deeper must be the point at which air is injected. Fig. 212 shows an airlift pump constructed for W. H. Maxwell at the Tunbridge Wells waterworks. There is a two-stage steam air compressor, compressing air to from 90 to 100 lb. per sq. in. The bore hole is 350 ft. deep, lined with steel pipes 15 in. diameter for 200 ft. and with perforated pipes 13½ in. diameter for the lower 150 ft. The rest level of the water is 96 ft. from the ground-level, and the level when pumping 32,000 gallons per hour is 120 ft. from the ground-level. The rising main is 7 in. diameter, and is carried nearly to the bottom of the bore hole and to 20 ft. above the ground-level. The air pipe is 2½ in. diameter. In a trial run 31,402 gallons per hour were raised 133 ft. above the level in the well. Trials of the efficiency of the system made at San Francisco with varying conditions will be found in a paper by E. A. Rix (_Journ. Amer. Assoc. Eng. Soc._ vol. 25, 1900). Maxwell found the best results when the ratio of immersion to lift was 3 to 1 at the start and 2.2 to 1 at the end of the trial. In these conditions the efficiency was 37% calculated on the indicated h.p. of the steam-engine, and 46% calculated on the indicated work of the compressor. 2.7 volumes of free air were used to 1 of water lifted. The system is suitable for temporary purposes, especially as the quantity of water raised is much greater than could be pumped by any other system in a bore hole of a given size. It is useful for clearing a boring of sand and may be advantageously used permanently when a boring is in sand or gravel which cannot be kept out of the bore hole. The initial cost is small.

§ 213. _Centrifugal Fans._--Centrifugal fans are constructed similarly to centrifugal pumps, and are used for compressing air to pressures not exceeding 10 to 15 in. of water-column. With this small variation of pressure the variation of volume and density of the air may be neglected without sensible error. The conditions of pressure and discharge for fans are generally less accurately known than in the case of pumps, and the design of fans is generally somewhat crude. They seldom have whirlpool chambers, though a large expanding outlet is provided in the case of the important Guibal fans used in mine ventilation.

It is usual to reckon the difference of pressure at the inlet and
outlet of a fan in inches of water-column. One inch of water-column =
64.4 ft. of air at average atmospheric pressure = 5.2lb. per sq. ft.

Roughly the pressure-head produced in a fan without means of utilizing
the kinetic energy of discharge would be v²/2g ft. of air, or 0.00024
v² in. of water, where v is the velocity of the tips of the fan blades
in feet per second. If d is the diameter of the fan and t the width at
the external circumference, then [pi]dt is the discharge area of the
fan disk. If Q is the discharge in cub. ft. per sec., u = Q/[pi]dt is
the radial velocity of discharge which is numerically equal to the
discharge per square foot of outlet in cubic feet per second. As both
the losses in the fan and the work done are roughly proportional to u²
in fans of the same type, and are also proportional to the gauge
pressure p, then if the losses are to be a constant percentage of the
work done u may be taken proportional to [root]p. In ordinary cases u
= about 22[root]p. The width t of the fan is generally from 0.35 to
0.45d. Hence if Q is given, the diameter of the fan should be:--

For t = 0.35d, d = 0.20 [root](Q/[root]p)
For t = 0.45d, d = 0.18 [root](Q/[root]p)

If p is the pressure difference in the fan in inches of water, and N
the revolutions of fan,

v = [pi]dN/60 ft. per sec.
N = 1230 [root]p/d revs. per min.

As the pressure difference is small, the work done in compressing the
air is almost exactly 5.2pQ foot-pounds per second. Usually, however,
the kinetic energy of the air in the discharge pipe is not
inconsiderable compared with the work done in compression. If w is the
velocity of the air where the discharge pressure is measured, the air
carries away w²/2g foot-pounds per lb. of air as kinetic energy. In Q
cubic feet or 0.0807 Qlb. the kinetic energy is 0.00125 Qw²
foot-pounds per second.

The efficiency of fans is reckoned in two ways. If B.H.P. is the
effective horse-power applied at the fan shaft, then the efficiency
reckoned on the work of compression is

[eta] = 5.2 pQ/550 B.H.P.

