COMPOSITION OF THE BASIC OXIDES AND HYDRA TES 65 



dessicator for some hours. When dry, cover and weigh as 

 quickly as possible. Replace in the dessicator for an hour, 

 and weigh again. Should the weight 

 have changed, continue the dessica- 

 tion till the weight is constant. Hav- 

 ing found the water combined with 

 the lime, heat the capsule again to a 

 low red heat to decompose the hy- 

 drate and drive off the water; cool 

 as before, and weigh. Does the loss 

 in weight tally with the previous gain Fig. 27. 



in weight ? If not, repeat the pro- 

 cesses until concordant results are obtained. Express the 

 result as percentage composition of slaked lime. 



Most basic hydrates can be decomposed by heat, but not all. Caustic 

 potash and caustic soda of commerce are the hydrates of potash and soda, 

 and it is not possible to decompose these by heating to redness. Since the 

 hydrates of basic oxides are compounds of metals with hydrogen and 

 oxygen, they are frequently spoken of as metallic hydroxides ; thus potas- 

 sium hydroxide or hydroxide of potassium. 



The approximate percentage composition of the more impor- 

 tant hydrates of the metallic oxides is as follows : — 



Basic Hydrate. 

 Hydrate of alumina (aluminium hydroxide) . 

 Hydrated oxide of copper (cupric hydroxide) 

 Hydrated oxide of iron (ferric hydroxide) 

 Slaked lime (calcium hydroxide) 

 Caustic potash (potassium hydroxide) . . 

 Caustic soda (sodium hydroxide) . 



It is possible that if compounds unite, the proportions in 

 which they combine will be the sum of the combining weights 

 of the elements contained, or simple multiples thereof. Assum- 

 ing that this be so, calculate the combining weights of water and 

 the metallic oxides, and find the number of combining weights 

 of each in each hydrate. Ascribe the simplest possible formulae 

 5 



