282 Prof. Tyndall on the Absorption and 



Conduction. — Notwithstanding the great accessions of late years 

 to our knowledge of the nature of heat, we are as yet, I believe, 

 quite ignorant of the atomic conditions on which radiation, 

 absorption, and conduction depend. What are the specific 

 qualities which cause one body to radiate copiously and another 

 feebly ? Why, on theoretic grounds, must the equivalence of 

 radiation and absorption exist? Why should a highly diather- 

 manous body, as shown by Mr. Balfour Stewart, be a bad radi- 

 ator, and an adiathermanous body a good radiator ? How is heat 

 conducted? and what is the strict physical meaning of good 

 conduction and bad conduction ? Why should good conductors 

 be, in general, bad radiators, and bad conductors good radia- 

 tors ? These, and other questions, referring to facts more or less 

 established, have still to receive their complete answers. It is 

 less with a hope of furnishing such than of shadowing forth the 

 possibility of uniting these various effects by a common bond, 

 that I submit the following reflections to the notice of the Royal 

 Society. 



In the experiments recorded in the foregoing pages, we have 

 dealt with free atoms, both simple and compound, and it has 

 been found that in all cases absorption takes place. The mean- 

 ing of this, according to the dynamical theory of heat, is that no 

 atom is capable of existing in vibrating ether without accepting 

 a portion of its motion. We may, if we wish, imagine a certain 

 roughness of the surface of the atoms which enables the ether 

 to bite them and carry the atom along with it. But no matter 

 what the quality may be which enables any atom to accept mo- 

 tion from the agitated ether, the same quality must enable it to 

 impart motion to still ether when it is plunged in the latter and 

 agitated. It is only necessary to imagine the case of a body 

 immersed in water to see that this must be the case. There is a 

 polarity here as rigid as that of magnetism. From the existence 

 of absorption, we \nay on theoretic grounds infallibly infer a 

 capacity for radiation ; from the existence of radiation, we may 

 with equal certainty infer a capacity for absorption ; and each of 

 them must be regarded as the measure of the other* 



This reasoning, founded simply on the mechanical relations 

 of the ether and the atoms immersed in it, is completely verified 

 by experiment. Great differences have been shown to exist 

 among gases as to their powers of absorption, and precisely 

 similar differences as regards their powers of radiation. But 

 what specific property is it which makes one free molecule a 

 strong absorber, while another offers scarcely any impediment 

 to the passage of radiant heat ? I think the experiments throw 



* This was written long before Kirchhoff's admirable papers on the 

 relation of emission to absorption were known to me. 