On the other hand, if the kinetic energy in the delivery pipe is taken
as part of the useful work the efficiency is

[eta]2 = (5.2 pQ + 0.00125 Qw²)/550 B.H.P.

Although the theory above is a rough one it agrees sufficiently with
experiment, with some merely numerical modifications.

An extremely interesting experimental investigation of the action of
centrifugal fans has been made by H. Heenan and W. Gilbert (_Proc.
Inst. Civ. Eng._ vol. 123, p. 272). The fans delivered through an air
trunk in which different resistances could be obtained by introducing
diaphragms with circular apertures of different sizes. Suppose a fan
run at constant speed with different resistances and the compression
pressure, discharge and brake horse-power measured. The results plot
in such a diagram as is shown in fig. 213. The less the resistance to
discharge, that is the larger the opening in the air trunk, the
greater the quantity of air discharged at the given speed of the fan.
On the other hand the compression pressure diminishes. The curve
marked total gauge is the compression pressure + the velocity head in
the discharge pipe, both in inches of water. This curve falls, but not
nearly so much as the compression curve, when the resistance in the
air trunk is diminished. The brake horse-power increases as the
resistance is diminished because the volume of discharge increases
very much. The curve marked efficiency is the efficiency calculated
on the work of compression only. It is zero for no discharge, and zero
also when there is no resistance and all the energy given to the air
is carried away as kinetic energy. There is a discharge for which this
efficiency is a maximum; it is about half the discharge which there is
when there is no resistance and the delivery pipe is full open. The
conditions of speed and discharge corresponding to the greatest
efficiency of compression are those ordinarily taken as the best
normal conditions of working. The curve marked total efficiency gives
the efficiency calculated on the work of compression and kinetic
energy of discharge. Messrs Gilbert and Heenan found the efficiencies
of ordinary fans calculated on the compression to be 40 to 60% when
working at about normal conditions.

Taking some of Messrs Heenan and Gilbert's results for ordinary fans
in normal conditions, they have been found to agree fairly with the
following approximate rules. Let p_c be the compression pressure and q
the volume discharged per second per square foot of outlet area of
fan. Then the total gauge pressure due to pressure of compression and
velocity of discharge is approximately: p = p_c + 0.0004 q² in. of
water, so that if p_c is given, p can be found approximately. The
pressure p depends on the circumferential speed v of the fan disk--

p = 0.00025 v² in. of water

v = 63 [root]p ft. per sec.

The discharge per square foot of outlet of fan is--

q = 15 to 18 [root]p cub. ft. per sec.

The total discharge is

Q = [pi] dt q = 47 to 56 dt [root]p

For

t = .35d, d = 0.22 to 0.25 [root](Q/[root]p) ft.

t = .45d, d = 0.20 to 0.22 [root](Q/[root]p) ft.

N = 1203 [root]p/d.

These approximate equations, which are derived purely from experiment,
do not differ greatly from those obtained by the rough theory given
above. The theory helps to explain the reason for the form of the
empirical results. (W. C. U.)

FOOTNOTES:

[1] Except where other units are given, the units throughout this
article are feet, pounds, pounds per sq. ft., feet per second.

[2] _Journal de M. Liouville_, t. xiii. (1868); _Mémoires de
l'Académie, des Sciences de l'Institut de France_, t. xxiii., xxiv.
(1877).

[3] The following theorem is taken from a paper by J. H. Cotterill,
"On the Distribution of Energy in a Mass of Fluid in Steady Motion,"
_Phil. Mag._, February 1876.

[4] The discharge per second varied from .461 to .665 cub. ft. in two
experiments. The coefficient .435 is derived from the mean value.

[5] "Formulae for the Flow of Water in Pipes," _Industries_
(Manchester, 1886).

[6] Boussinesq has shown that this mode of determining the corrective
factor [alpha] is not satisfactory.

[7] In general, because when the water leaves the turbine wheel it
ceases to act on the machine. If deflecting vanes or a whirlpool are
added to a turbine at the discharging side, then v1 may in part
depend on v2, and the statement above is no longer true.

