394 BIOT ON THE EMPLOYMENT OF POLARIZED LIGHT 
ture. When this reaction is effected at a moderate temperature, 
for example between 15° and 20° C., it is progressive; that is 
to say, it does not operate instantaneously on the whole active. 
mass, as may be ascertained by the gradual diminution of the: 
rotatory power, as also because the rest of the active substance 
is still precipitable, no longer by water, but by alcohol ; and we 
may easily prove also that, even in these advanced stages of the 
action, the property of acting on polarized light belongs exclu- 
sively to those starch molecules which are not yet destroyed. 
For if the acid be saturated by the addition of an alkaline solu- 
tion, introduced into the mixed liquid, in a known proportion 
by volume, the rotatory power is seen to preserve the same 
intensity as before; and, on evaporating slowly the solution at 
a gentle heat, we obtain a true salt, which, redissolved, again 
exhibits the rotatory power. . But besides its alkaline and acid 
elements, this salt is mixed with dextrine which the solution 
still contained. Thus it can never be made to crystallize in this 
mixed state, and this can only be effected by purifying it of that 
substance. At the end of this memoir will be found two tables 
of experiments in which the progress of the phenomenon was 
followed in all its stages, as long as the liquid formed could be 
observed by transmitted light. 
76. The rotatory phenomena have served in medicine as a cer- 
tain and readily applicable diagnostic for ascertaining the exist- 
ence and appreciating the different stages of that dreadful disease 
called saccharine diabetes, by showing in a moment the absolute 
quantity of sugar actually contained in the urines at all stages of 
the disease, and at all the periods of medical treatment opposed 
to it*. Another observation, which may also be very useful, is 
that by which M. Bouchardat discovered that all the vegetable 
alkalies possess rotatory powers, of various directions and inten- 
sities t. For not only will this property serve to characterize 
them visibly, and enable us to ascertain with facility all the 
adulterations to which they may have been subjected, but 
moreover, the temporary or lasting modifications which they ex- 
perience from the other alkalies and acids, as well as the various 
combinations into which they may be made to enter without de- 
* Comptes Rendus de l’ Académie des Sciences, tome xi. p. 1028. For the 
quantitative estimation of the saccharine matter, see vol. xv. pp. 633, 634. 
+ Sur les propriéiés optiques des alcalis végétaux, by M. Bouchardat. (Annales 
de Chimie et de Physique, 3rd Series, vol. ix. p. 213.) 
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