598 M. chevrkul'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, 