HYDRAZINE (DIAMIDOGEN), N2H4 or H2 N·NH2, a compound of hydrogen and nitrogen, first prepared by Th. Curtius in 1887 from diazo-acetic ester, N2CH·CO2C2H5. This ester, which is obtained by the action of potassium nitrate on the hydrochloride of amidoacetic ester, yields on hydrolysis with hot concentrated potassium hydroxide an acid, which Curtius regarded as C3H3N6(CO2H)3, but which A. Hantzsch and O. Silberrad (_Ber._, 1900, 33, p. 58) showed to be C2H2N4(CO2H)2, bisdiazoacetic acid. On digestion of its warm aqueous solution with warm dilute sulphuric acid, hydrazine sulphate and oxalic acid are obtained. C. A. Lobry de Bruyn (_Ber._, 1895, 28, p. 3085) prepared free hydrazine by dissolving its hydrochloride in methyl alcohol and adding sodium methylate; sodium chloride was precipitated and the residual liquid afterwards fractionated under reduced pressure. It can also be prepared by reducing potassium dinitrososulphonate in ice cold water by means of sodium amalgam:--

KSO3 \ KSO3 \
> N·NO --> > N·NH2 --> K2SO4 + N2H4.
KO / H /

P. J. Schestakov (_J. Russ. Phys. Chem. Soc._, 1905, 37, p. 1) obtained hydrazine by oxidizing urea with sodium hypochlorite in the presence of benzaldehyde, which, by combining with the hydrazine, protected it from oxidation. F. Raschig (German Patent 198307, 1908) obtained good yields by oxidizing ammonia with sodium hypochlorite in solutions made viscous with glue. Free hydrazine is a colourless liquid which boils at 113.5° C., and solidifies about 0° C. to colourless crystals; it is heavier than water, in which it dissolves with rise of temperature. It is rapidly oxidized on exposure, is a strong reducing agent, and reacts vigorously with the halogens. Under certain conditions it may be oxidized to azoimide (A. W. Browne and F. F. Shetterly, _J. Amer. C.S._, 1908, p. 53). By fractional distillation of its aqueous solution hydrazine hydrate N2H4·H2O (or perhaps H2N·NH3OH), a strong base, is obtained, which precipitates the metals from solutions of copper and silver salts at ordinary temperatures. It dissociates completely in a vacuum at 143°, and when heated under atmospheric pressure to 183° it decomposes into ammonia and nitrogen (A. Scott, _J. Chem. Soc._, 1904, 85, p. 913). The sulphate N2H4·H2SO4, crystallizes in tables which are slightly soluble in cold water and readily soluble in hot water; it is decomposed by heating above 250° C. with explosive evolution of gas and liberation of sulphur. By the addition of barium chloride to the sulphate, a solution of the hydrochloride is obtained, from which the crystallized salt may be obtained on evaporation.

Many organic derivatives of hydrazine are known, the most important
being phenylhydrazine, which was discovered by Emil Fischer in 1877.
It can be best prepared by V. Meyer and Lecco's method (_Ber._, 1883,
16, p. 2976), which consists in reducing phenyldiazonium chloride in
concentrated hydrochloric acid solution with stannous chloride also
dissolved in concentrated hydrochloric acid. Phenylhydrazine is
liberated from the hydrochloride so obtained by adding sodium
hydroxide, the solution being then extracted with ether, the ether
distilled off, and the residual oil purified by distillation under
reduced pressure. Another method is due to E. Bamberger. The diazonium
chloride, by the addition of an alkaline sulphite, is converted into a
diazosulphonate, which is then reduced by zinc dust and acetic acid to
phenylhydrazine potassium sulphite. This salt is then hydrolysed by
heating it with hydrochloric acid--

C6H5N2Cl + K2SO3 = KCl + C6H5N2·SO3K,

C6H5N2·SO3K + 2H = C6H5·NH·NH·SO3K,

C6H5NH·NH·SO3K + HCl + H2O = C6H5·NH·NH2·HCl + KHSO4.

