On CrystaliogfUphif, 351 



lion when the areometer remains in a fixed position. 

 Then we shall level it; and we shall in general seek for 

 the loss which the body has undergone of its weight in 

 the water. We shall attcrwards seek for ihe weight of the 

 quantity of water which it has inibihed, on weighing it 

 again in the air, .and in subtracting the first weight from 

 the second ; then we shall add the difference to the loss 

 formerly found : and the residt will give the true loss, or 

 that uhich would take place if the body was not suscepti- 

 ble of imbibition ; after which we shall form the propor- 

 tion indicated above. 



Let us suppose', for exatnple. a mesotype, the weight of 

 which in the air is nine granmies. Let us Suppose that . 

 the loss of weight which it experiences in the water, after 

 imbibition, is 4*3 grs. Let us suppose in fine, that being 

 again weighed in this state, it gives 9*13 grs. Subtracting 

 the first weight from the latter, we shall have 0*13 grs. for 

 the quantity of water which the mesotype has imbibed. - 

 The real loss, or that which the substance would have ex- 

 perienced of its weight in water, if it was not penetrable by 

 this liquid, will therefore be 4*3 grs. plus 0-13 grs. or 

 4*43 grs. ; which gives the following proportion, 4-43 grs, 

 : 9 : : \ : X. From which we shall conclude that the spe- 

 cific gravity is 2-0316. 



In fact, since bodies lose less of their weight: in water 

 in proportion as their absolute weight is more consider- 

 able, it results that the mesotype ought to have lost in the 

 water 0'.13 grs. less than if the imbibition had not taken 

 place. We nuist therefore add 0"13 grs. to the loss found 

 bv experiment, in order to have the loss corrected. 



The character which is drawn from tlie specific gravity 

 joins to the advantage of a great generality that of being 

 susceptible of a precise cstbmation, provided we do not 

 employ loo srtjall places of any substance. Its limit re- 

 latively to each mineral is the result of the operation made 

 on a piece chosen in the greatest possible state of purity. 

 It may vary bevond this limit on account of some colour- 

 ing principle of a metallic nature; or within this limit, by 

 the effect of the mixture of a substance less dense, or 

 whose presence relaxes the airgregation of tlie molecules. 

 Thus the specific gravity of the limpid hyahn-quanz, 

 called Madagascar crystal, is 2'Qb3 ; that of the red livalin- 

 quartz is 2-67(n: and that of the dull quArtz is only 2-t3439. 

 "i'he more do the limits relative to the species between 

 which it is requisite to determine escape being confounded, 

 tlie more decisive is the proof of the character. 



i\ Hard and soft bodies. The character tjuggrested by 



hardness 



