Sept, I, 1887] 



NATURE 



419 



some one of the other elements as an impurity, or to insufficient 

 observational power. This absence of coincident lines admits, 

 however, of two explanations — either that the elements are not 

 decomposed at the temperature of the elecric spark, or, what 

 ippears to me a much more improbable supposition, each one of 

 the numbers of bright lines exhibited by every element indicates 

 ;he existence of a separate constituent, no two of this enormous 

 number being identical. 



Terrestrial analysis having thus failed to furnish favourable 

 jvidence, we arc compelled to see if any information is forth- 

 ;omin5 from the chemistry of the sun and stars. And here I 

 vould remark that it is not my purpose now to dilate on the 

 Yonders which this branch of modern science has revealed. It 

 s sufficient to remind you that chemi-ts thus have the means 

 jiaced at their disposal of ascertaining with certainty the pre- 

 ;ence of elements well known on this earth in fixed stars so far 

 listant that we are now receiving the light which emanated from 

 hem perhaps even thousands of years ago. 



Since Bunsen and Kirchhoff 's original discovery in 1859, the 

 abours of many men of science of all countries have largely in- 

 ireased our knowledge of the chemical constitution of the sun 

 ind stars, and to no one does science owe more in this direction 

 han to Lockyer and Huggins in this country, and to Young in 

 he New England beyond the seas. Lockyer has of late years 

 levoted his attention chiefly to the varying nature of the bright 

 ines seen under different conditions of time and place on the 

 olar surface, and from these observations he has drawn the 

 aference that the matching observed by Kirchhoff between, for 

 nstance, the iron lines as seen in our laboratories and those 

 isible in the sun, has fallen to the ground. He further explains 

 Ills want of uniformity by the fact that at the higher transcen- 

 ental temperatures of the sun the substance which we know 

 ere as iron is resolved into separate components. Other expe- 

 imentalists, however, while accepting Lockyer's facts as to the 

 ariations in the solar spectrum, do not admit his conclusions, 

 nd would rather explain the phenomena by the well-known 

 ifferences which occur in the spectra of all the elements when 

 leir molecules are subject to change of temperature or change 

 f position. 



Further, arguments in favour of this idea of the evolution of 

 le elements have been adduced from the phenomena presented 

 y the spectra of the fixed stars. It is well known that some of 

 lese shine with a white, others with a red, and others again 

 ith a blue light ; and the spectroscope, especially under the 

 ands of Huggins, has shown that the chemical constitution of 

 lese stars is different. The white stars, of which Sirius may be 

 iken as a type, exhibit a much less complicated spectrum than 

 le orange and the red stars ; the spectra of the latter remind 

 s more of those of the metalloids and of chemical compounds 

 lan of the metals. Hence it has been argued that in the white, 

 resumably the hottest, stars a celestial d.ssociation of our ter- 

 jstrial elements may have taken place, whilst in the cooler stars, 

 robably the red, combination even may occur. But even in the 

 hite stars we have no direct evidence that a decomposition of 

 ay terrestrial atom has taken place ; indeed we learn that the 

 ydrogen atom, as we know it here, can endure unscathed the 

 iconceivably fierce temperature of stars presumably many times 

 lore fervent than our sun, as Sirius and Vega. 



Taking ail these matters into consideration, we need not be 

 irprised if the earth-bound chemist should, in the absence of 

 ilestial evidence which is incontestable, continue, for the pre- 

 ;nt at least, and until fresh evidence is forthcoming, to regard 

 le elements as the unalterable foundation-stones upon which his 

 ;ience is based. 



