284 Prof. Tyndall on the Absorption and 



the waves of the visible spectrum pass among them without sen- 

 sible absorption. Hence it is plain that their absorptive power 

 depends on the periodicity of the undulations which strike them. 

 At this point the present inquiry connects itself with the experi- 

 ments of Niepce, the observation of Foucault, the surmises of 

 Angstrom, Stokes, and Thomson, and those splendid researches 

 of Kirchhoff and Bunsen which so immeasurably extend our 

 experimental range. By Kirchhoff it has been conclusively 

 shown that every atom absorbs in a special degree those waves 

 which are synchronous with its own periods of vibration. Now, 

 besides presenting broader sides to the ether, the association of 

 simple atoms to form groups must, as a general rule, render 

 their motions through the ether more sluggish, and tend to 

 bring the periods of oscillation into isochronism with the slow 

 undulations of obscure heat, thus enabling the molecules to 

 absorb more effectually such rays as have been made use of in 

 our experiments. 



Let me here state briefly the grounds which induce me to 

 conclude that an agreement in period alone is not sufficient to 

 cause powerful absorption and radiation — that in addition to 

 this the molecules must be so constituted as to furnish points 

 d'appui to the ether. The heat of contact is accepted with 

 extreme freedom by rock-salt, but a plate of the substance once 

 heated requires a great length of time to cool. This surprised 

 me when I first noticed it. But the effect is explained by the 

 experiments of Mr. Balfour Stewart, by which it is proved that 

 the radiative power of heated rock-salt is extremely feeble. 

 Periodicity can have no influence here, for the ether is capable 

 of accepting and transmitting impulses of all periods; and the 

 fact that rock-salt requires more time to cool than alum, simply 

 proves that the molecules of the former glide through the ether 

 with comparatively small resistance, and thus continue moving 

 for a longer time; while those of the latter presenting broad 

 sides to the ether, speedily communicate to it the motion which 

 we call heat. This power of gliding through still ether pos- 

 sessed by the rock-salt molecules, must of course enable the 

 moving ether to glide round them, and no coincidence of period 

 could, I think, make such a body a powerful absorber. 



Many chemists, I believe, are disposed to reject the idea of 

 an atom, and to adhere to that of equivalent proportions merely. 

 They figure the act of combination as a kind of intcrpenetration 

 of one substance by another. But this is a mere masking of 

 the fundamental phenomenon. The value of the atomic theory 

 consists in its furnishing the physical explanation of the law of 

 equivalents : assuming the one, the other follows ; and assuming 

 the act of chemical union as Dalton figured it, we see that it 



