1881.] Radiant Heat, and its Conversion thereby into Sound. 35 



—q of an inch to 2 inches the absorption rose from 2 per cent, to 35 

 per cent, of the total radiation. Such experiments, recently verified, 

 entirely dispose of the hypothesis that liquid films were the cause of 

 the observed absorption. 



The " vapour-hesion " hypothesis involves the assumption that liquids 

 exert on radiant' heat an absorbent power which is denied to their 

 vapours. It assumes, in other words, that the seat of absorption is the 

 molecule considered as a whole, and not the constituent atoms of the 

 molecule. For were the absorption intra-molecular, the passage from 

 the liquid to the vaporous condition, which leaves the molecules intact, 

 could not abolish the absorption. So far back as 18(i4 the lecturer 

 had proved that when vapours, in quantities proportional to the 

 densities of their liquids, were examined in the experimental tube, the 

 order of their absorptions was precisely that of the liquids from which 

 they were derived. This result has been recently tested and verified 

 in the most ample manner by means of the apparatus in which in- 

 ternal reflection never comes into play. It furnishes, therefore, the 

 strongest presumptive evidence that the seat of absorption in liquids 

 and in vapours is the same. 



As a problem of molecular physics it was, however, in the highest 

 degree desirable to compare together equal quantities, instead of 

 proportional quantities, of liquids and vapours. Highly volatile 

 liquids alone lend themselves to this experiment, for only from such 

 liquids can vapours be obtained sufficient, when caused to assume the 

 liquid form, to produce layers of practicable thickness. Two cases, 

 however, have been very fully worked out, the substances employed 

 being the hydride of amyl and sulphuric ether. Careful and exact 

 experiments, many times repeated, lead to the result that when the 

 number of molecules traversed by the calorific rays in the vapour is 

 the same as that traversed in the liquid, the absorptions are identical. 

 In the silvered experimental tube, which, as stated, is 38 inches long, 

 hydride of amyl vapour, at a mercury pressure of 6 - 6 inches, is equi- 

 valent to a liquid layer 1 millim. in thickness, while a vapour column 

 of sulphuric ether, of the same length, and 7"2 inches pressure, would 

 also produce a liquid layer 1 millim. thick. The experiment has been 

 made with the utmost care, both with the lime-light and the incan- 

 descent platinum, with the result that it is impossible to say that 

 there is any difference between the vapour absorption and the liquid 

 absorption. In the face of such facts the "vapour-hesion " hypothesis, 

 as an explanation of the results published by the lecturer, cannot be 

 sustained. 



On the 29th of November, 1880, he had the pleasure of witnessing, 

 in the laboratory of the Royal Institution, the experiments of Mr. 

 Graham Bell, wherein a concentrated luminous beam, rendered inter- 

 mittent by a rotating perforated disk, was caused to impinge upon 



d 2 



