142 Prof. J. P. Oooke and Mr. F. A. Gooch on two 



only about 3 per cent., showing that it holds its water much 

 more firmly than the first. In order to institute a just compa- 

 rison between the different vermiculites, it is obviously important 

 to seek for each variety the point at which the mineral assumes 

 a constant condition and maintains it through a considerable 

 variation of temperature. Save only some practical convenience, 

 there is no peculiar virtue in 100°, as the temperature at which 

 a mineral should be dried for analysis. As in the case of crys- 

 talline salts, we should expect to find for each hydrous mineral 

 a certain point or points of temperature at which it loses the 

 whole or a part of its water of crystallization, and certain limits 

 between which it maintains a constant composition. Moreover 

 we should expect that these temperatures would be the more 

 definite in proportion as what we may call the hygroscopic power 

 is the more marked — that while in some cases the mineral 

 would lose its water at a nearly constant temperature, and the 

 intervals of definite hydration would be well marked, in others 

 the loss would extend over a considerable range of temperature, 

 and it would be more difficult to secure the states of definite 

 composition. That such differences as these are conspicuous 

 among the vermiculites the Tables given above abundantly show ; 

 but in addition to this evidence, the difference in the behaviour 

 of the several varieties, when heated, impressed upon us more 

 strongly the principle we have stated than the figures would in- 

 dicate. Nevertheless, as the following Table shows, we have 

 been able to bring all the vermiculites to essentially the same 

 condition. The Table is merely a summing-up of the results 

 already given, and exhibits a comparison of the atomic ratios* 

 of the several varieties. 



IV. VI. II. II. VI. II. 



Si. R+R, H. R. R. 



Hallite, at about 300° C 



2-49 



: 2-47 



. 1-25 



'77 



: 1-63 









2 



: 2 



1 



1 



: 2 



Pel ham vermiculite 



, at 100° . . 



275 



: 2-46 



T23 



ro5 



: 1*43 









2 



: 2 



! 1 



2 



: 3 



Lerni 



a 



» 



2-54 



: 2*50 



1-30 



1-01 



: 1'49 









2 



: 2 



: 1 



2 



: 3 



Culsagee 



a 



»> 



2-50 



: 2-66 



1-23 



1-37 



: 1-27 









2 



: 2 



1 



1 



: 1 



Millbury 



i> 



(Crossley) 



2-38 



: 274 



1-14 



1-37 



: 1-37 









2 



: 2 



1 



1 



: 1 



Jefferisite 



a 



at 100° . . 



2-56 



: 2-53 



1-17 



1-40 



: 1-13 



2 : 2 : 1 3 : 2 



* The atomic ratio is the same ratio which in most works on mineralogy 

 is still called the oxygen ratio. The numbers given in this paper are found 

 by dividing the percentage of each oxide by a divisor which is the quotient 

 of the molecular weight of the oxide divided by the quantivalence of the 

 radical. See the writer's ■ Chemical Philosophy,' p. 450, or paper " On 

 Atomic Ratios," Amer. Journ. Sci. vol. xlvii. May 1869. 



