652 THE POPULAR SCIENCE MONTHLY. 



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 1864 the lecturer 

 had proved that when vapors, in quantities proportional to the densi- 

 ties 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 internal re- 

 flection never comes into play. It furnishes, therefore, the strongest 

 presumptive evidence that the seat of absorption in liquids and in va- 

 pors is the same. 



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

 degree desirable to compare together equal quantities, instead of pro- 

 portional quantities, of liquids and vapors. Highly volatile liquids 

 alone lend themselves to this experiment, for only in the case of such 

 liquids can vapors 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 vapor 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 vapor, at a mercury pressure of 6*6 inches, is equiv- 

 alent to a liquid layer one millimetre in thickness, while a vapor col- 

 umn of sulphuric ether, of the same length, and 7*2 inches pressure, 

 would also produce a liquid layer one millimetre thick. The experiment 

 has been made with the utmost care, both with the lime-light and the 

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

 there is any difference between the vapor absorption and the liquid 

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

 as an explanation of the results published by the lecturer, can not 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 peforated disk, was caused to impinge upon 

 various solid substances, and to produce musical sounds. Mr. Bell's 

 previous experiments upon selenium naturally led him to conclude that 

 the effect was produced by the luminous rays of the spectrum. The 

 contemplation of these experiments produced in the lecturer the con- 

 viction that the results were due to the intermittent absorption of 

 radiant heat. He was experimenting on vapors at this time. Substi- 

 tuting in idea gaseous for solid matter, he clearly pictured the sudden 

 expansion of an absorbent gas or vapor at every stroke of the calorific 

 beam, and its contraction when the beam was intercepted. Pulses far 



