296 BIOT ON THE EMPLOYMENT OF POLARIZED LIGHT 
movements produced no appreciable change in the total devia- 
tions which its mass impressed on the planes of polarization of 
the luminous rays. The effect was still the same, and as con- 
stant, when a clock movement was applied to the rod, which 
caused the diaphragm to turn with such velecity that its periodical 
passage in the plane of the eye was not perceptible ; so that there 
was no perceptible intermittence in the continuity of the sen- 
sation*. Moreover, this artificial agitation, although so quick, 
was yet perhaps very slow in comparison to the internal vibra- 
tions which the molecules of liquid, and even of solid bodies, 
execute, without doubt, spontaneously either amongst them- 
selves, or around their centre of gravity in their usual state, 
under the influence of opposite forces which attract them to one 
another, and which keep them at a distance ; for in such cireum- 
stances the case of an exact and permanent equilibrium would 
be altogether exceptional. 
5. The preceding experiment was successfully made upon 
cane-sugar syrup, which possessed a very perceptible viscosity. 
But the molecular character of the action may likewise be 
made manifest in systems completely viscous, whose sensible 
parts possess relatively too little mobility for the same process 
of mechanical deplacement to be applied. Such, for example, 
are the semi-fluid secretions of the Conifere and the Terebin- 
thacee, commonly known under the general name of turpentines. 
If we take these substances as they are found in commerce, and 
purify them by filtration in a water-bath, or in a warm chamber 
at a moderate temperature, we obtain products which are still 
viscous and more or less coloured, but very limpid, and which act 
upon polarized light in different directions as well as different 
intensities, according to their origin and the preparations which 
they have undergone ; this proves that they are of very different 
naturest. All these products coagulate by repose and with a 
* Comptes Rendus des Séances de l' Académie des Sciences, t. xvii. p. 1209. 
+ I formerly had at my disposal similar products, which all impressed on 
polarized light deviations toward the left, as the pure essence of turpentine. 
The first time that I found one exercising right-handed deviation, I was very 
much astonished, and I begged M. Soubeiran, who had had the kindness to pu- 
rify it by filtration, to submit it again to distillation, in order to extract the es- 
sence: He did this in two ways; by distilling it with water, and without water. 
Now, to our mutual surprise, the two volatile liquids thus obtained exercised 
the deviation toward the left with different energies, and both much less than 
that of the pure essence of turpentine; so that they were dissimilar among 
