342 BIOT ON THE EMPLOYMENT OF POLARIZED LIGHT 
decreased; and as it cannot be liquefied by heat without removing 
from it, at least temporarily, a portion of its constituent water, 
it is impossible to tell whether, after cooling and solidification, 
it would preserve an appreciable rotatory power. But it would 
recover it, undoubtedly, on recombining with the water, or even 
without a complete restoration of that liquid, if it was again 
liquefied by heat alone, as this considerably increases its power. 
And, in fact, I have found it to be very energetic on observing it 
thus, in the state of fusion, before spontaneous solidification. 
If we compare these results with what I have said in § 11 re- 
specting the temporary modifications which the molecules of 
this acid experience under the influence of heat, it will, in my 
opinion, be regarded as highly probable that this influence, com- 
bined with that of the proportions of water added or removed 
from it, converts it into so many different bodies ; and that this 
continued mutability is the cause of the diversity of the chemical 
affinities which it has been found to possess, in the small number 
of terms of its progressive transformations in which we have 
been able to observe it in the state of definite combination. 
37. We shall subsequently see this singular acid manifesting, 
in its action upon polarized light, special properties which re- 
sult from the two principles of modification which I have just 
indicated. But before studying it by the optical characters 
which it produces in the combinations into which it enters, and 
concluding from them the fixed or variable nature of these com- 
binations, by the formule established above, I shall first prove 
the legitimacy of their applications, and confirm all their general 
consequences, by the employment of a different class of phy- 
sical facts, which I have not employed to establish them, and 
which agree completely with them in their results. 
Addition. 
Whilst the foregoing portion of this memoir was in the press, 
I attempted to effect the complete solidification of the turpen- 
tines, with the object explained in § 36. I repeated that expe- 
riment twice; and, although accessory circumstances, which in- 
tervened in the results, prevented their being perfectly suitable 
to the object proposed, they presented other characters suffi- 
ciently curious to merit being described. 
I first experimented on the kind of turpentine, the rotatory 
effects of which are mentioned in § 5. It was still in the same 
rectangular box in which I had measured them at the temperature 
