Chapter XXVIII: Section I: Physical Science

IT should be understood that the aim of this chapter is not to present a history of science, but chiefly to indicate what Scotland has contributed to the science of the period, and to show the importance of science as a factor in advancing civilisation. It will, therefore, in the first place, touch on the significance and progress of mathematical science; in the second, on physical science, or natural philosophy; and in the third, on mechanical science, or science in relation to its practical application.

From an early period various conceptions of the universe have been entertained; even Newton’s conception of it and his system founded on the theory of gravitation are not quite satisfactory. Hence his followers for a long time mainly occupied themselves in defending and explaining his system; they were loth to recognise that either his method or system was susceptible of improvement or extension――they adhered to his conclusions with extreme tenacity and superstitious veneration. While, in other parts of Europe, Descartes’ theory of the universe held the field, the earliest recorded recognition of Newton’s principles in France was in a memoir by Lonville, which appeared in a volume of the _Academy of Sciences_ for the year 1720; and the first French astronomer who ventured on a defence of the theory of gravitation was Maupertius, in his work on the figures of the celestial bodies, published in 1732. He compared the theories of Descartes and Newton, and came to a conclusion in favour of the latter. It was Voltaire, however, that really diffused a knowledge of Newton’s system in France, by the publication, in 1738, of his clear exposition of Newton’s discoveries in optics and astronomy. Henceforth in France the Newtonian system prevailed over the theory of Descartes.

The controversy touching the priority of the invention of the calculus between Newton and Leibnitz was an unfortunate affair, as it arose from trivial incidents. In short, from the first Leibnitz admitted Newton’s priority in forming the conception of the calculus; but he maintained, what was doubtless true, his own originality in the invention of the differential calculus. This branch of mathematical analysis was not much advanced by Newton and the English geometers of his time; as Newton himself was fond of geometrical forms and the synthetical method of statement. His treatise on _Fluxions_ was not published till after his death in 1736. When, however, it became known that the differential calculus was rapidly circulating on the Continent――and so little was Newton’s method known, that Leibnitz was regarded throughout Europe as its original inventor――the followers of Newton began to feel that this impression was unjust towards their great teacher, and hence the bitterness of the controversy which ensued. The result was a wide alienation between the English and foreign mathematicians, which had a pernicious effect on science. “Each party became the exclusive supporters of the two great luminaries of their respective countries. The British mathematicians, in particular, adhered with the most rigid pertinacity to the very letter of Newton’s methods; and were, with few exceptions, completely ignorant both of the original investigations of the other party, and of the improvements upon them which were being rapidly introduced.

“The difference in name and notation between the two methods, though in itself a trivial circumstance, was yet far from unimportant in some of the consequences which may be fairly traced to it. It tended in some measure to foster and increase the dissension between the two schools, and their ignorance of each other’s researches; while the diversity itself between the two methods, though in reality little more than nominal, became also a topic of no small dispute and controversy. But much as these differences were on all grounds to be lamented, the loss in point of scientific advantages, it must with shame be confessed, were almost entirely on the side of Britain.”¹

¹ Powell’s _History of the Physical and Mathematical Sciences_,
page 363. 1834.

If we reflect upon the past three centuries and ask what were the most requisite means of aiding men in their investigation of nature and the explanation of the phenomena of the universe, the answer must be mathematical science. In astronomy and in other branches of physical science, mathematics are indispensable; for as the universe exists in space and time――the two concepts which encircle all things――so it is a universal truth, that mathematics are not only the prime requisites in physical science, but also essential elements in navigation, engineering, shipbuilding, architecture, and in many other arts.

In the early part of the eighteenth century Scotland produced one eminent mathematician――Colin Maclaurin¹――and several others of lesser note. He was a native of Kilmodan, in Argyleshire, a son of a clergyman, was educated at the University of Glasgow, and early manifested an aptness for mathematics. In 1717, before he had attained his twentieth year, he was elected, after a ten days’ competition, Professor of Mathematics in Marischal College, Aberdeen; but in 1725, he was appointed assistant and successor to Dr. Gregory in the Chair of Mathematics in the University of Edinburgh. It is recorded that he was an able and successful instructor, and that in his public teaching he clearly explained the application of his science to purposes of utility and the perfecting of the mechanical arts.²

¹ Born in 1698; died in 1746.

