442 Prof. Tyndall on the Action of Fro: Molecules on 



the multiple rose successively from 4 to 20, 35, 47, 85, 

 ascending finally to 136 *. The behaviour of ozone was thus 

 proved to be similar to that of molecules composed of hetero- 

 geneous atoms. Hence the conclusion, drawn at the time, 

 that the molecule of ozone was formed of oxygen atoms so 

 grouped as to render their action upon radiant heat virtually 

 that of a compound body. This, it is needless to say, is the 

 constitution now assigned to ozone. 



With the view of including corrosive gases and vapours 

 among the number of those examined, and for other reasons, 

 the brass experimental tube was displaced by a tube of glass 

 of the same diameter and nearly 3 feet long. The source of 

 heat was also changed from a Leslie's cube containing boiling- 

 water, to a plate of copper against which a sheet of flame 

 was permitted to play. Extraordinary precautious were 

 found necessary to ensure perfect steadiness on the part of the 

 flame. With this arrangement the practical inability of tlie 

 elementary gases to absorb radiant heat was further illus- 

 trated and confirmed. Chlorine gas and bromine vapour, for 

 example, were proved to be highly diathermanous. 



At the pressure of an atmosphere it was found that the 

 diathermic range of colourless gases extended from 1 to about 

 1000. The portion of gas first entering the experimental 

 tube, having the whole heat to act upon, produced, as might 

 be expected, the greatest effect, the increment of absorption 

 (after a certain quantity of gas or vapour had entered) being- 

 infinitesimal t- It was therefore interesting to compare 

 together the various gases at very small pressures. When 

 the pressure was that of 1 inch of mercury the diathermic 

 range was greatly increased, the absorption by olefiant gas 

 being then at least 6000 times the absorption by atmospheric- 

 air. 



With the changed apparatus the action of the aqueous 



* Philosophical Transactions, vol. clii. pp. 84, 85. 

 t This is well illustrated by an experiment on sulphuric ether vapour 

 recorded in the Bakerian Lecture for 1861 : — 



Pressure. Absorption. 



1 inch. 214 



2 inches. 282 



3 „ 315 



4 „ 330 



5 „ 330 



The absorption of air being taken as unity, that of sulphuric ethei 

 vapour at 1 inch mercury pressure is here shown to be 214. When, 

 however, vapour corresponding- to a pressure of 4 inches was alreadv in 

 the experimental tube, the addition of another inch did not sensibly 

 augment the absorption. 



