120 FRAGMENTS OF SCIENCE 



line of vision was at right angles to the experimental 

 tube. The plate of selenite usually employed was a cir- 

 cle, thinnest at the centre, and augmenting uniformly in 

 thickness from the centre outward. When placed in its 

 proper position between the Nicol and the cloud, it ex- 

 hibited a system of splendidly- colored rings. 



The cloud here referred to was the first operated upon 

 in the manner described. It may, however, be greatly 

 improved upon by the choice of proper substances, and 

 by the application, in proper quantities, of the substances 

 chosen. Benzol, bisulphide of carbon, nitrite of amyl, 

 nitrite of butyl, iodide of allyl, iodide of isopropyl, and 

 many other substances may be employed. I will take the 

 nitrite of butyl as illustrative of the means adopted to 

 secure the best result, with reference to the present ques- 

 tion. 



And here it may be mentioned that a vapor, which 

 when alone, or mixed with air in the experimental tube, 

 resists the action of light, or shows but a feeble result of 

 this action, may, when placed in proximity with another 

 gas or vapor, exhibit vigorous, if not violent action. The 

 case is similar to that of carbonic acid gas, which, diffused 

 in the atmosphere, resists the decomposing action of solar 

 light, but when placed in contiguity with chlorophyl in 

 the leaves of plants has its molecules shaken asunder. 



Dry air was permitted to bubble through the liquid 

 nitrite of butyl, until the experimental tube, which had 

 been previously exhausted, was filled with the mixed air 

 and vapor. The visible action of' light upon the mixture 

 after fifteen minutes' exposure was slight. The tube was 

 afterward filled with half an atmosphere of the mixed air 

 and vapor, and a second half -atmosphere of air which had 