² He soon became the leading spirit in the University of
Edinburgh, and the celebrity which the Chair of Mathematics
had attained under the Gregories was admirably sustained
and even extended by him. In the _Scots Magazine_, in
1741, a full programme of Maclaurin’s courses of academical
instruction is given as follows:――“He gave every year three
courses, and sometimes a fourth, upon such of the ♦abstruse
parts of the science as were not explained in the former
three. The first course contained――Demonstrations of the
ground of Vulgar and Decimal Arithmetic; six books of Euclid;
Plane Trigonometry and use of the tables of Logarithms,
Sines, etc.; Surveying, Fortification, and other practical
parts; the Elements of Algebra; and a lecture on Geography
once a fortnight.

“The second course consisted of――Algebra; the Theory and
Mensuration of Solids; Spherical Trigonometry, the doctrine
of the Sphere, Dialling, and other practical parts; Conic
Sections, with the theory of Gunnery; the Elements of
Astronomy and Optics. He began the third course with
Perspective; then treated more fully of Astronomy and Optics.
After this he prelected on Sir Isaac Newton’s Principia, and
explained the direct and inverse method of Fluxions. At a
separate hour he gave a course of Experimental Philosophy,
beginning about the middle of December, which continued
thrice every week till the beginning of April; and at proper
hours of the night he described the constellations, and
showed the planets by telescopes of various kinds.” This was
a comprehensive course of teaching. He also exerted himself
to the utmost to provide an Observatory for the instruction
of students in the University of Edinburgh, and would
have been successful but for his early death. Although the
foundation of an Observatory was laid in Edinburgh by the
Town Council and Senatus, on the 25th June, 1776, it was not
until 1834 that the Observatory was rendered available for
the practical instruction of students of the University.

It has been said that in “Maclaurin’s time the teaching of
mathematics reached a point which it cannot be said to have
yet surpassed.”――Sir A. Grant’s _Story of the University of
Edinburgh_, Volume II., page 299.

♦ “abtruse” replaced with “abstruse”

His works are these:――(1) _Geometrica Organica_; (2) _A Complete System of Fluxions_; (3) _Treatise on Algebra_; (4) _Account of Sir Isaac Newton’s Philosophy_; (5) Various elegant and ingenious papers published in the _Transactions of the Royal Society_; and (6) _A Memoir on the Tides_.

In connection with his _Memoir on the Tides_, written in 1740, he gained equal honour with Euler and Daniel ♦Bernoulli, a famous Italian mathematician, as the prize of the French Academy of Sciences was equally divided among them. All the three adopted the principle of gravitation as the basis on which they attempted to explain the phenomena of the tides; and the results which they arrived at, presumed the earth to be at rest and the waters of the ocean also, and at every moment in a state of equilibrium between the force of gravity tending to the earth’s centre, and the lesser forces tending towards the sun and moon. This view is known as the equilibrium theory; and though it is far from perfect, it correctly indicates a part of the phenomena of the tides, notwithstanding that the problem of the tides is a dynamical and not a statical one. Subsequently the important subject of the tides was treated at great length by Laplace, but unfortunately his tidal theory was so profound that very few men have ever attempted to master its difficulties. The late Mr. Airy, the Astronomer Royal, however, gave to the public a connected and clear view of it.¹

♦ “Bernouilli” replaced with “Bernoulli”

¹ Mr. Airy summed up the merits of Laplace’s theory thus:――“If,
putting from our thoughts the details of the investigation,
we consider the general plan and objects, we must allow
it to be one of the most splendid works of the greatest
mathematician of the past age. To appreciate this, the
reader must consider――first, the boldness of the writer
who, having a clear understanding of the gross imperfection
of the methods of his predecessors, had also the courage
deliberately to take up the problem on grounds fundamentally
correct, however it might be limited by suppositions
afterwards introduced; secondly, the general difficulty
of treating the motions of fluids; thirdly, the peculiar
difficulty of treating the motions when the fluids cover
an area which is not plane but convex; and fourthly, the
sagacity of perceiving that it was necessary to consider
the earth as a revolving body, and the skill of directly
introducing this consideration. The last point alone, in
our opinion, gives a greater claim for reputation than the
boasted explanation of the long inequality of Jupiter and
Saturn.”――_Encyclopædia Metroa_, “Tides and Waves,” Article
117; compare Grant’s _History of Astronomy_, page 71,
_et seq._

After Laplace, the theory of the tides has been treated by
Dr. Thomas Young; and Dr. Whewell and Sir John Lubbock were
engaged for many years in determinating the laws of the
tides by observation, and in tracing their connection with
the positions of the sun and moon. The chief results of
their researches were published from time to time in a
series of papers, in the volumes of the _Royal Society_.