Phenylhydrazine is a colourless oily liquid which turns brown on
exposure. It boils at 241° C., and melts at 17.5° C. It is slightly
soluble in water, and is strongly basic, forming well-defined salts
with acids. For the detection of substances containing the carbonyl
group (such for example as aldehydes and ketones) phenylhydrazine is a
very important reagent, since it combines with them with elimination
of water and the formation of well-defined hydrazones (see ALDEHYDES,
KETONES and SUGARS). It is a strong reducing agent; it precipitates
cuprous oxide when heated with Fehling's solution, nitrogen and
benzene being formed at the same time--C6H5·NH·NH2 + 2CuO = Cu2O + N2
+ H2O + C6H5. By energetic reduction of phenylhydrazine (e.g. by use
of zinc dust and hydrochloric acid), ammonia and aniline are
produced--C6H5NH·NH2 + 2H = C6H5NH2 + NH3. It is also a most important
synthetic reagent. It combines with aceto-acetic ester to form
phenylmethylpyrazolone, from which antipyrine (q.v.) may be obtained.
Indoles (q.v.) are formed by heating certain hydrazones with anhydrous
zinc chloride; while semicarbazides, pyrrols (q.v.) and many other
types of organic compounds may be synthesized by the use of suitable
phenylhydrazine derivatives.

HYDRAZONE, in chemistry, a compound formed by the condensation of a hydrazine with a carbonyl group (see ALDEHYDES; KETONES).

HYDROCARBON, in chemistry, a compound of carbon and hydrogen. Many occur in nature in the free state: for example, natural gas, petroleum and paraffin are entirely composed of such bodies; other natural sources are india-rubber, turpentine and certain essential oils. They are also revealed by the spectroscope in stars, comets and the sun. Of artificial productions the most fruitful and important is provided by the destructive or dry distillation of many organic substances; familiar examples are the distillation of coal, which yields ordinary lighting gas, composed of gaseous hydrocarbons, and also coal tar, which, on subsequent fractional distillations, yields many liquid and solid hydrocarbons, all of high industrial value. For details reference should be made to the articles wherein the above subjects are treated. From the chemical point of view the hydrocarbons are of fundamental importance, and, on account of their great number, and still greater number of derivatives, they are studied as a separate branch of the science, namely, organic chemistry.

See CHEMISTRY for an account of their classification, &c.

HYDROCELE (Gr. [Greek: hydôr], water, and [Greek: kêlê], tumour), the medical term for any collection of fluid other than pus or blood in the neighbourhood of the testis or cord. The fluid is usually serous. Hydrocele may be congenital or arise in the middle-aged without apparent cause, but it is usually associated with chronic orchitis or with tertiary syphilitic enlargements. The hydrocele appears as a rounded, fluctuating translucent swelling in the scrotum, and when greatly distended causes a dragging pain. Palliative treatment consists in tapping aseptically and removing the fluid, the patient afterwards wearing a suspender. The condition frequently recurs and necessitates radical treatment. Various substances may be injected; or the hydrocele is incised, the tunica partly removed and the cavity drained.

HYDROCEPHALUS (Gr. [Greek: hydôr], water, and [Greek: kephalê], head), a term applied to disease of the brain which is attended with excessive effusion of fluid into its cavities. It exists in two forms--_acute_ and _chronic hydrocephalus_. Acute hydrocephalus is another name for tuberculous meningitis (see MENINGITIS).

_Chronic hydrocephalus_, or "water on the brain," consists in an effusion of fluid into the lateral ventricles of the brain. It is not preceded by tuberculous deposit or acute inflammation, but depends upon congenital malformation or upon chronic inflammatory changes affecting the membranes. When the disease is congenital, its presence in the foetus is apt to be a source of difficulty in parturition. It is however more commonly developed in the first six months of life; but it occasionally arises in older children, or even in adults. The chief symptom is the gradual increase in size of the upper part of the head out of all proportion to the face or the rest of the body. Occurring at an age when as yet the bones of the skull have not become welded together, the enlargement may go on to an enormous extent, the Spaces between the bones becoming more and more expanded. In a well-marked case the deformity is very striking; the upper part of the forehead projects abnormally, and the orbital plates of the frontal bone being inclined forwards give a downward tilt to the eyes, which have also peculiar rolling movements. The face is small, and this, with the enlarged head, gives a remarkable aged expression to the child. The body is ill-nourished, the bones are thin, the hair is scanty and fine and the teeth carious or absent.

