62 ESSAYS BIOGRAPHICAL AND CHEMICAL 



on the phosphoric acids, Graham next advanced the sug- 

 gestion that certain salts may be substituted, molecule for 

 molecule, for water of crystallisation. Thus, sulphate of 

 zinc ordinarily crystallises with seven molecules of water, 

 forming the heptahydrate, ZnS0 4 .7H 2 0. It is possible to 

 replace one of these molecules of water with a mole- 

 cule of potassium sulphate, obtaining the double salt, 

 ZnS0 4 .K 2 S0 4 .6H 2 0. It appeared, too, with this and 

 similar salts, that six molecules of water may be expelled 

 at a lower temperature than the seventh, which may be 

 supposed to be the one which is replaced by the potassium 

 sulphate in the double salt. 



Experiments were also made on the heat evolved on 

 neutralising bases with acids, and on the solution of salts 

 in water. Such experiments on salt were carried on until 

 1843. 



But Graham had all the while another set of re- 

 searches in progress, in which he attempted to arrive 

 at some definite knowledge regarding the constitution of 

 matter. Recognising that the gaseous state represents 

 matter in a simpler condition than that of liquid or solid, 

 his experiments were largely directed towards elucidating 

 the properties of gases. These experiments were started 

 in 1836. From an observation of Doebereiner's, that in a 

 cracked cylinder, containing hydrogen, and standing over 

 water, more gas escaped than entered, so that the level of 

 the water rose in the cylinder, Graham was led to make 

 his experiments on the diffusion of gases, and also on the 

 rate of the escape of gases through narrow openings. 

 Both sets of experiments led to the same law, viz. that 

 the rates of escape are inversely proportional to the square 

 roots of the densities of the gases. Under equal physical 

 conditions, hydrogen moves four times as quickly as 

 oxygen, which is sixteen times as heavy as the former. 

 And since the densities are proportional to the weights of 



