302 FRAGMENTS OF SCIENCE. 
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 C., 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 fetter. 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- 