The average circumference of the adult head is 22 in., and in the normal child it is of course much less. In chronic hydrocephalus the head of an infant three months old has measured 29 in.; and in the case of the man Cardinal, who died in Guy's Hospital, the head measured 33 in. In such cases the head cannot be supported by the neck, and the patient has to keep mostly in the recumbent posture. The expansibility of the skull prevents destructive pressure on the brain, yet this organ is materially affected by the presence of the fluid. The cerebral ventricles are distended, and the convolutions are flattened. Occasionally the fluid escapes into the cavity of the cranium, which it fills, pressing down the brain to the base of the skull. As a consequence, the functions of the brain are interfered with, and the mental condition is impaired. The child is dull, listless and irritable, and sometimes imbecile. The special senses become affected as the disease advances; sight is often lost, as is also hearing. Hydrocephalic children generally sink in a few years; nevertheless there have been instances of persons with this disease living to old age. There are, of course, grades of the affection, and children may present many of the symptoms of it in a slight degree, and yet recover, the head ceasing to expand, and becoming in due course firmly ossified.

Various methods of treatment have been employed, but the results are unsatisfactory. Compression of the head by bandages, and the administration of mercury with the view of promoting absorption of the fluid, are now little resorted to. Tapping the fluid from time to time through one of the spaces between the bones, drawing off a little, and thereafter employing gentle pressure, has been tried, but rarely with benefit. Attempts have also been made to establish a permanent drainage between the interior of the lateral ventricle and the sub-dural space, and between the lumbar region of the spine and the abdomen, but without satisfactory results. On the whole, the plan of treatment which aims at maintaining the patient's nutrition by appropriate food and tonics is the most rational and successful. (E. O.*)

HYDROCHARIDEAE, in botany, a natural order of Monocotyledons, belonging to the series Helobieae. They are water-plants, represented in Britain by frog-bit (_Hydrocharis Morsusranae_) and water-soldier (_Stratiotes aloïdes_). The order contains about fifty species in fifteen genera, twelve of which occur in fresh water while three are marine: and includes both floating and submerged forms. _Hydrocharis_ floats on the surface of still water, and has rosettes of kidney-shaped leaves, from among which spring the flower-stalks; stolons bearing new leaf-rosettes are sent out on all sides, the plant thus propagating itself on the same way as the strawberry. _Stratiotes aloïdes_ has a rosette of stiff sword-like leaves, which when the plant is in flower project above the surface; it is also stoloniferous, the young rosettes sinking to the bottom at the beginning of winter and rising again to the surface in the spring. _Vallisneria_ (eel-grass) contains two species, one native of tropical Asia, the other inhabiting the warmer parts of both hemispheres and reaching as far north as south Europe. It grows in the mud at the bottom of fresh water, and the short stem bears a cluster of long, narrow grass-like leaves; new plants are formed at the end of horizontal runners. Another type is represented by _Elodea canadensis_ or water-thyme, which has been introduced into the British Isles from North America. It is a small, submerged plant with long, slender branching stems bearing whorls of narrow toothed leaves; the flowers appear at the surface when mature. _Halophila_, _Enhalus_ and _Thalassia_ are submerged maritime plants found on tropical coasts, mainly in the Indian and Pacific oceans; _Halophila_ has an elongated stem rooting at the nodes; _Enhalus_ a short, thick rhizome, clothed with black threads resembling horse-hair, the persistent hard-bast strands of the leaves; _Thalassia_ has a creeping rooting stem with upright branches bearing crowded strap-shaped leaves in two rows. The flowers spring from, or are enclosed in, a spathe, and are unisexual and regular, with generally a calyx and corolla, each of three members; the stamens are in whorls of three, the inner whorls are often barren; the two to fifteen carpels form an inferior ovary containing generally numerous ovules on often large, produced, parietal placentas. The fruit is leathery or fleshy, opening irregularly. The seeds contain a large embryo and no endosperm. In _Hydrocharis_ (fig. 1), which is dioecious, the flowers are borne above the surface of the water, have conspicuous white petals, contain honey and are pollinated by insects. _Stratiotes_ has similar flowers which come above the surface only for pollination, becoming submerged again during ripening of the fruit. In _Vallisneria_ (fig. 2), which is also dioecious, the small male flowers are borne in large numbers in short-stalked spathes; the petals are minute and scale-like, and only two of the three stamens are fertile; the flowers become detached before opening and rise to the surface, where the sepals expand and form a float bearing the two projecting semi-erect stamens. The female flowers are solitary and are raised to the surface on a long, spiral stalk; the ovary bears three broad styles, on which some of the large, sticky pollen-grains from the floating male flowers get deposited, (fig. 3). After pollination the female flower becomes drawn below the surface by the spiral contraction of the long stalk, and the fruit ripens near the bottom. _Elodea_ has polygamous flowers (that is, male, female and hermaphrodite), solitary, in slender, tubular spathes; the male flowers become detached and rise to the surface; the females are raised to the surface when mature, and receive the floating pollen from the male. The flowers of _Halophila_ are submerged and apetalous.

