IN STUDYING QUESTIONS OF CHEMICAL MECHANICS. 305 
are not heated to the point of conversion into vapour. They are 
very easily applicable to diaphanous liquids, except solutions of 
tartaric acid, because the law of dispersion of the planes of 
polarization which is nearly common to them, always occa- 
sions, in a certain azimuth of the analysing prism, an extra- 
ordinary violet-blue (gris de lin) tint, which follows the deep 
blue and precedes the red by an immediate transition, the per- 
ception of which furnishes a fixed index of excessive delicacy. 
In proof of this I shall relate the following experiment, made 
upon essence of turpentine, although it only presents a simple 
approximation. I filled a glass tube of a somewhat large calibre, 
1463 millimetres long, and capable of being closed by caps of 
thin glasses with parallel faces, with a certain sample of this 
essence. I observed the optical power of the essence through 
this tube at the temperature of 7°5 C. The extraordinary violet- 
blue tint which I took for index became evident in the azimuth 
— 55°"; that is to say, when the principal section of the 
analysing prism was removed by this amount from the primi- 
tive plane of polarization, toward the left of the observer. I 
then left the alidade of the prism in this position; and, having 
closed the tube with its metallic mounting in a tin case of the 
same calibre, I put the whole into a water-bath, the temperature 
of which was gradually raised; then, after a certain time, I re- 
moved the case and withdrew the tube, which I replaced anew, 
as quickly as possible, upon the optical apparatus, the alidade of 
which remained fixed. I cannot exactly state the temperature 
which the essence had acquired, not having taken the necessary 
means of measuring it, nor above all to render it fixed; I can, 
however, form an approximate valuation, because in the first 
moment when I replaced the tube in observation, it was not too 
hot to touch it with the naked hands, so that I suppose its tem- 
perature may have been about 50° or 60°. The extraordinary tint, 
which was previously violet-blue, immediately appeared a beau- 
tiful red ; and to restore to it its first character, I was obliged to 
bring back the alidade, and consequently the principal section of 
the analysing prism, 2° toward the primitive plane of polarization. 
Thus the hot essence caused the planes of polarization to deviate 
toward the left, less than the cold essence by that quantity 
through an equal thickness; and this diminution of 2°, visibly 
manifested by the change of tint, consequently expressed the 
total change which had been effected in the power of the liquid 
