vin THE GENERAL PHYSICAL PROPERTIES OF ALBUMINS 261 



solution which is more rapid in an open vessel than if the solution be 

 preserved in a closed bottle. 1 



On the assumption that As 2 S 3 really dissociates into As 2 3 ooc 

 and 3H 2 S', and these in their turn into As(OH) 2 / + H and 

 H + HS', the complexity of the colloidal solution must be very 

 great, and therefore its equilibrium must be very unstable. By 

 the addition of strong acids the dissociation of As(OH) 2 O / + H 

 and that of H + HS' will be prevented, and therefore As 2 3 + 3H 2 S 

 are formed first, and ultimately the completely undissociated As 2 S 3 . 

 By the addition of alkalies, on the other hand, either ortho-arsenites 

 (e.g. Ca 3 (As0 3 ) 2 ) or meta-arsenites (e.g. Ca 2 As 2 5 or KAs0 2 ) result, 

 which keep the arsenic in solution because both potassium and 

 calcium possess much greater electro-affinities 2 than does arsenic. 

 The author has thought it necessary to enter into this questrorr 

 because other writers always speak of spontaneous changes taking 

 place in colloidal solutions without having taken proper precau- 

 tions for excluding the possibility of chemical action. The author 

 has found that using paraffined vessels to exclude the chemical 

 action of glass, and keeping bottles well filled with colloidal solutions 

 to diminish the air space, and using paraffin stoppers, that colloidal 

 solutions keep with very little alteration after a preliminary change, 

 due to mechanical disturbance, has taken place (see p. 274). 



When a colloidal solution becomes semi-soluble, or, in other words, 

 more colloidal, when, for example, Picton's 8-arsenic sulphide solution 

 is changed into y-, then /3-, and ultimately into the a-variety, the 

 following changes occur : In a freshly prepared non-colloidal arsenic 

 sulphide solution, As 2 S 3 is dissociated into [As 2 3 ] 000 and 3[H 2 S]', 

 and this dissociation is also met with in colloidal solutions, as has 

 been shown by Freundlich. 3 When the colloidal solution becomes less 

 colloidal there occurs, according to the theory of the author, a diminu- 

 tion in the amount of electrical dissociation ; and this diminution is 

 accompanied by a gradual increase in the size of colloidal particles. 

 Picton and Linder 4 were the first to notice that the size of the colloidal 

 particles increases when the point of coagulation is neared, and that 

 there is a reaction other than mechanical between solvent and solid, 

 even in these cases of colloidal solution. 



1 Freundlich has omitted to analyse the remaining solution, and the possibility of the 

 arsenic solution which was kept in an open vessel having absorbed C0 2 from the air, and 

 having thereby become precipitated, must also be taken into account, for by acids 

 arsenic sulphide is precipitated completely. 



2 Mann, Physiological Histology, 1902, p. 14. 



8 Herbert Freundlich, Zeit. f. physik. Chetn. 44. 129 (1903). 

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



