viii THE GENERAL PHYSICAL PROPERTIES OF ALBUMINS 263 



When arsenic sulphide, As 2 S 3 , is in solution, it partly dissociates 

 into [As 2 3 ] and 3[H 2 S]"'. The latter then dissociates still further 

 into H + HS', and thereby renders the whole solution acid. Compar- 

 ing various salts which undergo hydrolysis and in doing so form acid 

 solutions, we find, on the one hand, solutions which are almost colloidal, 

 namely, HgCl 2 = HgOH + 2HC1 ; next, colloidal compounds such as 

 arsenic trisulphide, As 2 S 3 .= As 2 3 + 3H 2 S; and finally, salts which are 

 thrown down as insoluble basic compounds if their saturated solutions 

 be diluted, as happens in the case of the nitrate of bismuth [Bi(NO 3 ) + 

 5H 2 0] = Bi(OH) 2 N0 3 + 2HN0 3 . Whether a salt undergoing hydrolysis 

 does or does not form insoluble basic salts depends on the strength of 

 the base, on the power of dissociation possessed by the acid radical to 

 which the base is joined, and as to whether the acid is an oxy-acid, for 

 while, as just stated, mercuric chloride forms clear solutions, mercuric 

 sulphate and mercuric nitrate form insoluble basic salts if they are 

 diluted sufficiently. 



In the case of arsenic sulphide, As 2 S 3 , we are dealing with both a 

 feeble kation [As] and a feeble anion [S]', but the S of the anion is 

 stronger than is the As of the kation, in consequence of which the 

 compound As 2 S 3 , on conling into contact with water hydrolyses, which 

 means the stronger anionic S' links on to two kations, H, of the water 

 to form SH 2 or sulphuretted hydrogen, while the kationic As links on 

 to three anions, OH', of the water to form As(OH) 3 . Both SH 2 and 

 As(OH) 3 have the tendency to dissociate electrolytically in such a way 

 as to liberate free, acid, hydrogen kations, but the SH 2 , possessing 

 greater electrolytic power, only allows of a very partial electrolytic 

 dissociation of the As(OH) 3 radical, the dissociation of the As(OH) 3 

 being just sufficient to keep it in solution in the form of particles of 

 such size as to be capable of polarising light, i.e. to produce a colloidal 

 solution. Another way of looking at this question is to say that the 

 water + H 2 S forms one phase, and that the As(OH) 3 forms a second 

 phase, and that As(OH) 3 is soluble in the H 2 S + water phase because a 

 certain amount of difference of potential can be set up at the inter- 

 faces, in consequence of which the two phases tend to mix with one 

 another, i.e. tend to pass into solution. (See the author's definition of 

 ; solution ' on p. 254.) 



The colloidal nature of arsenic sulphide depends thus on the 

 partial solution of the As(OH) 3 complex. As arsenic sulphide in pure 

 water tends to split up into the two acid solutions [SH]' + H and 

 [As(OH) 2 0] / + H, it will be readily understood that the formation of 

 these two acids will be prevented by the presence of any stronger 

 acid, i.e. of any other acid which, if present in equivalent amount, 



