316 EARTHS PROPER. 



derable care, it will show how near the truth I 

 was able to come. 



Bihydrate. L A quantity of alumina left upon the filter 

 in a temperature not exceeding 60, for a couple 

 of months, was* found to weigh 36 '66 grains. 

 Being strongly ignited in a platinum crucible, 

 the weight was reduced to 17*85 grains. The 

 loss of weight, therefore, was 18*81 grains, 

 which gives the amount of the water in the hy- 

 drate. Now, 17-85 : 18-81 : : 2-25 : 2-42624, 

 which exceeds 2 atoms of water by 0-17624 ; 

 and this excess of water I find it difficult to get 

 rid of in a damp room. If we were to consider 

 the water in this hydrate as amounting to exactly 

 2 atoms, and to calculate the atomic weight of 

 alumina on that supposition, we have 18-81 : 

 , 17-85 : : 2-25 : 2-1352 = atomic weight of alu- 

 mina. But the analysis of alum, given in a pre- 

 ceding part of this section, shows that there is 

 really an excess of water. Besides, I have fre- 

 quently obtained a hydrate of alumina composed 

 exactly of equal parts of alumina and water, 

 showing that the atomic weight of alumina is 

 exactly 2-25, and that this hydrate contains 2 

 atoms water. 



Hydrate. 2. When the bihydrate of alumina is dried in 

 a temperature of about 100, it is reduced to the 

 state of a hydrate at least very nearly, for I 

 have not always been able to obtain exactly the 

 atomic proportions, though the deviation was 



