396 BIOT ON THE EMPLOYMENT OF POLARIZED LIGHT 
fying calorific rays existed for liquids exercising a rotatory power 
on polarized light ; and it would be very interesting to ascer- 
tain the identity or the dissimilarity of the action in this latter 
case, where it is purely molecular. 
Tables containing the results of two experiments, in which the 
progress of the reaction of concentrated nitric acid on the starch, 
in closed vessels, was continued during several days. 
First Experiment.—The mixture was made on the morning of 
Monday, April 8th, 1844, by M. Pelouze, with some starch of 
commerce and very concentrated fuming nitric acid. As soon 
as the liquefaction appeared sufficient, the decanted solution was 
introduced into a glass tube whose length was 146™™-25, and 
which was closed by thin glasses with parallel faces. It was nine 
o’clock in the morning. Through this thickness it appeared 
slightly turbid. Nevertheless its action on polarized light was 
distinguishable, and the deviation which it produced in the planes 
of polarization was estimated, by approximation, at between 
+205 and + 21°5 towards the right of the observer. The rest 
of the solution was preserved in a flask with ground stopper, 
under conditions of temperature similar to those to which the 
tube was exposed ; and this reserved portion was used to replace 
at need the small portion of liquid dissipated by evaporation at 
the surfaces of junction of the glasses, or to enable us to follow 
its action in other tubes a little different in length, when the cor- 
rosion of the metallic envelopes required this change. But all 
these results have been rendered comparable by reducing them, 
by the law of proportionality, to what they would have been in 
the primitive tube. As the especial object was to follow the pro- 
gress of the reaction, and not to determine its precise course, the 
deviations were not observed through red glass, which would 
have been tedious and difficult, but only determined by the naked 
eye, by the passage of the extraordinary image from the vanish- 
ing blue tint to the rising yellowish red, adding the indication 
of the actual tint of the liquid observed. This renders the mea- 
surement of the deviations thus obtained relatively a little too 
small in proportion as the tint of the liquid is a deeper red. 
But this relative inferiority will only render in the present case 
more sensible the slowness of the molecular modifications expe- 
rienced by the substance of the starch on which the acid acts. 