Maclaurin also entered the arena in defence of the Fluxional system, which was boldly attacked by Berkeley, the famous Idealist, in _The Analyst_, published in 1734. Berkeley argued that the fundamental idea of supposing a finite ratio to exist between terms absolutely evanescent is completely absurd and unintelligible, and with biting sarcasm called these ratios “the ghosts of departed quantities.” Dr. Irwin produced a reply, and several others appeared on both sides; but Maclaurin and Robins made the most satisfactory defence of the principle of limiting ratios. Still the point was not thoroughly cleared up, till D’Alembert showed the real application of the principle of limits in the simplest form; and at last Lagrange, in his theory of Functions, discarded all idea of infinitesimals and limits, and reduced the whole to a simple algebraical investigation, by the development of functions in series.

Maclaurin’s method and style in the solution of problems were greatly admired. He showed in his applications of mathematics to physical problems a rare power of seizing the really important points amidst a mass of irrelevant details. In private life he was one of the best and worthiest of men.

James Stirling,¹ another distinguished Scottish mathematician, was a native of Stirlingshire. He was educated at the University of Glasgow, and afterwards on Snell’s foundation at Oxford. While studying at Oxford, he printed, in 1717, a small tract on “lines of the third order, with new solutions of some difficult problems by the fluxionary calculus.” Subsequently he accepted an invitation to settle at Venice, where he remained for several years and taught mathematics.

¹ Born about the end of the seventeenth century, and died in
1772.

Having returned home, he opened a mathematical school on Tower Hill, and maintained a correspondence with philosophers at home and abroad. In 1730, he published his well-known work on _The Differential Method and Series_. After toiling in his school for several years, he was induced to leave London, and undertake the direction of the mines at Leadhills in Scotland. In that elevated region, near Sanquhar, he resided during the rest of his life; and by his skill, intelligence, and energy, greatly improved the operations of extracting the lead ore. He now held a good position; but his high mathematical fame would have secured to him the honour of succeeding Maclaurin in the chair of mathematics in Edinburgh, in 1746, if he had not at that unhappy time been tainted with Jacobite opinions.¹ In his later years he seems to have confined himself to practical concerns.

¹ Leslie’s _Dissertation for the Encyclopædia Britannica_.

Matthew Stewart was elected to the vacant chair of mathematics in Edinburgh. He was a mathematician of note in the department of geometry; and is the author of _Tracts, Physical and Mathematical_. Robert Simson, who long held the chair of mathematics in the University of Glasgow, was a distinguished geometer. The names of Playfair and Leslie may also be mentioned as distinguished professors and learned mathematicians.

Passing to physical science, it is necessary to observe that in the eighteenth century the several branches of this department were not then so clearly distinguished from each other as they are now. The subject of heat was treated as a branch of chemistry; while chemistry itself was usually conceived as a mere appendage to medicine. Dr. Cullen was the first in Britain who assigned to chemistry its proper place as a science.

Dr. Black, the eminent professor of chemistry, and the discoverer of latent heat, was born in France, in 1728, where his father was then engaged in the wine trade. He received the rudiments of his education at Belfast; and in 1746, he entered the University of Glasgow, and under Dr. Cullen, was instructed in the science of chemistry, in which he showed much aptitude. As he intended to follow the medical profession, he went to Edinburgh to complete his studies, and graduated in 1754. Before this, he had prosecuted a series of chemical experiments touching the causticity of many earthy bodies, which resulted in his first discovery of the existence of fixed air or carbonic acid gas as an essential constituent of marble and other solids, along with a train of important consequences.

In 1755, Dr. Cullen removed to Edinburgh, and in 1756, Dr. Black succeeded him as professor of medicine and chemistry in the University of Glasgow. At this time he directed special attention to the subject of heat. He had discovered the phenomenon of “Latent Heat” at least as early as 1758, and taught the doctrine in his lectures at Glasgow.¹ He was the first who formed distinct ideas on the subject.

¹ Dr. Black himself, writing in 1780, said:――“I began to give
the doctrine of latent heat in my lectures at Glasgow, in
the winter 1757‒58, which I believe was the first winter of
my lecturing there, or, if I did not give it that winter,
I certainly gave it in 1758‒59, and I have delivered it
every year since that time in my winter lectures, which I
continued to give at Glasgow until winter 1766‒67, when I
began to lecture in Edinburgh.”――Letter of Dr. Black to Mr.
Watt, 1708, quoted by Muirhead in his _Life of James Watt_,
pages 309‒310.

