1862.] 163 



phide of carbon. Finally, terchloride and oxy chloride of phosphorus, 

 although perfectly colourless and limpid, arrest all the chemical rays. 



The experiments upon aeriform bodies yielded important results ; 

 they show but little coincidence with those of Tyndall on the 

 absorptive power of the gases for radiant heat. These experiments 

 were made by interposing in the track of the ray between the ver- 

 tical slit and the quartz prism, a brass tube two feet long, closed at 

 each end air-tight by means of a plate of quartz. Each gas or 

 vapour in succession was introduced into the tube, and the results 

 compared with those produced by causing the rays to traverse the 

 tube when filled with atmospheric air. 



Amongst the colourless gases, oxygen, hydrogen, nitrogen, car- 

 bonic acid, and carbonic oxide exhibit no absorptive power. 



Olefiant gas, protoxide of nitrogen, cyanogen, and hydrochloric 

 acid exert a slight but perceptible absorbent effect. But in the case 

 of coal-gas the absorptive action is extremely marked, the more 

 refrangible half of the spectrum being cut off by it abruptly. The 

 absorption exerted by sulphurous acid is still more powerful and as 

 sharply defined ; sulphuretted hydrogen and the vapour of bisulphide 

 of carbon exhibit a still more decided absorbent action ; the effect of 

 the terchloride and oxy chloride of phosphorus is not less marked. 

 This absorbent action of these different compounds of sulphur and 

 phosphorus is very striking. 



Coal-gas appears to owe its remarkable power of arresting the 

 chemical rays to the presence of the vapour of benzol and other 

 heavy hydrocarbons ; since the vapour of benzol at 65, diffused to 

 saturation through a column of atmospheric air two feet long, exerts 

 a still more powerful absorptive effect than coal-gas. 



On the other hand, the effect of a similar arrangement, in which 

 the vapour of ether, of chloroform, and of oil of turpentine was sub- 

 stituted for that of benzol, gave effects which, though perceptible, 

 were much less marked. An arbitrary scale is laid down, by which 

 a comparative estimate of the absorptive power of each compound, 

 whether solid, liquid, or gaseous, may be effected with tolerable 

 accuracy. 



With a view of facilitating the production of a spectrum on a flat 

 field, at a uniform distance at all points from the prism, the author 

 instituted a series of experiments, in which a small metallic speculum 



