596 M. CHEVREUL’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 7e- 
agents* . 
17. There are physical properties 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; 
6. Those the existence of which necessarily involves that of others ; 
ce. 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, according to the views that I have just ex- 
plained, the optical character 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 that this 
property is not fundamental, since it is found in the same species with 
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. Mrom this it 
is evident that the optical character proposed by M. Biot does not apply 
to one of those properties the existence of which necessarily involves that 
* Rapport de M. Chevreul sur un Mémoire de M. Donnée. Annales de 
Chimie et de Physique, vol. xxxviii. p. 89. 
