34 



Dr. Tyndall. Action of Free Molecules on [Nov. 24, 



most distant plate of rock-salt. To obtain the image with clearness, 

 the spiral was first rendered highly luminous, and afterwards reduced, 

 by the introduction of resistance, to the required temperature. In 

 this way a calorific beam was sent along the axis of the experimental 

 tube without at all impinging upon its interior surface. No reflection 

 came into play; no absorption by hypothetical liquid films, coating 

 the internal surface, 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 

 cylinder of lime, a portion of which was rendered incandescent by an 

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

 of those obtained with the spiral. The order of absorption in bot 

 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 results 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 

 interior surface was rough and tarnished, and when warmed dynami- 

 cally 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 alleged to be deposited, remain to be ex- 

 amined. In this case also an experiment um crucis 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 being 

 copiously supplied with vapour, ought to produce no effect ; but if the 

 absorption, as alleged by the lecturer, be the act of the vapour mole- 

 cules, then the deepening of the absorbing stratum ought to produce an 

 augmented effect. For many gases and some vapours 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 contact with each 

 other, and then gradually separated, until the gaseous stratum between 

 them was some inches in depth. With sulphuric ether vapour the 

 distance between the plates being of an inch, an absorption of 

 2 per cent, was observed. With a thinner stratum, or a weaker 

 vapour, even this small absorption vanished ; while in passing from 



