17 



IV. V. VI. VII. VIIL 



Oxide of zinc, . 74-173 . 74-262 . 74-247 . 74-092 . 73-427 



Carbonic acid, ) 



Hydrated water, 25-827 . 25-738 . 25-753 . 25*908 . 26-573 

 Hygroscopic water, ) 



100-000 100-000 100-000 ilOO-000 100-000 



IX. X. XI. XII. XIII. 



Oxide of zinc, . 74-232 . 74-284 . 74-391 . 74-437 . 74-480 

 Carbonic acid, j 



Hydrated water, 25-778 . 25-716 . 25-609 . 25-563 . 25 520 

 Hygroscopic water, ) 



100 000 100-000 100-000 100-000 100-000 



So far as these results go, they prove that the change assumed by 

 Messrs. Peterson and Yeit to take place in the composition of the 

 mineral by exposure to the air does not occur. It is probable that the 

 mineral may have been when first formed more highly hydrated, and 

 that, according as it hardened, in consequence of the gradual evaporation 

 of the mechanically-adhering water, it likewise lost part of its hydrated 

 water, — thereby giving rise to the formation of a sufficiently stable com- 

 pouud to remain unaltered in the air. We generally consider that hy- 

 drated gelatinous precipitates have the composition which the analyses 

 of the bodies formed by throwing them upon filters, pressing and drying 

 the filtered masses, give us ; it is, however, very probable, that the moist 

 gelatinous mass is a diiferent hydrate from that which we get upon 

 the dried filter. It is quite possible that all bodies capable of combining 

 with water may do so in a great many proportions, some of which 

 only possess the necessary degree of stability to enable us to isolate 

 them — of this we have a striking example in the two, if not three, hy- 

 drates which common salt forms. We also know that in bodies which 

 contain several equivalents of hydrated water, each equivalent may not 

 always be held with the same amount of force. All the specimens ex- 

 amined by us were thoroughly air- dried, having been in a dry, warm 

 room, during more than eight months, and had all consequently arrived 

 at the stage of greatest stability, whatever may have been the original 

 degree of hydration. It does not appear that any carbonic acid was 

 lost. 



If we consider the part of the water which is driven off in the water- 

 bath as hygroscopic, the formula 8ZnO,3C025HO = 3 (ZnO,C03) + 5 

 (ZnO,HO), represents the composition of the Spanish hydrocarbonate. 

 The following table, which contains the results of the analyses I., II., 

 III., from which the hygroscopic water, lime, &c., have been deducted, 

 shows the agreement between the composition calculated from this for- 

 mula and that deduced from experiment : — 



I. A. PEOC. VOL. VIII. D 



