Sulphate of Lime with Water. S27 



off with nearly the same ease as any of the water is driven off 

 from gypsum. It may however be a compound of anhydrite 



with gypsum or with the monohydrate Ca SH, in which case 

 its constitution may be reconciled with the theoretical views 

 of Professor Graham. 



That the relations of sulphate of lime to water do not form 

 an exception to the general laws by which those of the other 

 sulphates belonging to the same isomorphous group are re- 

 gulated, may be inferred from the discovery by Mitscherlich, 



of a sulphate of iron (Fe S + 2H) analogous in constitution 

 with gypsum, and like it possessed only of a sparing solubility. 

 This analogy leads us rather to expect other compounds of 

 sulphate of lime with water analogous to those observed in 

 the sulphates of iron, magnesia, &c. and that the changes 

 produced on the analogous hydrated salts of each of the sul- 

 phates by heat and other agents should be generally the 

 same. 



There are strong grounds for accepting it as a general con- 

 clusion that whatever compound has been formed by one 

 member of an isomorphic, may be formed, under other circum- 

 stances perhaps, by every other member of the same group. 

 If we compare what we know with what on this prinicple is 

 possible, we shall find our real knowledge to be unexpectedly 

 small. It is something, however, to know how much in a 

 particular line remains yet to be discovered. Even in the 

 branch of saline compounds which has received so general 

 an attention from chemists, and for so long a period, a com- 

 parison of this kind is calculated to astonish us by the vast 

 number of compounds it exhibits as remaining yet to be 

 sought for. 



Let us take for example the class of hydrated sulphates to 

 which gypsum belongs, and compare the known with the pos- 

 sible, as is done in the following table. 



In this table 81 compounds are indicated, while of these 

 only 18 are actually known to us, or less than one fourth 

 part, while many more may still be possible belonging to the 



formula R S + 4 H, and the various formulae intermediate be- 



tween seven and twelve atoms of water (RS + 7H and RS 



+ 12H). 



The general formulae at the heads of the columns indi- 

 cate the crystalline compounds believed to be possible, the 

 special formulae underneath the crystalline compounds actu- 

 ally known. 