He deduced the discovery of latent heat from experiments showing that ice in being melted absorbs 140° of heat, which becomes latent in the water produced, thus:――“The melting ice receives heat very fast, but the only effect of this heat is to change it into water, which is not in the least sensibly warmer than the ice was before. A thermometer, applied to the drops or small streams of water, immediately as it comes from the melting ice, will point to the same degree as when it is applied to the ice itself.... A great quantity, therefore, of the heat, or of the matter of heat, which enters into the melting ice, produces no other effect but to give it fluidity, without augmenting its sensible heat; it appears to be absorbed and concealed within the water, so as not to be discoverable by the application of a thermometer.”¹ By comparing the time required to change the ice from 28° to 32°, with the subsequent time required for its complete liquefaction, he found that it absorbed about 140 times as much heat as would raise its temperature one degree; and he also found that one pound of ice, when mixed with one pound of water, was just melted, but not raised in its temperature above 32°. So he concluded that water differed from ice of the same temperature by containing a great quantity of heat or the cause of heat, which would not quit it for another colder body, nor go into the liquor of the thermometer and expand it. This phenomenon, considered as the possible cause of heat, was latent, and Black accordingly called it “latent heat.”²

¹ Black’s _Lectures on the Elements of Chemistry_, Volume I.,
page 119. 1803.

² Black’s _Lectures on the Elements of Chemistry_, Volume I.,
pages 120‒132.

This discovery was connected with his experiments and researches on boiling and evaporation, which ultimately resulted in laying the foundation of the practical application of steam. He concluded that a great quantity of heat becomes latent during the conversion of water into vapour or steam; and he endeavoured to determine this quantity by experiment. He found that the latent heat in steam, which balanced the pressure of the atmosphere, was upwards of 800°. He also directed Dr. Irvine of Glasgow, one of his own pupils, to make an experiment for measuring the heat actually extricated from steam during its condensation in the refrigeratory of a still, which was found to be 774°. A few weeks after, Mr. James Watt made similar experiments on steam with a similar still; and the medium result of these trials gave 825°.¹ It may be observed that, in these early experiments, the latent heat of steam was considerably underrated.

¹ _Ibid._, pages 144‒174. I deem it of interest to give a few
brief quotations from Dr. Black’s lectures on the latent
heat of steam, to indicate his views. “I immediately set
about boiling off small quantities of water, and I found
that it was accomplished in times very nearly proportional
to the quantities, even although the fire was sensibly
irregular.

“My conjecture, when put into form, was to this purpose:
――I imagined that during the boiling, heat is absorbed by
the water, and enters into the composition of the vapour
produced from it, in the same manner as it is absorbed
by ice in melting, and enters into the composition of the
produced water. And, as the ostensible effect of the heat,
in this last case, consists not in warming the surrounding
bodies, but in rendering the ice fluid, so in the case
of boiling, the heat absorbed does not warm surrounding
bodies, but converts the water into vapour. In both cases,
considered as the cause of warmth, we do not perceive its
presence: it is concealed, or latent, and I give it the name
of Latent Heat....

“I put into a very strong phial about as much water as half
filled it, and I corked it close. The phial was placed in a
sand-pot, which was gradually heated, until the sand and the
phial were several degrees above the common vaporific point
of water. I was curious to know what would be the effect
of suddenly removing the pressure of the air, which is well
known to prevent water from boiling. The water boiled a very
short while, but the ebullition gradually decreased, till
it was almost insensible. Here the formation of more vapour
was opposed by a very strong pressure proceeding from the
quantity of vapour already accumulated and confined in the
upper part of the phial, and from the increased elasticity
of this vapour, by the increase of its heat. When matters
were in this state, I drew the cork. Now, according to the
common opinion of the formation of vapour by heat, it was
to be expected that the whole of the water would suddenly
assume the vaporous form, because it was all heated above
the vaporific point. But I was beginning by this time to
expect a different event, because I could not see whence
the heat was to be supplied, which the water must contain
when in the form of vapour. Accordingly, it happened as I
expected: a portion only of the water was converted into
vapour, which rushed out of the phial with a considerable
explosion, carrying along with it some drops of water. But,
what was most interesting to me in this experiment, was,
that the heat of what remained was reduced in an instant to
the ordinary boiling point. Here, therefore, it was evident
that all that excess of heat which the water had contained
above the boiling point, was spent in converting only a
portion of it into vapour. This is plainly inconsistent
with the common opinion, that nothing more is necessary
for water’s existing in a vaporous form under the pressure
of the atmosphere, than its being raised to a certain
temperature....

