262 CHEMISTRY OF THE PROTEIDS CHAP. 



A non-colloidal arsenic sulphide solution, i.e. one in which the 

 component particles are smaller than half a wave-length (see p. 258), is 

 only capable of existence in the presence of free hydroxyl-ions, and if 

 these be removed there occurs at once a change which expresses itself 

 optically by light being polarised, and this depends, as has already been 

 shown, on particles being formed in the solution which are larger than 

 half a mean wave-length. 



The removal of the hydroxyl-ions from the arsenic sulphide 

 solution may produce one of two changes. If it required one hydroxyl- 

 ion, OH", for each arsenic sulphide molecule, As 2 S 3 (or an arsenite 

 molecule), to remain in solution, then with the removal of each 

 hydroxyl-ion one arsenic sulphide molecule will cease to exist as an 

 ion, and form a sediment as quickly as the viscosity of the water 

 allows it to do so, provided it cannot form a binary system with water 

 (see under solution, p. 255). If all the hydrogen-ions, each in custody 

 of one arsenic sulphide molecule, were neutralised, then the whole of 

 the sulphide would settle as an insoluble mass. The other possibility, 

 and the one which the author believes to be actually at work, is that 

 the As 2 S 3 molecule itself undergoes a dissociation as soon as the 

 hydroxyl radicals are removed, and that in this way are formed 

 [As 2 O 3 ] 000 + SfHsjS]"'. 1 If all the As 2 S 3 molecules underwent this 

 hydrolytic dissociation the arsenic sulphide solution would still be non- 

 colloidal, as it is improbable that molecules having such low molecular 

 weights as As 4 S 6 = 492, 2 or As 2 3 =198, or H 2 S = 34, are capable of 

 polarising light. We must therefore assume that the particles in a 

 colloidal solution of arsenic sulphide are composed of at least several 

 molecules, and that these aggregates are kept in solution by an electrical 

 charge. It is immaterial whether we assume that the colloidal particles 

 are formed by a mechanical conglutination (see p. 274) of molecules 

 or by such chemical action as anhydride -formation, as seen, for 

 example, in the case of dextrose when it is converted into colloidal 

 starch. 



The essential point is that each colloidal aggregate must carry a 

 definite electrical load, because it can move with or against an elec- 

 trical stream, and that there must be other ions in the solvent, possess- 

 ing an electrical sign the opposite from that carried by the colloid. 

 As such a colloidal solution is precipitated by certain agencies, we 

 must next inquire as to how a colloidal solution consisting of smaller 

 particles is transformed into one containing larger units. 



1 The dissociation may be such that the positive load is carried originally by the 

 H 2 S, and the negative load by the As 2 3 or [As 2 3 ]'" + 3[H 2 S] 000 . 



3 The correct formula for arsenic sulphide is As 4 S 6 , and is not As 2 S 3 . 



