CELESTINE AND BARYTO-CELESTINE Ol C 1,11 TON. 299 



above show. This difference in composition is best shown by 

 summing up Mr. Stoddart's and my own results as follows : — 



From these numbers it is obvious tliat the celestine of this 

 neighbourhood is an isomorphous mixture, in varying proportions 

 of the two sulphates, and on the supposition that it has been slowly 

 deposited from aqueous solution, we should expect this to be the 

 case, the amount of each sulphate varying with the amount of the 

 sulphate in solution. Thus, if we obtain crystals from a mixture 

 of aqueous solutions of chrome alum, and ordinary alum, the 

 colour of the crystals is darker the more of the coloured chrome 

 alum we take ; whereas, if we obtain crystals from a saturated 

 solution of ordinary alum and a very little chrome alum, their colour 

 is only very slightly purplish, showing that the ordinary (colourless) 

 alum largely preponderates. 



Now, as sulphate of strontium is much more soluble than 

 sulphate of barium, we should expect the former to preponderate 

 in a mixture separating from solution, and that is exactly what we 

 observe in the case of the Bristol celestine. The occurrence of 

 baryto-celestine, that is to say, of a mineral containing such a large 

 proportion of sulphate of barium, is remarkable, considering the 

 fact that so much sulphate of strontium occurs in the neighbour- 

 hood 5 and it can only be accounted for by assuming that the bed 

 from which the sulphates were originally dissolved out, contained 

 a very large proportion of sulphate of barium as compared with 

 sulphate of strontium. One would scarcely have expected specimens 

 to vary so widely in composition where they occur so close to each 



