5Y8 M. CHEVREUL’S EXAMINATION OF AN OPTICAL CHARACTER 
greatest degree of purity, it is incontestably important to investigate 
the action which it is capable of exercising upon the plane of polariza- 
tion, when it is dissolved in any liquid whatever, compared with the 
action which it is capable of exercising after having been exposed to 
the influence of some agent, such as light, heat, or electricity. 
25. I shall show that the importance of these researches to the che- 
mist does not arise from their affording proof that an observable altera- 
tion has taken place in the rotatory power of a substance submitted 
to the action of a certain agent, when the nature of this substance has 
been evidently changed ; that is to say, when it has been converted into 
a substance absolutely distinct from what it was before the experiment ; 
but from their enabling him to ascertain whether an alteration in the 
arrangements of the particles has really occurred, in cases in which the 
substances submitted to experiment appear at first sight not to have 
undergone any such alteration, and in which, without the test of cir- 
cular polarization, we should be led to conclude that they had absolutely 
not experienced any. 
26. The following example will illustrate my proposition: 
A solution of starch in boiling water is converted into sugar by sul- 
phuric acid. The starch dissolved in water being insoluble in alcohol, 
whilst the sugar into which it becomes converted is soluble in that sub- 
stance, we have a means of distinguishing in the action of sulphuric 
acid upon starch the moment when the conversion of this principle into 
saccharine matter commences, and the moment when it is completed. 
If it be now discovered that the solution of starch is possessed of a pro- 
perty of causing the plane of polarized light to deviate to the right in a 
much greater degree than is effected by its sugar, is it not true that the 
observation of the diminution of the rotatory power of the solution of 
starch submitted to the action of sulphuric acid teaches nothing more 
than the preceding facts relative to the alteration effected in the pro- 
perties of the starch? And the conversion of a substance essentially in- 
sipid and incapable of producing alcohol into a fermentable saccharine 
substance, gives a much more exact idea of the change effected in its 
composition than that derived from the variation of its rotatory power. 
In cases in which a substance submitted to an agent has sustained an 
alteration in its rotatory power, which, far from being, as in the exam- 
ple of the starch, the result of the conversion of one substance into an- 
other perfectly distinct from the first, has on the contrary sustained so 
slight an alteration in the distribution of its particles that without 
having verified it we should conclude that the substance had not under- 
gone any alteration whatever in its properties ; it is then, I repeat, that 
the observation of the optical character becomes interesting, as leading 
to researches which may render other alterations discernible which 
without them might escape the notice of the observer. 
