162 Prof. Tyndall — Contributions to Molecular Physics, [Mar. 17, 



vapour, the position of water fixes that of aqueous vapour. Water had 

 been compared with other liquids in a multitude of experiments, and it was 

 found that as a radiant and as an absorbent it transcends them all. Thus, 

 for example, a layer of bisulphide of carbon, 0*02 of an inch in thickness, 

 absorbs 6 per cent., and allows 94 per cent, of the radiation from the red- 

 hot platinum spiral to pass through it ; benzol absorbs 43, and transmits 

 57 per cent, of the same radiation ; alcohol absorbs 67, and transmits 33 

 per cent., and it stands at the head of all hquids except one in point of 

 power as an absorber. The exception is water. A layer of this substance, 

 of the thickness above given, absorbs 81- per cent., and permits only 19 per 

 cent, of the radiation to pass through it. Had no single experiment ever 

 been made upon the vapour of water, we might infer with certainty from the 

 deportment of the liquid, that weight for weight this vapour transcends all 

 others in its power of absorbing and emitting radiant heat. 



The relation of absorption and radiation to the chemical constitution of 

 the radiant and absorbent substances was next briefly considered. 



For the first six substances in the list of those examined, the radiant 

 and absorbent powers augment as the number of atoms in the compound 

 molecule augments. Thus, bisulphide of carbon has 3 atoms, chloroform 

 5, iodide of ethyl 8, benzol 12, and amylene 15 atoms in their respective 

 molecules ; and the order of their powers as radiants and absorbents is 

 that here indicated — bisulphide of carbon being the feeblest, and amylene 

 the strongest of the six. Alcohol, however, excels benzol as an absorber, 

 though it has but 9 atoms in its molecule ; but, on the other hand, its 

 molecule is rendered more com'plex than that of benzol by the introduc- 

 tion of a new element. Benzol contains carbon and hydrogen, while alco- 

 hol contains carbon, hydrogen, and oxygen. Thus, not only does the 

 idea of multitude come into play in absorption and radiation, that of com- 

 plexity must also be taken into account. The author directed the parti- 

 cular attention of chemists to the molecule of water ; the deportment of 

 this substance towards radiant heat being perfectly anomalous, if the che- 

 mical formula at present ascribed to it be correct. 



Sir William Herschel made the important discovery that beyond the 

 limits of the red end of the solar spectrum, rays of high heating power 

 exist which are incompetent to excite vision. The author has examined 

 the deportment of those rays towards certain bodies which are perfectly 

 opaque to light. Dissolving iodine in the bisulphide of carbon, he ob- 

 tained a solution which entirely intercepted the light of the most brilliant 

 flames, while to the extra-red rays of the spectrum the same iodine was 

 found to be perfectly diathermic. The transparent bisulphide, which is 

 highly pervious to the heat here employed, exercised the same absorption 

 as the opaque solution. A hollow prism filled with the opaque liquid was 

 placed in the path of the beam from an electric lamp ; the light-spectrum 

 was completely intercepted, but the heat-spectrum was received upon a 



