84 FRAGMENTS oP 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 tlie probabilities are so much 
in its favor, and it is in my 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. 
