On the Photographic Transparency of various Bodies. 307 



these organic bodies, particularly the acetates, than in other cases. 

 The different varieties of sugar are freely diactinic. 



Amongst the salts of inorganic acids, the nitrates are the most 

 remarkable for their power of arresting the chemical rays. A solu- , 

 tion of each of these salts, in all the instances tried, cut off all the 

 more refrangible rays, and reduced the spectrum to less than a sixth 

 of its ordinary length. The chlorates, however, do not participate 

 in this absorptive power to nearly the same extent. 



Although the sulphates, as a class, are largely diactinic, the sul- 

 phites are much less so ; and the hyposulphites cut off about three- 

 fourths of the length of the spectrum, leaving only the less re- 

 frangible portion. 



Of eighteen different liquids tried by the author, two only can be 

 regarded as tolerably diactinic, viz. water, which is eminently so, 

 and absolute alcohol, which, however, exhibits a considerable falling 

 off. The liquids which follow are mentioned in the order of their 

 chemical transparency, those most transparent being mentioned first : 

 — Dutch liquid, chloroform, ether; then benzol and distilled gly- 

 cerin, which differ but little ; then fousel oil, wood-spirit, and oxalic 

 ether, which are also nearly alike; acetic acid, oil of turpentine, 

 glycol, carbolic acid, liquid paraffin, boiling at 360° F., and bisul- 

 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 oxychloride 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 



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