IN STUDYING QUESTIONS OF CHEMICAL MECHANICS, 297 
slight refrigeration ; so that the bottle containing them may be 
reversed without their being detached, and even without the ac- 
tion of their own weight disuniting their mass, except after a 
certain time, proportioned to the lowness of the temperature. 
Nevertheless, under these circumstances, where the sensible ele- 
ments of the total mass can continue to receive only very slow 
and very crampt relative movements, we may still perceive that 
the action exerted upon the polarized light is molecular, and in- 
dependent.of the occasional aggregation which the approxima- 
tion of the elements has caused them to contract. For in the 
first place, for the same mass, this action has the same direction 
as in the liquid state. In the next place it has an energy dif- 
fering little for equal thickness ; and lastly (a fact which is de- 
cisive), its total effect is always proportional to this thickness, 
whatever be the internal direction of the transmitted ray. These 
characters may be easily verified, if not with complete accuracy, 
at least with an evident approximation, by receiving immediately 
the filtered products into flasks sensibly cylindrical, through 
which they are observed while they are still liquid, and again 
after they are coagulated. For, in this latter state, as in the 
first, if the flask is turned around its axis, so that the polarized 
ray traverses it successively by its different diameters, the de- 
viation observed remains constant in direction, as in amplitude, 
with the exception of some small variations which may be pro- 
duced in this amplitude by the inequalities of the internal dia- 
meters; and through flasks of unequal size the deviation im- 
pressed by the same substance on the same type ray is propor- 
themselves and with the latter, as their odour equally indicated. The devia- 
tions observed by the naked eye, through a thickness of 99"™-5, were, for the 
essence obtained by distillation with water, —7° ~<~_ ; for the essence de- 
rived by distillation without water, —]9° ~«—_ ; whilst the pure essence of 
turpentine, observed under the same circumstances, exercised a deviation equal 
to —34° 2. The matter observed originally was therefore a compound which 
was destroyed by distillation, either by removing the essences from it, or deter- 
mining their formation. Various analogous products, known in commerce 
under the name of Swiss turpentine, also presented to us right-handed devia- 
tions; but that which is extracted from Pinus sylvestris exercises a deviation 
toward the left, to judge by a specimen from that species which has been given 
me by M. Dubail. It would be interesting to extend these chemical and optical 
tests to the different products thus obtained, if we could procure them from 
certain sources, and M. Soubeiran proposes to take an opportunity of doing 
this. This study might above all be undertaken with advantage by chemists 
residing in the very places where these kinds of substances are obtained, because 
they could then determine the variations of their nature, not only for the dif- 
ferent species of trees, but for the same species in its different parts, at differ- 
ent seasons of the year, and at the different periods of its life. 
