Aug. 27, 1874] 



NATURE 



349 



with the sun, luminous radiation becomes afifinity in the imme- 

 diate organic principles which are formed and accumulated 

 in vegetable cellules. That mode of motion of ether which was 

 " light " is become another mode of motion which is " affinity," 

 and sways the atoms of an organic compound. In its turn this force 

 thus stored up is expended again wlien the organic compounds 

 are destroyed in the phenomena of combustion. Affinity, satisfied 

 and as it were lost by the combination of combustible elements 

 with oxygen, again becomes heat or electricity. Wood in 

 burning, and carbon in becoming oxidised, produce sparks or 

 flames : a metal which exhausts its affinities in decomposing an 

 acid warms the liquid, or, under other conditions, produces an 

 electric current, warming it less when the current is exterior. And 

 in another order of phenomena, heat which distributes or propa- 

 gates itself unequally between two surfaces, rubbing one against 

 the other, or in a crystal that is warmed, or in two metals united 

 by solder, disappears partially as such and manifests itself 

 as static electricity or as an electric current. Thus all these 

 forces are equivalent to one another and appear under diverse 

 forms, whether they are passing from atoms to ether or from 

 ether to atoms ; but we never see them disappear or lose their 

 force — only transform themselves and perpetually renew their 

 youth. 



And this is not all. These vibratory movements which sway 

 atoms and which whirl about in ether can cause movements of 

 the mass, displacement either of the bodies or of [the molecules. 

 Warm a bar of iron, it will dilate with a force almost irresistible ; 

 a part of the heat will be employed in producing a certain puUing 

 asunder of the molecules. Warm a gas, it will in like way dilate, 

 and a part of the heat disappearing as such, will produce a separa- 

 tion very considerable in this case between the gaseous molecules ; 

 and the proof of the consumption of heat in the work of dilation 

 is not difficult to give, for if you warm the same gas to the same 

 degree, but prevent it from dilating, less heat need be given to 

 it than in the former case. The difference between the two 

 quantities of heat corresponds exactly to the mechanical work 

 performed by the molecules in dilatation. That is one of the 

 most simple considerations, on which is founded the principle of 

 the mechanical equivalent of heat so often now referred to in 

 mechanics, in physics, and in physiologj-. 



In physics it explains the mystery of latent heat, of fusion, and 

 of volatilisation. But how is it that heat supplied continuously 

 to a boiling liquid to maintain ebullition does not ever raise the 

 temperature of the liquid above a point which under similar 

 pressure remains fixed ? The reason is that this heat'is continually 

 absorbed, and disappears as such to produce the mechanical work 

 of driving apart the molecules. And so in the phenomena of 

 fusion, the constancy of the temperature indicates the absorption 

 of the heat consumed in molecular work. These conceptions 

 have modified and throw-n much light on the definitions which 

 physicists have applied to different states of matter, and it is seen 

 that they are in harmony with chemical theories of the constitu- 

 tion of bodies. These are formed of molecules which i-epresent 

 systems of atoms anmiated by harmonic movements, and whose 

 equilibrium is exactly maintained and strengthened by these 

 movements. 



Applied to molecules thus constituted, heat can produce 

 three different effects. In the first place, an elevation of 

 temperature by the increase of vibratory energy ; in the 

 second place, an increase of volume by the driving apart 

 of atoms and molecules, and this augmentation becoming 

 very considerable, a change of condition, solid becoming 

 liquid, and liquid becoming gas ; in the last, the driving apart 

 of the molecules is become immense in relation to their di- 

 mensions. Thus acting on the atoms which compose the 

 molecule and amplifying their trajectories, heat can disturb the 

 equilibrium which exists in the system, causing a conflict of these 

 atoms with those of another molecule in such a way that this 

 disturbance or this conflict leads to fresh systems of equilibrium, 

 that is to new molecules. There commence the phenomena of 

 decomposition and dissociation, or, inversely, of combination, 

 which is the mainspring of chemistry, and it is seen they are but 

 the continuation or consequence of the physical phenomena we 

 have just analysed, the same hypothesis, that of atoms, applied 

 to one and the other with an equal simplicity. 



I ask, will it not be easy to conceive that the physical and 

 chemical forces which act on ponderable bodies are applied also 

 to diffuse continuous matter in some way, and is it not natural to 

 suppose that there are limited and definite particles which repre- 

 sent the points of application of all these forces ? And this view 

 ought to apply to the two sorts of matter which form the uni- 



verse, ether and atomic matter, the one infinitely rarefied but 

 homogeneous, filling all space, and in consequence enormous in 

 its mass, both unseizable and imponderable ; the other mm- 

 continuous, heterogeneous, and only occupying a very limited 

 portion of space, ahhough it forms all worlds. 



