Mr. J. J. Coleman on Liquid Diffusion, 



Height diffused? 

 in millimetres. 



22 

 44 



66 



Rubidium 



Potassic 



Sodic 



chloride, 



chloride, 



chloride, 



per cent. 



per cent. 



per cent. 



. 68 



64 



60 



. 39 



39 



31 



. 19 



19 



10 



. 7 



7 



3 



. 3-5 



3-5 





110 



It will be seen that rubidium chloride is quite as diffusive 

 as potassic chloride, a salt which has hitherto been the most 

 diffusive salt examined. This I anticipated from the large 

 atomic volume of the metal, which is very much greater than 

 even that of potassium ; but, on the other hand, its atomic 

 weight is greater. 



If a large molecular volume indicates a tendency to rapidity 

 of diffusion, it may be suspected that a large atomic or mole- 

 cular weight has a tendency to retard it ; this, however, is 

 not so easy to prove, from the extreme difficulty of getting 

 groups of soluble compounds, the molecular volumes of which 

 are identical, but the molecular weights of which differ. 

 Moreover, it appears necessary to look for such groups in the 

 family groups of MendelejefFs vertical series, or the isomor- 

 phous groups of the older chemists. The sulphates of zinc 

 and magnesia are strictly isomorphous, and possess an identical 

 molecular volume, viz. 44 when anhydrous, their molecular 

 weights being as 120 to 161. 



Graham not only in this case, but in several other cases of 

 isomorphous bodies, strove hard to prove that the rate of dif- 

 fusion was identical, returning again and again to the subject, 

 on the last occasion making seventy-two experiments with 

 magnesic and zincic sulphates ; which he sums up by stating 

 that the approach to equality becomes close in the 4-per-cent. 

 and larger portions of salt, but differed as much as 8*75 per 

 cent, in favour of the sulphate of magnesia in the 1-per-cent. 

 solutions. 



I have recently diffused these substances for the long period 

 of 50 days, and at a temperature of 15° C, in the apparatus 

 described in an earlier part of this paper as an improvement 

 upon Graham's. The results are that — 



9 per cent, of the magnesic sulphate rose 100 millim. 

 7 „ „ zincic „ „ 100 „ 



It therefore appears that these substances are not equi- 

 diffusive, and that the one possessing the least molecular 

 weight is the most diffusive. These experiments are very 



