486 Prof. Tyndall on the Action of Free Molecules on 



Here, then, where no trace of reflected beat is sent to the 

 pile, we have substantially the same results as those obtained 

 when by far the greater portion of the heat reaching the pile 

 had undergone reflection. The agreement shows that the 

 films, the action of which had been posited, but never proved, 

 are, in regard to this action, what I always knew them to be, 

 imaginary. 



b. The Plates of Rock-salt. 



Having thus, I trust effectually, disposed of the hypothesis 

 which ascribed my results to liquid layers covering the in- 

 terior surface of the experimental tube, I proceed to the 

 examination of the plates of rock-salt, which have also been 

 credited with a liquid deposit. The hypothesis is to a great 

 extent disposed of in a paper of mine published in the Philo- 

 sophical Transactions for 1864, which, like other papers, has 

 been overlooked by writers on this question. An apparatus 

 is there described which enabled me to operate upon gaseous 

 strata of different thicknesses, the thickness in each case 

 being measured with great accuracy by aid of a vernier. Let 

 two polished plates of rock-salt be supposed in close contact 

 with each other, and let them be gradually separated by a 

 suitable rack-and -pinion movement. From the first moment 

 of their separation let the space between them be copiously 

 supplied with vapour. Liquid films, if they form at all, will 

 be deposited when the distance between the plates of salt is 

 but small. As far as the films are concerned, they will be as 

 influential when the plates are ^ of an inch apart as when 

 they are 2 inches apart. Hence, if the hypothesis of my 

 opponents be correct, the absorption ought to declare itself 

 with the former amount of separation as clearly as with the 

 latter. But if, as I allege, the absorption be the act of the 

 vapour molecules, then the deepening of the vapour stratum 

 Avill be accompanied by an increase of the absorption. The 

 following experiment, bearing directly on this subject, was 

 executed in 1864. My plates of salt were first fixed at a 

 distance of ^ of an inch asunder ; and the space between 

 them was copiously supplied with air saturated with sul- 

 phuric-ether vapour. The distance between the plates of salt 

 was then augmented by steps, as shown in the following table; 

 and at each step the absorption was determined. Here are 

 the results : — 



