112 FRAGMENTS OF SCIENCE. 



From the illuminated bluish cloud, therefore, polarised 

 light was discharged, the direction of maximum polari- 

 sation being at right angles to the illuminating beam ; 

 the plane of vibration of the polarised light was per- 

 pendicular to the beam. 1 



Thin plates of selenite or of quartz, placed between 

 the Nicol and the actinic cloud, displayed the colours of 

 polarised light, these colours 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 centre, and augmenting uniformly 

 in thickness from the centre outwards. When placed 

 in its proper position between the Nicol and the cloud, 

 it exhibited a system of splendidly-coloured 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 sub- 

 stances 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 vapour, 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 vapour, exhibit vigorous, if not violent 

 action. The case is similar to that of carbonic acid 

 gas, which, diffused in the atmosphere, resists the de- 



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

 much in its favour, 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. 



