302 FRAGMENTS Of 1 SCttitfCfl. 



chloroform was most opaque. For the radiation from a 

 very small gas-flame, consisting of a blue base and a small 

 white tip, the bisulphide was also most opaque, and its 

 opacity very decidedly exceeded that of the chloroform 

 when the source of heat was the flame of bisulphide of 

 carbon. Comparing the radiation from a Leslie's cube 

 coated with isinglass with that from a similar cube coated 

 with lampblack, at the common temperature of 100 de- 

 grees 0., it was found that, out of eleven vapors, all but one 

 absorbed the radiation from the isinglass most powerfully; 

 the single exception was chloroform. 



It is worthy of remark that whenever, through a change 

 of source, the position of a vapor as an absorber of radiant 

 heat was altered, the position of the liquid from which the 

 vapor was derived underwent a similar change. 



It is still a point of difference between eminent investi- 

 gators whether radiant heat, up to a temperature of 100 de- 

 grees C., is monochromatic or not. Some affirm this; some 

 deny it. A long series of experiments enables me to state 

 that probably no two substances at a temperature of 100 de- 

 grees C. emit heat of the same quality. The heat emitted by 

 isinglass, for example, is different from that emitted by 

 lampblack, and the heat emitted by cloth, or paper, differs 

 from both. It also a subject of discussion whether rock-salt 

 is equally diathermic to all kinds of calorific rays; the differ- 

 ences affirmed to exist by some investigators being ascribed 

 by others to differences of incidence from the various 

 sources employed. MM. de la Provostaye and Desains 

 maintain the former view, Melloni and M. Knoblauch 

 maintain the latter. I tested this point without changing 

 anything but the temperature of the source; its size, dis- 

 tance, and surroundings remaining the same. The experi- 

 ments proved rock-salt to be colored thermally. It is 

 more opaque, for example, to the radiation from a barely 

 visible spiral than to that from a white-hot one. 



In regard to the relation of radiation to conduction, if 

 we define radiation, internal as well as external, as the 

 communication of motion from the vibrating atoms to the 

 ether, we may, I think, by fair theoretic reasoning, reach 

 the conclusion that the best radiators ought to prove the 

 worst conductors. A broad consideration of the subject 

 shows at once the general harmony of this conclusion with 

 observed facts. Organic substances are all excellent radi- 