“This experiment was afterwards made by my friend Mr.
Watt, in a very satisfactory manner. His studies for the
improvement of the steam-engine gave him a great interest
in everything relating to the production of steam.”――Pages
159‒160.

In 1781, Dr. Black said to the students of his class:――“I
think it sufficient to inform you that Mr. Watt, in the
course of his studies on the steam-engine, has made all the
necessary experiments with the most scrupulous care, knowing
that the improvement of that noble engine must depend
entirely on an exact knowledge of the procedure of nature in
the formation and condensation of steam. Mr. Watt informs me
that he has observed as exact coincidence between the heat
rendered latent in the vapour, and that which emerges from
it, as can be desired; and that the heat obtainable from
steam, capable of sustaining the ordinary pressure of the
atmosphere, is not less than 900° of Fahrenheit’s scale, and
that it does not exceed 950°.”――_Ibid._, page 174.

Dr. Black also contributed to advance the knowledge of Specific Heat; but he chiefly left the development of this branch in the hands of his pupil Dr. Irvine, who was professor of chemistry in the University of Glasgow from 1769 to 1786, and to Mr. Watt, for both of whom he had the greatest respect.

In 1766, Dr. Black was appointed professor of chemistry in the University of ♦Edinburgh, in succession to Dr. Cullen; and he filled this chair with much credit to himself and advantage to the University, until his death in 1799. He was a very successful instructor; his lectures in the class-room were described by those who heard them as inimitable, and so interesting that they never failed to rivet attention.¹ Thus his influence on the progress and the diffusion of science by his teaching for the long period of forty-three years, and his intercourse with society, was great and highly beneficial to his country and to the world.

♦ “Ediuburgh” replaced with “Edinburgh”

¹ Professor Robson, one of his pupils, and the editor of his
lectures, says that Dr. Black endeavoured every year to make
his course of lectures more plain, and illustrated them by
more examples in the way of experiment. So the students in
his class “were not only instructed, but delighted; and he
became a favourite lecturer, and many were induced, by the
report of his students, to attend his courses.”

Another branch of the science of heat was taken up by Sir John Leslie;¹ he directed his attention to “Radiant Heat”――heat propagated from hot bodies to sensible distances. Sir John was educated at the University of St. Andrews, and early manifested a bent for mathematical studies. His work on the _Nature and Propagation of Heat_, which appeared in 1804, first brought him into notice; and the following year he was appointed to the chair of mathematics in the University of Edinburgh.

¹ Born in 1766, and died in 1832.

The fact that heat is radiant, passing through space like light, was known at an early period; and various experiments had been made, and some of the phenomena which characterise it indicated; but heat in its radiant form was not systematically investigated till towards the end of the eighteenth century. The band of scientific men then engaged on this subject were Pictet, Prevost, Rumford, and Herschel; the first two were professors in Geneva, and were earlier in the field than Leslie. In 1791, Pictet’s work entitled _Essai sur le Feu_ appeared, which contains observations on latent and specific heat, and on the power of different surfaces to reflect and absorb it. He showed that radiant heat moves with great velocity. His treatise also embraced observations on hygrometry, on various points of meteorology, and on friction heat. He has the merit of establishing the meteorological observations at the convent of the Great St. Bernard, and thus commenced a series which has proved exceedingly interesting to scientific men.

Prevost is the author of the theory termed the “Movable Equilibrium of Heat.” His fundamental idea is that heat is a substance related with bodies of a highly elastic nature, continually given off from them in proportion to their temperature, which may represent the tension of the imaginary elastic fluid. Thus, when the temperature of a body is stationary, it is because it receives by radiation from surrounding bodies exactly as much heat as it parts with in the same way.¹ His views were first published in 1791.

¹ Dr. Forbes’s _Dissertation for the Encyclopædia Britannica_,
page 944. 1856.