1, Female flower.
2, Stamens, enlarged.
3, Barren pistil of male flower, enlarged.
4, Pistil of female flower.
5, Fruit.
6, Fruit cut transversely.
7, Seed.
8, 9, Floral diagrams of male and female flowers respectively.
s, Rudimentary stamens.]

The order is a widely distributed one; the marine forms are tropical or subtropical, but the fresh-water genera occur also in the temperate zones.

HYDROCHLORIC ACID, also known in commerce as "spirits of salts" and "muriatic acid," a compound of hydrogen and chlorine. Its chemistry is discussed under CHLORINE, and its manufacture under ALKALI MANUFACTURE.

HYDRODYNAMICS (Gr. [Greek: hydôr], water, [Greek: dynamis], strength), the branch of hydromechanics which discusses the motion of fluids (see HYDROMECHANICS).

HYDROGEN [symbol H, atomic weight 1.008 (o = 16)], one of the chemical elements. Its name is derived from Gr. [Greek: hydôr], water, and [Greek: gennaein], to produce, in allusion to the fact that water is produced when the gas burns in air. Hydrogen appears to have been recognized by Paracelsus in the 16th century; the combustibility of the gas was noticed by Turquet de Mayenne in the 17th century, whilst in 1700 N. Lémery showed that a mixture of hydrogen and air detonated on the application of a light. The first definite experiments concerning the nature of hydrogen were made in 1766 by H. Cavendish, who showed that it was formed when various metals were acted upon by dilute sulphuric or hydrochloric acids. Cavendish called it "inflammable air," and for some time it was confused with other inflammable gases, all of which were supposed to contain the same inflammable principle, "phlogiston," in combination with varying amounts of other substances. In 1781 Cavendish showed that water was the only substance produced when hydrogen was burned in air or oxygen, it having been thought previously to this date that other substances were formed during the reaction, A. L. Lavoisier making many experiments with the object of finding an acid among the products of combustion.

Hydrogen is found in the free state in some volcanic gases, in fumaroles, in the carnallite of the Stassfurt potash mines (H. Precht, _Ber._, 1886, 19, p. 2326), in some meteorites, in certain stars and nebulae, and also in the envelopes of the sun. In combination it is found as a constituent of water, of the gases from certain mineral springs, in many minerals, and in most animal and vegetable tissues. It may be prepared by the electrolysis of acidulated water, by the decomposition of water by various metals or metallic hydrides, and by the action of many metals on acids or on bases. The alkali metals and alkaline earth metals decompose water at ordinary temperatures; magnesium begins to react above 70° C., and zinc at a dull red heat. The decomposition of steam by red hot iron has been studied by H. Sainte-Claire Deville (_Comptes rendus_, 1870, 70, p. 1105) and by H. Debray (ibid., 1879, 88, p. 1341), who found that at about 1500° C. a condition of equilibrium is reached. H. Moissan (_Bull. soc. chim._, 1902, 27, p. 1141) has shown that potassium hydride decomposes cold water, with evolution of hydrogen, KH + H2O = KOH + H2. Calcium hydride or hydrolite, prepared by passing hydrogen over heated calcium, decomposes water similarly, 1 gram giving 1 litre of gas; it has been proposed as a commercial source (Prats Aymerich, _Abst. J.C.S._, 1907, ii. p. 543), as has also aluminium turnings moistened with potassium cyanide and mercuric chloride, which decomposes water regularly at 70°, 1 gram giving 1.3 litres of gas (Mauricheau-Beaupré, _Comptes rendus_, 1908, 147, p. 310). Strontium hydride behaves similarly. In preparing the gas by the action of metals on acids, dilute sulphuric or hydrochloric acid is taken, and the metals commonly used are zinc or iron. So obtained, it contains many impurities, such as carbon dioxide, nitrogen, oxides of nitrogen, phosphoretted hydrogen, arseniuretted hydrogen, &c., the removal of which is a matter of great difficulty (see E. W. Morley, _Amer. Chem. Journ._, 1890, 12, p. 460). When prepared by the action of metals on bases, zinc or aluminium and caustic soda or caustic potash are used. Hydrogen may also be obtained by the action of zinc on ammonium salts (the nitrate excepted) (Lorin, _Comptes rendus_, 1865, 60, p. 745) and by heating the alkali formates or oxalates with caustic potash or soda, Na2C2O4 + 2NaOH = H2 + 2Na2CO3. Technically it is prepared by the action of superheated steam on incandescent coke (see F. Hembert and Henry, _Comptes rendus_, 1885, 101, p. 797; A. Naumann and C. Pistor, _Ber._, 1885, 18, p. 1647), or by the electrolysis of a dilute solution of caustic soda (C. Winssinger, _Chem. Zeit._, 1898, 22, p. 609; "Die Elektrizitäts-Aktiengesellschaft," _Zeit. f. Elektrochem._, 1901, 7, p. 857). In the latter method a 15% solution of caustic soda is used, and the electrodes are made of iron; the cell is packed in a wooden box, surrounded with sand, so that the temperature is kept at about 70° C.; the solution is replenished, when necessary, with distilled water. The purity of the gas obtained is about 97%.

