264 CHEMISTRY OF THE PROTEIDS CHAP. 



will give rise to a greater number of free acid H ions in the same 

 bulk of fluid. 



The precipitation of the colloidal arsenic sulphide solution is there- 

 fore in every respect comparable to the precipitation of any non- 

 colloidal acid by the addition to its saturated solution of a second 

 stronger acid. 



The next questions are difficult, namely, whether the basic radical 

 As 2 3 is appreciably dissociated into As(OH) 3 ^ [As(OH) 2 0] / + H, and 

 whether As 2 S 3 may be considered capable of giving rise to such 

 complex-ions as [As 2 2 S]" and [As 2 OS 2 ]"" or to more complex arsenic- 

 sulph-hydroxyl-ions. As in an arsenic sulphide solution the colloidal 

 particles are repelled more from the negative than from the positive 

 pole, according to Linder and Picton x they must carry negative loads. 

 The author again supposes that the colloidal particles are hydroxyl 

 compounds, comparable to colloidal gold, mentioned above on p. 260, 

 and that each particle is composed of a sufficient number of arsenic- 

 sulph-hydroxyl radicals to have assumed dimensions larger than one- 

 half a mean wave-length (see p. 258). 



Provided the author's hypothesis is correct, " we may consider the 

 solution of an ordinary dissociated electrolyte as representing a double 

 ' colloidal ' solution, and the formation of insoluble hydrates during 

 hydrolysis as the precipitation of one of the two colloidal solutions." 2 

 "If the aggregated particles in a colloidal solution are completely 

 broken up into the composing units by acquiring definite charges, then 

 the colloidal solution loses its colloidal character and becomes a so]ution 

 of a completely dissociated electrolyte, as happens, for example, in the 

 case of silicic acid, which in the presence of HC1 gives negative results 

 with Tyndall's experiment, but which in the absence of hydrochloric 

 acid soon undergoes a change by means of which an aggregation into 

 particles is brought about, and now Tyndall's experiment gives positive 

 results." 



The change from a so-called 'electrolytic' into a so-called 

 'colloidal' solution the author explained as follows : "If to a solu- 

 tion containing a definite number of electro-positive (colloid + H)-ions 

 there is added an alkali containing the same number of electro-negative 

 hydroxyl-ions, then the H of the colloid and the OH' of the alkali 

 unite to form electrically neutral water, and the colloid, having lost its 

 electrical charge, is precipitated ; if, however, not a sufficient number 

 of OH'-ions were added to bind all the hydrogen-atoms, then the 

 colloid-aggregates re-arrange themselves into larger aggregates, which 



1 S. E. Linder and H. Picton, Chem. Journ. 61. 137 (1892). 

 2 Mann, Physiological Histology, 1902, p. 46. 