Leslie was an ingenious and able experimenter. But unfortunately he started his researches with some rather dogmatic preconceptions; he had a notion that the pressure of air is essential to the propagation of heat; nevertheless, many of his experiments are interesting and valuable. He used a thermoscopic instrument constructed by himself, which he called the differential thermometer; it is an ingenious modification of the common air thermometer. He showed that the radiating or emissive effect of different surfaces varied from 100° to 12°. He also showed by experiment that the radiation of heat from a plane surface proceeds with unequal force in different directions. When the specific heating power of the colorific rays is measured in a direction perpendicular to the surface whence it emanates, it is found to be at a maximum; and at any other angle with the surface, it varies as the sine of the angle. Afterwards this was also found to prevail in the case of light. His experiments to prove that the law of radiation of heat varies inversely as the square of the distance were not quite conclusive.

He considered the influence of colour on the heating of bodies by original experiment; and it was found to be effectual only when the radiations were luminous. He engaged in long and ingenious researches touching the law of cooling bodies, embracing the effects of mass, surface, contact of air, currents of air, the cooling effects of different gases, and of air of different degrees of rarefaction.

Besides his work on heat, his _Dissertation on the Progress of Physical and Mathematical Science_, and the articles on “Cold” and “Meteorology,” in the seventh edition of the _Encyclopædia Britannica_, he is the author of _Elements of Natural Philosophy_ (left unfinished), a _Treatise on Geometry_, and _Philosophy of Arithmetic_.

He held the mathematical chair from 1805 to 1819, and in the latter year he was appointed to the chair of natural philosophy. He had a large and fine collection of apparatus, as indicated above, and devised many ingenious experiments. He was elected a corresponding member of the Institute of France in 1820; and, on the recommendation of Lord Brougham, he received the honour of knighthood in 1832.

Since Leslie’s time the science of heat has been greatly advanced; the dynamical theory of heat has been developed in the present century, and Scotsmen have contributed their share to the definite advancement of this branch of science. But it has been advanced to its present stage by a long list of scientific men. In 1812, Davy enounced that the direct cause of the phenomenon of heat is motion, and that the laws of its communication are precisely the same as the laws of the communication of motion. The researches into the radiation and absorption of heat mainly form the physical basis of Spectrum Analysis, which has greatly extended the power of ascertaining the constituent elements of the celestial bodies, the sun and the fixed stars.

In the researches which ultimately led to these results, several Scotsmen have taken an honourable part. Professor Forbes discovered and demonstrated the polarisation of heat, and thus showed that radiant heat and light are the same. Among others who have contributed to advance Spectrum Analysis, I may mention Professor Stokes, Professor Balfour Stewart, and Sir William Thomson, of the University of Glasgow. Sir William Thomson (now Lord Kelvin) has taught the doctrine that there is sodium vapour in the sun’s atmosphere, in his public lectures in the University of Glasgow, since the year 1852.

Interesting conclusions touching the composition of the sun and of some of the stars have been reached:――“When we compare the spectra of different stars with that of the sun, we come to some very curious conclusions. We find four classes of spectra, as a rule, among the different fixed stars which have seemed of importance enough to be separately examined. The first class of spectra are those of white stars. You see an admirable example in Vega, and another in Sirius or the dog-star. All these white stars have the characteristic that they have an almost continuous spectrum with few dark lines crossing it, and these for the most part lines of hydrogen. These stars are in all probability at a considerably higher temperature than the sun. Then you come to the class of yellow stars, of which our sun is an example. In their spectra you have many more dark lines than in those of the white stars, but you have nothing of the nature of nebulous bands crossing the spectrum, such as you find in the third class; still less have you certain curious joins of shaded lines which you have in the fourth class of stars. This classification seems to point out the period of life, or phase of life, of each particular star or sun. When it is first formed, by the impact of enormous quantities of matter coming together by gravitation, you have very nearly continuous spectrum of a glowing white-hot liquid or solid body, or it may be dense gas, the sole, or nearly sole, absorbent being gaseous hydrogen in comparatively small quantity, and the spectrum having therefore few absorption lines. As it gradually cools, more and more of these gases surrounding its glowing surface become absorbent, and so you have a greater number and variety of lines. Then, as it still further cools, you have those nebulous bands which seem to indicate the presence of compound substances; which could not exist in the first two classes, because their temperature is so high as to produce dissociation. Still further complexity of compounds will be found in the atmosphere of the fourth class.”¹

¹ _Recent Advances in Physical Science_, by P. G. Tait, pages
230‒231.

After the publication of Newton’s _Optics_ in 1704, the history of this branch of science was almost a blank in Britain, till 1803, when the researches of Dr. Young on the undulatory theory of light appeared; and since the subject has been treated with increasing interest and success.

