308 BIOT ON THE EMPLOYMENT OF POLARIZED LIGHT 
perature must have been not far from 50°; and, as the density 
of the essence of turpentine decreases about 4 in passing from 
0 to 100°, its diminution, for half of this interval, is also nearly 
1 : 
5g? that is to say, equal to the decrease of the rotatory power. 
Thus, judging from these numbers, the whole specific heat of 
the essence would be employed in changing its volume, without 
its molecular constitution experiencing any alteration. But it 
may be imagined that such results ought to be established by 
precise measurements, placing the tubes of observation in me- 
tallic coverings, which would allow them to be kept at constant 
and known temperatures. I have long since constructed an 
apparatus of this kind, but have not had leisure to make use of 
it, and I fear that I shall not find an opportunity. 
11. The aqueous solutions of tartaric acid present an oppo- 
site efiect to that just mentioned. The deviations « of one and 
the same ray are seen in them increasing for one and the same 
thickness, in proportion as the temperature rises, notwithstand- 
ing the diminution in density which should weaken them. This 
may be instantly proved by heating for some minutes, with a 
spirit-lamp, the tube which contains them, whilst it is placed 
upon the optical apparatus. The increase of the deviations is im- 
mediately manifested ; it then stops after the lamp is withdrawn ; 
and they finally return slowly to their primitive amplitude, in 
proportion as the previous temperature is re-established. The 
specific heat introduced into these solutions is therefore far from 
being wholly consumed in their changes of volume. The mole- 
cular constitution of the acid which they contain is momentane- 
ously modified by it, since it is that which communicates to 
water the rotatory power, by forming with it a true combination, 
as I shall prove hereafter. We see, by these two examples, how 
much this class of observations, which allows us, so to speak, to 
witness the calorific modifications of material particles, may 
give us new ideas on the part which should be attributed to the 
imponderable principle of heat in the constitution of bodies,— 
a question which has hitherto remained inaccessible to other 
physical experiments. But, in order to be able to place reliance 
upon the indications which the optical characters may furnish 
on these invisible phenomena, we must extend their theory to 
