596 M. CHEVREUi's EXAMINATION OF AN OPTICAL CHARACTER 



This distinction is very important when in our researches into the im- 

 mediate principles of organized beings we are desirous of ascertaining 

 the value of indications furnished by what in chemistry are called re- 

 agents*. 



17. There are physical propei'ties which furnish characters for di- 

 stinguishing bodies in analytical researches, which are valuable in pro- 

 portion to the limitation of the number of species possessing them, and 

 the facility with which these species may be distinguished among them- 

 selves by other characters. Such is the property of producing a violet 

 vapour, which belongs only to iodine and indigo, bodies very distinct, 

 since the vapour of the first does not undergo any alteration, even at 

 the most elevated temperatures, while the vapour of the second is com- 

 pletely altered even below 560d." 



18. Definitively, the properties which furnish the chemist with cha- 

 racters the best adapted for the classification, definition, and recogni- 

 tion of chemical species in analyses, are 



a. Those which are the most constantly found in a certain species, 

 whatever be the diversity of circumstances in which it may be placed; 



b. Those the existence of which necessarily involves that of others ; 



c. Those which are in general concomitants ; 



d. Those, easily verified, which belonging only to a very small num- 

 ber of species, differing widely in other respects, are valuable for ana- 

 lytical researches, or to concur with other properties in characterizing 

 these species, but whose existence does not lead to any conjecture rela- 

 tive to an analogy of properties between the bodies to which they 

 belong. 



We shall now examine, accoi'ding to the views that I have just ex- 

 plained, the optical chai-acter proposed by M. Biot. 



19. Grape sugar which has not been solidified directs the plane of 

 polarization to the left ; and as, according to M. Biot, its chemical na- 

 ture is unaltered when it becomes crystallized in grape juice, and as it 

 then directs the plane of polarization to the right, it follows (hat this 

 properti/ is tiot fundamental, since it is found in the same species tvith 

 two different signs ; it does not therefore fulfill the condition 18 a. 



20. Cane sugar has certainly less analogy with sugar of starch of the 

 first formation than the latter has with sugar of starch of the second 

 formation ; the action however of the two first is equal or nearly equal, 

 while the action of sugar of starch of the second formation is much 

 feebler than that of sugar of starch of the first formation. Fro>n this it 

 is evident (hat the optical character proposed by M. Biot does not apply 

 to one cf those properties the existence of which necessarily involves that 



* Rapport de M. Chevreul aur tin Mcmnire tie M. Doiuiee. Atuudes Je 

 C/iimie et th' Physique, vol. xxxviii. p. 89. 