Sir David Brewster,¹ who attained distinction in this branch of science, and in other fields of intellectual effort, was a native of Jedburgh. He was educated at the University of Edinburgh, and had the advantage of the instruction of Professor Robison, and other eminent teachers who then spread the rays of light with consummate ability. He devoted himself to science; and in 1805, he edited Ferguson’s _Lectures on Astronomy_; and, as already mentioned, he commenced the _Edinburgh Encyclopædia_ in 1810. His first separate work, _On New Philosophical Instruments_, appeared in 1813, which also contained observations on refractive and dispersive powers. From this date onwards he became a regular contributor to the London Philosophical _Transactions_, and also those of Edinburgh; he commenced the _Edinburgh Philosophical Journal_ and the _Edinburgh Journal of Science_. His contributions to scientific societies and journals would fill many volumes. One list of his scientific papers extends to the number of three hundred and fifteen, and the following is only the briefest indication of some of the more important subjects treated in them:――

¹ Born in 1781; died in 1868.

(1) “The Laws of Polarisation by Reflection and Refraction, and other Quantitative Laws of Phenomena;” (2) “The Discovery of the Polarising Structure induced by Heat and Pressure;” (3) “The Discovery of Crystals with Two Axes of Double Refraction, and many of the Laws of their Phenomena, comprising the Connection of Optical Structure and Crystalline Forms;” (4) “The Laws of Metallic Reflection;” (5) “Experiments on the Absorption of Light.”

The more important of his other works are――(1) _A Treatise on the Kaleidoscope_, published in 1819; (2) _A Treatise on Optics_, 1831; (3) _A Treatise on the Microscope_; (4) _A Treatise on the Stereoscope_; (5) an Article on “Magnetism,” reprinted from the _Encyclopædia Britannica_; (6) _The Martyrs of Science――Galileo, Tycho Brahe, and Kepler_; (7) _Life of Sir Isaac Newton_; (8) _Letters on Natural Magic_; (9) _More Worlds than One_. He also wrote a _Life of Euler_ and edited his Lectures; edited Robison’s _System of Mechanical Philosophy_, with a preface and notes, which appeared in 1822, in four large volumes; he also contributed seventy-four articles to the _North British Review_.

This enumeration of his writings, though far from complete, is sufficient to show his great mental energy, and his scientific and literary talents. He was a man of remarkable intellectual resource, his imaginative and elaborative faculties were of a high order, and, for industry and observation he has rarely been surpassed. His style is clear and flowing, he has a copious command of expressive language.

Besides his discoveries of the law of polarisation, of biaxal crystals, of optical mineralogy, and of double refraction by compression, he invented a dioptric apparatus for the illumination of lighthouses, which he described in 1812. In 1820, he endeavoured to get the dioptric system adopted, but failed; at length, however, on the motion of Mr. Hume, a Committee of the House of Commons was appointed to consider the subject; and, in 1836, this system was applied to a Scotch lighthouse, and has since been universally extended. In 1816, he invented the kaleidoscope, which soon became popular over Europe; afterwards, he made an important improvement on the principle of constructing stereoscopes. In the words of Professor Forbes――“Few persons have made with their own eyes so vast a number of independent observations; few have ever observed better, or recorded their observations more faithfully.” He was an honour to his country and a benefactor to mankind.

In the interesting science of the earth――geology――Dr. Hutton, in 1788, enounced his theory that the changes in the earth’s crust have been mainly caused by the agency of fire; but though his views were ingenious and well argued, they have long ago been superseded by conclusions more in accordance with the observed phenomena. This branch of knowledge has been much investigated in the present century, and several Scotsmen have attained distinction in advancing it.

Sir Charles Lyell¹ was a native of Forfarshire, and studied at Oxford. His _Principles of Geology_, being an attempt to explain the former changes of the earth’s surface by a reference to causes now in operation, appeared in 1830‒32, in two volumes. He made additions to it, and alterations from time to time, and the eighth edition of the work, thoroughly revised, was published in 1850. Though he recognised new facts, he continued to hold his theory that we may dispense with sudden and general catastrophes, and consider the past and present fluctuations of the organic and inorganic world as forming one continuous and regular series of phenomena.

¹ Born in 1797; died in 1875.

