SOUND AND RADIANT HEAT. 651 



could occur ; and yet experiments made with this arrangement entirely 

 confirmed the preceding ones, wherein by far the greater quantity of 

 heat which reached the pile had undergone reflection. 



When the source of heat was changed to a carefully worked cylin- 

 der of lime, a portion of which was rendered incandescent by an ig- 

 nited stream of coal-gas and oxygen, the results were confirmatory of 

 those obtained with the spiral. The order of absorption in both cases 

 was the same, the only difference being that the fractional part of the 

 total radiation absorbed in the case of the lime-light was less than 

 that absorbed in the case of the spiral. 



To condense the radiation from the lime-light, concave mirrors 

 were sometimes employed, and sometimes rock-salt lenses. The re- 

 sults in both cases were identical. 



An experimental tube of the dimensions here given was employed 

 by the lecturer to check his results more than ten years ago. Its in- 

 terior surface was rough and tarnished, and when warmed dynamically 

 by the entrance of a gas its power as a radiator enabled it to disturb, 

 to some slight extent, the purity of the results. To obviate this, the 

 experimental tube recently employed was provided with an internal 

 silver surface, deposited electrolytically and highly polished. By this 

 arrangement the radiation of the tube itself, as well as its absorption, 

 was rendered quite insensible. 



The rock-salt plates used to close the experimental tube, and on 

 which liquid films are also alleged to be deposited, remain to be ex- 

 amined. In this case also an experimentum cruets is possible. If the 

 observed absorptions be due to such liquid films, then the separation 

 of the salts more widely from each other, the space between them be- 

 ing copiously supplied with vapor, ought to produce no effect ; but if 

 the absorption, as alleged by the lecturer, be the act of the vapor- 

 molecules, then the deepening of the absorbing stratum ought to pro- 

 duce an augmented effect. For many gases and some vapors this 

 problem was solved as far back as 1863. By means of an apparatus 

 then described, polished plates of rock-salt could be brought into con- 

 tact with each other, and then gradually separated, until the gaseous 

 stratum between them was some inches in depth. With sulphuric- 

 ether vapor, the distance between the plates being one twentieth of an 

 inch, an absorption of two per cent was observed. With a thinner 

 stratum, or a weaker vapor, even this small absorption vanished ; while 

 in passing from one twentieth of an inch to two inches the absorption 

 rose from two per cent to thirty-five per cent. Such experiments, 

 recently verified, entirely dispose of the hypothesis that liquid films 

 were the cause of the observed absorption. 



The vapor-hesion hypothesis involves the assumption that liquids 

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

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

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



