306 BIOT ON THE EMPLOYMENT OF POLARIZED LIGHT 
column, as well by the rarefaction due to the elevation of tempe- 
rature, as by the peculiar modification which the constituent 
molecules had undergone. In proportion as the temperature 
fell, the new violet-blue image became gradually more and more 
blue; and the following day, when the liquid had resumed the 
temperature of 7°'5, I found that, to bring it back to its primitive 
violet-blue character, it was necessary to move again the azimuth 
2° toward the left, that is to say, to bring it back to the same 
deviation in which it was originally. The change effected in 
the action of the essence was therefore temporary, and its par- 
ticles had not undergone any permanent ‘modification in their 
constitution. I commenced a second time the same experiment, 
endeavouring to render all the conditions of heating similar ; it 
re-produced exactly the same results*, 
Imperfect as this experiment is, as regards the measurement 
of the temperatures, it may serve as an example of calculation. 
Let [«] be the molecular power of the essence at the lowest 
temperature ¢ at which the experiment is commenced, [a] being 
defined as in § 8. It is observed in the tube of the length /, 
under its actual density 8, and it has been found to produce there 
the deviation « upon the type ray, which was in this case the 
pure yellow ray ; for in the general law of dispersion reciprocal 
to the square of the length of the fits, the azimuth of the violet 
blue or blue-violet image is exactly that of the deviation peculiar 
to this kind of rayst. Now, according to § 8, the theoretical 
* To perform these experiments with the essence conveniently, I first fixed 
one of the caps of the glass tube by the interposition of a very thin layer of 
a concentrated solution of gum-arabic, which I applied only near its exterior 
border, in order that on fixing the tube it might not extend into the interior of 
the surface which the active liquid should occupy. When this layer was dry it 
adhered completely. Then, before pouring the essence into the tube by the 
orifice which had remained open, I applied a similar layer on its margin, and 
filled it with care, until the column of liquid introduced rose somewhat above it, 
forming at its free extremity a convex meniscus, I then broke this meniscus by 
applying the cap, which caused it to adhere; and I completed this adherence 
by means of a tightening screw terminating the enveloping metallictube. For 
these experiments it is well to use a glass tube of a rather large calibre, when a 
bubble of air may be left in it, which offers a sufficient space for the liquid to 
expand. If we have specifically in view these experiments, it would be more 
safe and convenient to adapt a sort of lateral reservoir to the tube for this same 
purpose. But every one will readily imagine the details which will be most 
convenient, and I only mention here those which I employed in order to pre- 
pare the way for the best arrangements. 
+ This fundamental proposition has been established theoretically, and 
afterwards confirmed by experiment, in the memoir, Sur la Polarisation Circu-— 
laire, inserted in vol. xiii. of the Collection of the Academy. 
