84 FRAGMKNTS OF SCIENCE. 



the illuminating beam; the plane of vibration of the 

 polarized light was perpendicular to the beam.* 



Thin plates of selenite or of quartz, placed between the 

 Nicol and the actinic cloud, displayed the colors of polar- 

 ized light, these colors being most vivid when the line of 

 vision was at right Angles to the experimental tube. The 

 plate of selenite usually employed was a circle, thinnest at 

 the center, and augmenting uniformly in thickness from 

 the center outward. When placed in its proper position 

 between the Nicol and the cloud, it exhibited 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 im- 

 proved 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 question. 



And here it may be mentioned that a vapor, which when 

 alone, or mixed with air in the experimental tube, re- 

 sists 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 chlorophyll 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 after- 

 ward filled with half an atmosphere of the mixed air and 

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

 permitted to bubble through fresh commercial hydrochloric 

 acid. On sending the beam through this mixture, the tube, 



* This is still an undecided point; but the probabilities are so much 

 in its favor, and it is in ray opinion so much preferable to have a 

 physical image on which the mind can rest, that I do not hesitate to 

 employ the phraseology in the text. 



