1864.] Prof. Tyndall — Contributions to Molecular Physics, 167 



is more than three times that by bisulphide of carbon ; for the radiation from 

 the most luminous portion of a gas flame the absorption by chloroform is 

 also considerably in excess of that by bisulphide of carbon ; while for the 

 flame of a Bunsen's burner, from which the incandescent carbon particles 

 are removed by the free admixture of air, the absorption by bisulphide of 

 carbon is nearly twice that by chloroform ; the removal of the incandescent 

 carbon particles more than doubled in this instance the relative transparency 

 of the chloroform. Testing, moreover, the radiation from various parts of 

 the same flame, it was found that for the blue base of the flame the bisulphide 

 was the most opaque, while for all other portions of the flame the chloroform 

 was most opaque. For the radiation from a very small gas flame, consisting 

 of a blue base and a small white top, the bisulphide was also most opaque, 

 and its opacity very decidedly exceeded that of the chloroform when the 

 flame of bisulphide of carbon was employed as a source. Comparing the 

 radiation from a Leslie's cube coated with isinglass with that from a similar 

 cube coated with lampblack, at a common temperature of 100° C, it was 

 found that out of eleven vapours all but one absorbed the radiation from the 

 isinglass most powerfully ; the single exception was chloroform. It may 

 be remarked that whenever, through a change of source, the position of a 

 vapour as an absorber of radiant heat was altered, the position of the liquid 

 from which the vapour was derived was changed in the same manner. 



It is still a point of difi^erence between eminent investigators as to whether 

 radiant heat up to a temperature of 100° C. is monochromatic or not. 

 Some affirm this, others deny it. A long series of experiments enables the 

 author to state that probably no two substances at a temperature of 100° 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 is also a subject of discussion whether rock- 

 salt is equally diathermic to all kinds of calorific rays, — the differences 

 affirmed to exist by one investigator 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 main- 

 tain the latter. The question was examined by the author without changing 

 anything but the temperature of the source. Its size, distance, and sur- 

 roundings remained the same, and the experiments proved that rock-salt 

 shared in some degree the defect of all other substances ; it is not perfectly 

 diathermic, and it is more opaque to the radiation from a barely visible 

 spiral than to that from a white-hot one. 



The author devotes a section of his memoir to the relation of radiation to 

 conduction. Defining radiation, internal as well as external, as the com- 

 munication of motion from the vibrating molecules to the ether, he arrives 

 by theoretic reasoning at 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 the conclusion with observed facts. 

 Organic substances are all excellent radiators j they are also extremely bad 