Pursuing another line of inquiry on this subject, Crookes has 

 elded a remarkable contribution to the question of the possibility 

 f decomposing the elements. With his well-known experi- 

 lental prowess, he has discovered a new and beautiful series of 

 henomena, and has shown that the phosphorescent lights emitted 

 y certain chemical compounds, especially the rare earths, under 

 n electric discharge in a high vacuum exhibit peculiar and 

 daracteristic lines. For the purpose of obtaining his material 

 Tookes started from a substance believed by chemists to be 

 omogeneous, such, for example, as the rare earth yttria, and 

 acceeded by a long series of fractional precipitations in obtain- 

 ig products which yield different phosphorescent spectra, 

 [though when tested by the ordinary methods of what we may 

 ;rm high temperature spectroscopy, they appear to be the one 

 ibstance employed at the starting-point. The other touchstone 

 y which the identity, or otherwise, of these various products 



might be ascertained, viz, the determination of their atomic 

 weights, has not, as yet, engaged Crookess attentijn. In ex- 

 planation of these singular phenomena, the discoverer suggests 

 two possibilities. First, that the bodies yielding the different 

 phosphorescent spectra are different elementary constituents 

 of the substance which we call yttria Or, if this be objected 

 to because they all yield the same spark-spectrum, he adopts the 

 very reasonable view that the Daltonian ato.n is probably, as we 

 have seen, a system of chemical complexity ; and adds to this 

 the idea that these complex atoms are not all of exactly the same 

 constitution and weight, the differences, however, being so slight 

 that their detection has hitherto elu led our most delicate tests, 

 with the exception of this one of phosphorescence in a vacuum. 

 To these two explanations, Marignac, in a discussion of Crookes's 

 results-, adds a third. It having been shown by Crookes him- 

 self that the presence of the minutest traces of foreign bodies 

 produce remarkable alterations in the phosphorescent spectra, 

 Marignac suggests that in the course of the thousands of separa- 

 tions which must be made before these differences become mani- 

 fest, traces of foreign bodies may have been accidently introduced, 

 or, being present in the original material, may have accumulated 

 to a different extent in the various fractions, their presence being 

 indicated by the only test by which they can now be detected. 

 Which of these three explanations is the true one must be left to 

 future experiment to decide. 



We must now pass from the statics to the dynamics of 

 chemistry ; that is, from the consideration of the atoms at rest to 

 that of the atoms in motion. Here, again, we are indebted to 

 John Dalton for the first step in this direction, for he showed 

 that the particles of a gas are constantly flying about in all direc- 

 tions ; that is, that gases diffuse into one another, as an escape 

 of coal gas from a burner, for example, soon makes itself per- 

 ceptible throughout the room. Dalton, whose mind was con- 

 stantly engaged in studying the molecular condition of gases, 

 first showed that a light gas cannot rest upon a heavier gas a-; 

 oil upon water, but that an interpenetration of each gas by the 

 other takes place. It is, however, to Graham's experiments, 

 made rather more than half a century ago, that we are indebted 

 for the discovery of the law regulating these molecular motions 

 of gases, proving that their relative rates of diffusion are inversely 

 proportional to the square roots of their densities, so that oxygen 

 being 16 times heavier than hydrogen, their relative rates of 

 diffusion are I and 4. 



But whilst Dalton and Graham indicated that the atoms are in 

 a continual state of motion, it is to Joule that we owe the first 

 accurate determination of the rate of that motion. At the 

 Swansea meeting, in 1848, Joule read a paper before Section A 

 on the " Mechanical Equivalent of Heat and on the Constitution 

 of Elastic Fluids." In this paper Joule remarks that whether we 

 conceive the particles to be revolving round one another accord- 

 ing to the hypothesis of Davy, or flying about in every direction 

 according to Herapath's view, the pressure of the gas will be in 

 proportion to the vis viva of its particles. "Thus it maybe 

 shown that the particles of hydrogen at the barometrical pres- 

 sure of 30 inches at a temperature of 6o° mu t move with a 

 velocity of 6225 '54 feet per second in order to produce a pres- 

 sure of I47i4lbs. on the square inch," or, to put it in other 

 words, a molecular cannonade or hailstorm of particles, at the 

 above rate — a rate, we must remember, far exceeding that of a 

 cannon ball — is maintained against the bounding surface. 



We can, however, go a step further and calculate with Clerk 

 Maxwell the number of times in which this hydrogen molecule, 

 moving at the rate of 70 miles per minute, strikes against others 

 of the vibrating swarm, and we learn that in one second of time 

 it must knock against others nj less than 18 thousand million 

 times. 



And here we may pause and dwell for a moment on the reflec- 

 tion that in Nature there is no such thing as great or small, and 

 that the structure of the smallest particle, invisible even to our 

 most searching vision, may be as complicated as that of any one 

 of the heavenly bodies which circle round our sun. 



But how does this wonderful atomic motion affect our 

 chemistry ? Can chemical science or chemical phenomena 

 throw light upon this motion, or can this motion explain any of 

 the known phenomena of our science ? I have already said 

 that Lavoisier left untouched the dynamics of combustion. He 

 could not explain why a fixed and unalterable amount of heat is 

 in most cases emitted, but in some cases absorbed, when chemi- 

 cal combination takes place. What Lavoisier left unexplained 

 Joule has made clear. On August 25, 1843, Joule read a short 