In 1838, he published his _Elements of Geology_, which was afterwards enlarged to two volumes. He is also the author of _Travels in North America, with Geological Observations on the United States, Canada, and Nova Scotia_, published in 1845. He was twice elected president of the Geological Society, and he received the honour of knighthood in 1848. His style is attractive, easy, and fluent, and his writings were popular. The following is a short specimen of his manner:――

“The analogy, however, of the monuments consulted in geology, and those available in history, extends no further than to one class of historical monuments――those which may be said to be undesignedly commemorative of former events. The canoes, for example, and stone hatchets found in the peat bogs afford an insight into the rude arts and manners of the earliest inhabitants of our island; the buried coin fixes the date of the reign of some Roman emperor; the ancient encampment indicates the districts once occupied by invading armies, and the former method of constructing military defences; the Egyptian mummies throw light on the art of embalming, the rites of sepulture, or the average stature of the human race in ancient Egypt. This class of memorials yields to no other in authenticity, but it constitutes a small part only of the resources on which the historian relies; whereas in geology it forms the only kind of evidence which is at our command. For this reason we must not expect to obtain a full and connected account of any series of events beyond the reach of history. But the testimony of geological monuments, if frequently imperfect, possess at least the advantage of being free from all suspicion of misrepresentation. We may be deceived in the inferences which we draw, in the same manner as we often mistake the nature and import of the phenomena observed in the daily course of nature; but our liability to err is confined to the interpretation, and, if this be correct, our information is certain.”

Sir Roderick Murchison,¹ a distinguished geologist, was a native of Ross-shire, and served in the army from 1807 to 1816. He directed his attention chiefly to a series of strata in the district bordering on England and Wales, inhabited in early times by the British tribe of the Silures; and after working four years in classifying the rocks and deposits, he separated them into four formations, and showed that each is characterised by peculiar organic remains: and, in 1835, he divided them into a lower and upper group, both of which he anticipated would be found applicable to wide regions of the earth, and named them the Silurian System, the details of which he published under this title in 1839. In 1854, his later researches were published under the title of _Siluria: the History of the Oldest Known Rocks containing Organic Remains_.

¹ Born in 1792; died 1871.

He spent many years in Russia and in other countries in geologic explorations; and, in 1846, he published _The Geology of Russia and the Ural Mountains_, in which he was assisted by Count A. von Keyserling and E. de Verneuil. He is also the author of upwards of one hundred separate memoirs presented to scientific societies. In 1844, after examining some specimens of Australian rocks brought to this country, and comparing them with those of the auriferous Ural Mountains, he came to the conclusion that gold existed in Australia. Two years later, he urged the Cornish tin miners to emigrate to the colony of New South Wales, where they could obtain gold from the alluvial soil in the same way as they extracted tin from the gravel of their own country.

The following is a summary of the Siluria strata as they occur in the district mentioned above, upon which Sir Roderick mainly founded his system; they represent a thickness of about nine thousand feet:――

{ Finely laminated reddish sandstone and shales.
{ Micaceous grey sandstones of varying thickness.
Upper { Argillaceous limestone.
strata. { Calcareous shale, with concretions of limestone.
{ Concretionary limestone and argillaceous shale.
{ Shelly limestone and sandstone.
{ Gritty sandstones and shales.

{ Grits and sandy shales.
Lower { Thick-bedded white freestone.
strata. { Dark-coloured flagstones and slates.
{ Dark-coloured calcareous flags, bands of limestone,
{ and gritty flagstones.

Excepting a few indistinct fragments of land plants in the uppermost beds, the whole remains are characteristically marine, and evince conditions favourable to a variety of invertebrate life. Among the prevailing and distinctive fossils are fucoids or seaweed plants, corals, radiate animals, sea-worms, and shell-fish in great variety. And strata characterised by these fossils are largely developed in many countries, especially along the flanks of the older mountain-chains. “They occur in Wales, in Cumberland, in Westmoreland; along the south of Scotland; south-east of Ireland; the south of France, Spain, Scandinavia, Russia, and Bohemia; in Asia Minor; along the Himalaya and Altai ranges; in Australia and New Zealand; along the Andes, Rocky Mountains, and Appalachians in America.”

I have already mentioned Hugh Miller as a geologist, and no one was more ready than Sir Roderick Murchison to recognise his merits and to applaud his genius. Murchison received the honour of knighthood in 1846. The first editor of the _Scotsman_, Mr. Maclaren, was a student of geology, and published an _Account of the Geology of Fife and the Lothians_ in 1839. It was, however, Hugh Miller that made geology popular in Scotland, and gave a great impulse to its study.