Yes, it forms all worlds, and the elements of ours have been 

 discovered in the sun and in the stars. Yes, the radiations given 

 off by incandescent atomic matter which forms these stars are also, 

 for the most part, those which are prodnced by the simple bodies 

 of our planet. Marvellous conquest of physics which reveals at 

 once to us the abundance of forces which environ the sun and the 

 simplicity of the constitution of the universe ! 



A solar ray falls upon a prism and is turned aside in its path 

 and decomposed into an infinity of different radiations. These 

 take each a particular direction, and all range themselves in 

 bands in juxtaposition, and spread themselves out in the spectrum 

 if the light thus received and decomposed is thrown on to a 

 screen. The visible part of this spectrum shines with all the 

 colours of the rainbow ; but besides this, beyond both ends of the 

 coloured bands the radiations are not absent. The heat-rays can 

 be made to reveal themselves beyond the red ; the chemical 

 rays, more powerful than the others to make and destroy the 

 chemical combinations, are known beyond the violet. All the 

 forces which manifest themselves on the surface of our globe, as 

 heat, light, and chemical energy, are sent to us in a ray of white 

 light. 



But this brilliant spectrum is not continuous. Fraunhofer has 

 discovered in it an infinity of black lines cutting the shining 

 band ; these are the "dark fines " of the spectrum, and Ivirchhoff 

 has found that a certain number of them occupy the same posi- 

 tion as the "bright lines " which occur m the spectra of metallic 

 substances when In a state of incandescence. This last physicist, 

 generalising an observation of Foucault, has seen, further, that 

 under given circumstances these bright lines can be obscured 

 and " reversed," coincidmg then with the dark lines of the solar 

 spectrum. 



We have been able to conclude that these have an identical origin 

 and are due to radiations given off by metallic substances spread 

 in vapour over the solar globe, radiations which are obscured by 

 these same vapours in the atmosphere of the sun. Tlius the 

 star which gives us heat, light, and life, is formed of elements 

 like thos:: which form our globe. These elements are hydrogen 

 and metals in a state of vapour. They are not distributed 

 equally in the mass of the sun and in his rarefied envelopes ; the 

 hydrogen and most volatile metals are raised to a greater height 

 on the surface of the sun than are the other metals. They are 

 never in repose ; this ocean of incandescent gas is continually 

 agitated by tremendous tempests. The trombes tlirow themselves 

 out in immense columns to the height of 50,000 leagues above 

 the gaseous sphere; these are the "protuberances," and they 

 shine with a rose light peculiar to themselves ; and they are 

 formed according to Janssen and Lockyer by hydrogen, vei-y 

 rarefied, and also by an unknown substance — "helium." The 

 luminous globe itself, the photosphere, gives the spectra of 

 our ordinary metals, except gold, silver, platinum, and mercury ; 

 the precious metals, those which have little affinity for oxygen, 

 being wanting. But, on the contrary, in the solar spectrum there 

 are "lines" different from those which the metals of our earth 

 give, but which are like them. The lines of the metalloids are 

 wanting, as are the lines which are characteristic of compound 

 bodies. The gaseous mass has such an incandescence that no 

 chemical combination could withstand it. 



Tlie lines of Fraunhofer are dark, only the lines of the protu- 

 berances and those seen a moment after the disappearance of the 

 sun in an eclipse, and a moment before its reappearance, are 

 bright, like those which characterise the spectra of incandescent 

 metallic vapours. Here we have a curious relationship which 

 has furnished most important and precise indications on the 

 physical constitution of the sun. 



I have spoken of the chemistry of the sun, but the spectro- 

 scope has explored all the far-off space of heaven. The light of 

 hundreds of stars has been analysed, and nebula, scarcely 

 visible, have had the quality of their radiations revealed by its aid. 

 The light, in some cases very feeble, with which a number of 

 stars shine, gives a spectrum with dark lines like the solar 

 spectrum, and this fact proves to us that the constitution of 

 these stars is like that of our sun. Aldebaran sends us records 

 of hydrogen, magnesium, and calcium, which abound in solar 

 light, but also those of metals which are rare or absent, as 

 tellurium, antimony, and mercury. 



Nebulx, twenty thousand times less brilliant than a candle a 