Pure hydrogen is a tasteless, colourless and odourless gas of specific gravity 0.06947 (air = 1) (Lord Rayleigh, _Proc. Roy. Soc._, 1893, p. 319). It may be liquefied, the liquid boiling at -252.68° C. to -252.84° C., and it has also been solidified, the solid melting at -264° C. (J. Dewar, _Comptes rendus_, 1899, 129, p. 451; _Chem. News_, 1901, 84, p. 49; see also LIQUID GASES). The specific heat of gaseous hydrogen (at constant pressure) is 3.4041 (water = 1), and the ratio of the specific heat at constant pressure to the specific heat at constant volume is 1.3852 (W. C. Röntgen, _Pogg. Ann._, 1873, 148, p. 580). On the spectrum see SPECTROSCOPY. Hydrogen is only very slightly soluble in water. It diffuses very rapidly through a porous membrane, and through some metals at a red heat (T. Graham, _Proc. Roy. Soc._, 1867, 15, p. 223; H. Sainte-Claire Deville and L. Troost, _Comptes rendus_, 1863, 56, p. 977). Palladium and some other metals are capable of absorbing large volumes of hydrogen (especially when the metal is used as a cathode in a water electrolysis apparatus). L. Troost and P. Hautefeuille (_Ann. chim. phys._, 1874, (5) 2, p. 279) considered that a palladium hydride of composition Pd2H was formed, but the investigations of C. Hoitsema (_Zeit. phys. Chem._, 1895, 17, p. 1), from the standpoint of the phase rule, do not favour this view, Hoitsema being of the opinion that the occlusion of hydrogen by palladium is a process of continuous absorption. Hydrogen burns with a pale blue non-luminous flame, but will not support the combustion of ordinary combustibles. It forms a highly explosive mixture with air or oxygen, especially when in the proportion of two volumes of hydrogen to one volume of oxygen. H. B. Baker (_Proc. Chem. Soc._, 1902, 18, p. 40) has shown that perfectly dry hydrogen will not unite with perfectly dry oxygen. Hydrogen combines with fluorine, even at very low temperatures, with great violence; it also combines with carbon, at the temperature of the electric arc. The alkali metals when warmed in a current of hydrogen, at about 360° C., form hydrides of composition RH (R = Na, K, Rb, Cs), (H. Moissan, _Bull. soc. chim._, 1902, 27, p. 1141); calcium and strontium similarly form hydrides CaH2, SrH2 at a dull red heat (A. Guntz, _Comptes rendus_, 1901, 133, p. 1209). Hydrogen is a very powerful reducing agent; the gas occluded by palladium being very active in this respect, readily reducing ferric salts to ferrous salts, nitrates to nitrites and ammonia, chlorates to chlorides, &c.

