346 On Cri/stallographp. 



which has remained without subdivisions; or rather they wili 

 form by themselves a kiud ot" system, which will have the 

 advantage of midtiplying the points ot" view under whieh 

 minerals n)ay be regarded. 



AN.NOTA'TIONS KKLATIVl^ TO THE GENKRAL TABLE OF 

 WINEUALOGICAL CHAllACTKRS. 



Physical cliaructers, 



1. Specific gravity^ Lei us stippose a series of bodies 

 of different natures, which h^n'e equal volumes. If we 

 weigh all these bodies successively by means of common 

 scales, it will be necessary, in order to e*labli$h the equili- 

 brium, to employ weights more or less considerable, ac- 

 cording as these bodies are more or less dense. Let us 

 suppose, moreover, that, having taken as a term of com- 

 parison one of these bodies, for instance the lightest, we 

 represent its weight by unity, and we express the weight of 

 all the other bodies by nun)bers proportional to that unity. 

 We shall have the relations between the weights of the 

 diflerent bodies of equal volume, or the specific gravities 

 of these bodies. 



Bui the hypothesis, that all the bodies of which we 

 would propose to determine the specific gravities have equal 

 volumes, not being capable of being reahzt-d, it will be- 

 come necessary to seek for another method in order to at- 

 tain the same object. We might succeed equally well, were 

 we to estimate exactly the volume oF each body ; after 

 which it would be easy to bring the results of the different 

 weighings to what they would have been on the hypothesis 

 of there having been an unity in the volutTie, But as this 

 method presents obstacles which are insurmountable in 

 practice, we supply its place bv an ingenious process, whieh 

 consists in seeking for the relation between the weight of 

 each body when weighed in the air, and the loss of weight 

 Avhich appears when the same body is weighed in water, 

 which we here suppose to be respectively ligliter than it. This 

 loss proceeds from the eflbrt made by the water to sustain 

 the body in part ; and this effort being equal to that which 

 it exercised in order to keej) in equilibrium the volume of 

 the same liquid displaced by the body, it results that the 

 loss in question represents the weioht of a volume of wa- 

 ter equal to that of the body. We liave therefore the re- 

 lation between the weight of the body and that of the 

 water in equal volumes ; and this liquid thus serves as a 

 common measurement, in order to compare with each 

 Other the specific crravities of the different bodies. 



It 



