446 Prof. Tyndall on the Action of Free Molecules on 



Any reasoning, therefore, which ascribes a potent absorption 

 to perfectly impalpable films, condensed on the surface of 

 my apparatus, and which denies that absorption to the free 

 molecules within the experimental tube, is in my opinion 

 untenable. 



The relation between liquids and their vapours here indi- 

 cated is very thorough. It extends beyond the field of 

 experiment which we have hitherto had in view. I have, for 

 example, published some researches on the action of rays of 

 high refrangibility on gaseous matter, and have shown, in a 

 great variety of cases, that the molecules are shaken asunder 

 by such rays. The actinic clouds, as I have called them, 

 produced by this decomposition reveal vividly the track of the 

 beam by which they are generated, and render it easy to 

 observe the distance to which the action penetrates. In the 

 case of nitrite of amyl, for example, the power of decompo- 

 sition is soon exhausted, the actinic cloud ceasing abruptly at 

 a point about 18 inches from the place where the beam enters 

 the vapour. An experimental tube 3 feet long has therefore 

 one half of its vapour shielded by the other half; and on re- 

 versing the tube, the shielded half comes instantly down as an 

 actinic cloud. In the case of iodide-of-allyl vapour, on the 

 other hand, the beam may pass through a charged experi- 

 mental tube 5 feet long, fill it with an actinic cloud, and still 

 effect decomposition in another tube placed beyond it. What 

 is true of these vapours is true equally of their liquids ; for 

 while a layer of the liquid nitrite £ of an inch thick prevents, 

 when placed in the track of the beam, the decomposition of 

 its vapour, a layer of the liquid iodide, of quadruple thickness, 

 does not arrest the decomposition. The power, and the lack of 

 power, to be penetrated to considerable depths is shared alike 

 by the liquids and their vapours. Other, and still more subtle 

 and penetrating illustrations of parallelism between liquid and 

 vaporous absorption are mentioned in the Bakerian Lecture 

 for 1864* 



§ 3. Researches of Magnus. 



Prompted by the experiment of Grove, illustrating the 

 chilling action of hydrogen, Magnus, in 1860, began an in- 



* Carbonic acid is one of the feeblest of the compound gases, as regards 

 the radiation from solid bodies : but for the radiation from a carbonic 

 oxide flame it transcends all other gases in absorbent power. The action 

 of aqueous vapour is also enhanced -when it acts upon the rays emitted 

 by a hydi-ogen-flame. _ The _ enhancement extends to water* Curious 

 reversals of diathermic position, when heat from different sources is 

 employed, are moreover shown to occur simultaneously with liquids and 

 vapours. 