For determinations of the volume ratio with which hydrogen and oxygen
combine, see J. B. Dumas, _Ann. chim. phys._, 1843 (3), 8, p. 189; O.
Erdmann and R. F. Marchand, ibid., p. 212; E. H. Keiser, _Ber._, 1887,
20, p. 2323; J. P. Cooke and T. W. Richards, _Amer. Chem. Journ._,
1888, 10, p. 191; Lord Rayleigh, _Chem. News_, 1889, 59, p. 147; E. W.
Morley, _Zeit. phys. Chem._, 1890, 20, p. 417; and S. A. Leduc,
_Comptes rendus_, 1899, 128, p. 1158.

Hydrogen combines with oxygen to form two definite compounds, namely, water (q.v.), H2O, and hydrogen peroxide, H2O2, whilst the existence of a third oxide, ozonic acid, has been indicated.

_Hydrogen peroxide_, H2O2, was discovered by L. J. Thénard in 1818 (_Ann. chim. phys._, 8, p. 306). It occurs in small quantities in the atmosphere. It may be prepared by passing a current of carbon dioxide through ice-cold water, to which small quantities of barium peroxide are added from time to time (F. Duprey, _Comptes rendus_, 1862, 55, p. 736; A. J. Balard, ibid., p. 758), BaO2 + CO2 + H2O = H2O2 + BaCO3. E. Merck (_Abst. J.C.S._, 1907, ii., p. 859) showed that barium percarbonate, BaCO4, is formed when the gas is in excess; this substance readily yields the peroxide with an acid. Or barium peroxide may be decomposed by hydrochloric, hydrofluoric, sulphuric or silicofluoric acids (L. Crismer, _Bull. soc. chim._, 1891 (3), 6, p. 24; Hanriot, _Comptes rendus_, 1885, 100, pp. 56, 172), the peroxide being added in small quantities to a cold dilute solution of the acid. It is necessary that it should be as pure as possible since the commercial product usually contains traces of ferric, manganic and aluminium oxides, together with some silica. To purify the oxide, it is dissolved in dilute hydrochloric acid until the acid is neatly neutralized, the solution is cooled, filtered, and baryta water is added until a faint permanent white precipitate of hydrated barium peroxide appears; the solution is now filtered, and a concentrated solution of baryta water is added to the filtrate, when a crystalline precipitate of hydrated barium peroxide, BaO2·H2O, is thrown down. This is filtered off and well washed with water. The above methods give a dilute aqueous solution of hydrogen peroxide, which may be concentrated somewhat by evaporation over sulphuric acid in vacuo. H. P. Talbot and H. R. Moody (_Jour. Anal. Chem._, 1892, 6, p. 650) prepared a more concentrated solution from the commercial product, by the addition of a 10% solution of alcohol and baryta water. The solution is filtered, and the barium precipitated by sulphuric acid. The alcohol is removed by distillation _in vacuo_, and by further concentration _in vacuo_ a solution may be obtained which evolves 580 volumes of oxygen. R. Wolffenstein (_Ber._, 1894, 27, p. 2307) prepared practically anhydrous hydrogen peroxide (containing 99.1% H2O2) by first removing all traces of dust, heavy metals and alkali from the commercial 3% solution. The solution is then concentrated in an open basis on the water-bath until it contains 48% H2O2. The liquid so obtained is extracted with ether and the ethereal solution distilled under diminished pressure, and finally purified by repeated distillations. W. Staedel (_Zeit. f. angew. Chem._, 1902, 15, p. 642) has described solid hydrogen peroxide, obtained by freezing concentrated solutions.

Hydrogen peroxide is also found as a product in many chemical actions, being formed when carbon monoxide and cyanogen burn in air (H. B. Dixon); by passing air through solutions of strong bases in the presence of such metals as do not react with the bases to liberate hydrogen; by shaking zinc amalgam with alcoholic sulphuric acid and air (M. Traube, _Ber._, 1882, 15, p. 659); in the oxidation of zinc, lead and copper in presence of water, and in the electrolysis of sulphuric acid of such strength that it contains two molecules of water to one molecule of sulphuric acid (M. Berthelot, _Comptes rendus_, 1878, 86, p. 71).