344< Prof. Tyndall on the Absorption and 



that Laplace had no right to his correction, inasmuch as the 

 heat developed by the local compression of a mass of air of inde- 

 finite extension would be instantly wasted by radiation. Expe- 

 riments, he argued, conducted in confined vessels furnish no 

 ground for drawing conclusions regarding what occurs in the 

 atmosphere, where the heat developed has an indefinite space to 

 lose itself. In our experimental tube, though it is mechanically 

 closed, indefinite extension, as regards the radiation of heat, is 

 secured in one direction, and the means also exist of measuring 

 the flux of heat in this direction. What is true for one direction 

 is of course true for all, so that the apparatus will inform us of 

 what must occur in the open atmosphere. Now, with the most 

 powerful radiating gases which I have examined, the radiation 

 continues a very sensible time, while the heat acquired by air on 

 entering the tube is often a source of inconvenience on account 

 of the inability of the air to disperse its heat by radiation. The 

 question is therefore experimentally decided in favour of Laplace 

 and his supporter. 



T would here dwell for a moment on this comparative absence 

 of radiating power on the part of air, and of the elementary gases 

 generally. The air is the sole source of the heat which has 

 warmed the vapours in our experiments on dynamic radiation ; 

 it is related to them precisely as a hot polished metal plate is to 

 the coat of varnish which makes it a radiator. The air and the 

 metal (both elements or mixtures of elements) are incompetent 

 to impart motion to the luminiferous ether without the interme- 

 diation of a second body. They possess the motion, but they 

 are so related to the ether that they cannot communicate this 

 motion to it directly, or only in an extremely feeble degree. 

 The atoms of air oscillate, but the ether does not swell. We 

 have here a definite picture before the mind's eye, which, if the 

 theory of an ether be true, is as certain as any conclusion of 

 mathematics, and would hardly be rendered more certain if the 

 physical vision were so sharpened as to be able to see the oscil- 

 lating atom and the fluid in which it swings. I write thus strongly 

 and definitely lest it should be imagined that I am dealing in 

 vague conjectures in connexion with this subject. If I am vague, 

 the mechanical theory of an ether must in reality bear the reproach. 

 So far, however, from having a reproach to bear, the whole body 

 of facts is in complete harmony with this theory. 



Further, if, as all the facts declare, radiation and absorption 

 are complementary acts, the one consisting in communication, 

 the other in reception, and the one being strictly proportional to 

 the other, no coincidence in period between the vibrations of a 

 radiating body and those of oxygen, hydrogen, or air could 

 make any one of these substances a good absorber. The form 



