8 Mr. J. J. Coleman on Liquid Diffusion. 



decisive also on the matter of solubility not influencing dif- 

 fusibility, since the sulphate of zinc is well known to be much 

 more soluble in water than the sulphate of magnesia. 



Two substances from MendelejeiFs 6th group, viz. chromic 

 acid and tungstic acid, were selected for diffusion ; the 

 molecular volumes of which are identical, and the molecular 

 weights of which are as 100*5 to 232. They were diffused as 

 normal salts of soda for thirty days, at a temperature of 

 12°*5 C, with this result (in which it will be again observed 

 the one possessing the least atomic weight is most diffusive): — 



25 per cent, of chromate of soda rose 75 millim. 

 17 „ tungstate „ 75 „ 



Molybdate of soda was diffused also at the same time, but 

 as the atomic volume of molybdic acid is not identical but 

 larger than that of either of the other two, there was a 

 greater diffusibility, viz. : — 



28 per cent, molybdate of soda rose 75 millim. 



In the case of Mendelejeff's 7th group we have chlorides, 

 bromides, and iodides of the following molecular weights and 

 volumes : — 



Sodic chloride . 

 „ bromide . 

 „ iodide 



Potassic chloride 

 „ bromide 

 ,, iodide . 



Mol. wt. Mol. vol. 



58*5 26-0 



103-0 33-0 



150*0 43-5 



74-6 37-0 



119-1 44-0 



166-1 54-0 



In all these cases it will be noticed that there is an increase 

 of molecular volume simultaneous with an increase of 

 molecular weight, so that it is possible retardation of diffusi- 

 bility, owing to an increased molecular weight, may be 

 counterbalanced by an increased energy, indicated by the 

 larger molecular volume ; and may account for the singular 

 fact discovered by Graham that these substances are practi- 

 cally equally diffusive, though further experiment may show 

 some little variations. Similar remarks may be made in 

 regard to the chlorides and nitrates of calcium, strontium, and 

 barium, whicn Grraham also showed to be practically equally 

 diffusive. Mol. wt. Mol. vol. 



Calcic chloride .... Ill 44 



Baric chloride 208 54 



Calcic oxide 56 18 



Strontic oxide 103*6 22 



Baric oxide 1530 28 



