46 HILLEBRAND, MERWIN AND WRIGHT. [April 25, 



Change of Color Due to Heating. — Metahewettite, dark maroon 

 in color at first, becomes progressively darker red till the last mole- 

 cule of water begins to escape, then the color becomes gradually 

 lighter and finally yellow-brown. Concurrent with the last change 

 is a breaking up of each crystal into a crystalline aggregate which 

 retains the form of the original crystal. 



Hewettite also darkens when water is lost. At the start, with 

 nine molecules of water, it is mahogany red. The hydrate with 

 three molecules is between carob-brown and liver-brown, the com- 

 pound with one molecule chestnut-brown, and the anhydrous pow- 

 der medal-bronze. No breaking down of structure was observed 

 till the last molecule of water was lost. 



Changes in Weight and Color after Dehydration. — The powders 

 of both minerals were exposed after complete dehydration to sulphuric 

 acid of sp. g. 1. 10. Metahewettite from Utah, after dehydration by 

 heating in air, regained its original weight and color in a few days, 

 but after treatment under greatly reduced pressure the color was not 

 restored, even after moistening, although the observed loss of weight 

 had been the same in both cases. Hewettite, on the other hand, 

 after beating to only 270° and while it still retained one molecule 

 of water, neither regained its original color nor quite its original 

 weight when placed in moist air. In one experiment, after heating 

 to 350° and losing 20.75 P^^ cent, in weight, it regained 19 per 

 cent., but on again fractionating over the acids first used the rate of 

 loss and the amount lost at each step were markedly different from 

 those noted in the first fractionation. 



Probable Formula. 



Manifestly the empirical formula indicated for metahewettite 

 nearly saturated with water at 35° is CaO, 3V2O5, 9H2O. What it 

 may have been when the minerals were originally deposited can not 

 at present be said, although from the curve for water content under 

 varying hygrometric conditions at summer temperatures (Fig. 2), 

 which shows only very slight changes when the humidity is high, 

 it might be argued that the limit of hydration has been reached 

 with nine molecules of water. The effect of low temperatures is. 



