IN 8TUDYING QUESTIONS OF CHEMICAL MECHANICS. 393 
longer it stands the more the precipitation produced by the 
water diminishes, and at length it ceases entirely, after which 
the liquid remains for some time limpid; then it becomes 
coloured more and more, and finally there are formed in it 
various products which have not yet been completely studied. 
But, up to this last phase of the chemical reaction, the starch 
has only become progressively disaggregated without changing 
its molecular state. For if the solution be taken as soon as it 
has become quite liquid, and a glass tube closed at its extremities 
by parallel glasses be filled with it, and this transferred upon 
the polarizing apparatus, it is found to possess a considerable 
rotatory power, directed like that of the dextrine towards the 
right of the observer; and this power remains exactly constant 
for several days, notwithstanding all the variations which the 
substance undergoes during this interval in its power of being 
precipitated by water. This proves therefore the constancy of 
its molecular state throughout this diversity of physical phases. 
Thus, during the whole time that it undergoes them, if we collect 
the precipitate formed by the water and observe it under the 
microscope, we discover in it the fragments of the starch pro- 
gressively lacerated by the acids impregnating them, and mixed 
with a multitude of granules of dimensions still perceptible. 
Even when the reaction is not far advanced, if Nicol’s prisms 
crossed rectangularly be used in the observation, and combined 
with a sensible lamina of gypsum, we see several of the shreds, 
and the larger granules themselves, still preserve their primitive 
organization so slightly modified as to act visibly on polarized 
light, and change the proper tint of the lamina into red or green. 
These phenomena only cease with the precipitation by water ; 
and it is likewise then that the rotatory power commences to 
change perceptibly, the acid having sufficiently disaggregated 
the substance of the starch globules to modify its chemical na- 
preceding note. At the time when it was composed, it was not known what 
use might be made of the rotatory power, as a molecular character of bodies 
possessing it. Ifthe author had been able to apply this process of observation 
to the substance he obtained by acting with concentrated nitric acid on starch, 
lignine, cotton, or hemp linen, he would easily have ascertained that it was not 
produced, but only extracted from those bodies in a state of disaggregation, 
which modified merely its physical properties without changing its molecular 
condition, and that it was nothing else than the element of the starch globules 
contained in them. The important remark of M. Pelouze on the simultaneous 
presence of the two elements of the system in the matter precipitated by the 
water, is given in a note inserted in the Comptes Rendus de l’ Académie des 
Sciences, vol. vii. p. 713. 
VOL. IV. PART XV. QF 
