148 



Dr. T. Ewan. On the 



the ferric chloride solution, and one would imagine that if all the 

 iron exists as hydroxide in a solution containing an equivalent quan- 

 tity of chlorine this should be certainly the case in one containing 

 only some 12 per cent, of that quantity of chlorine. An explanation 

 is, I think, suggested by an observation made by Picton.* He 

 noticed that a solution of antimony hydrosulphide, obtained by 

 pouring a solution of tartar emetic into one of sulphuretted hydrogen, 

 possessed only a faint fluorescence, and, under the microscope, no 

 particles were visible in it. On dialysing the solution the fluorescence 

 increased, and gradually minute particles became visible under the 

 microscope; finally the solution coagulated. Picton and Linder 

 ( k J. Chem. Soc.,' 1892, 154) found also that silicic acid behaves in 

 quite a similar way. In a colloid solution of silicic acid containing 

 hydrochloric acid the particles were too small to reflect light, while 

 after dialysis they were large enough to do so. Krecke (loc. cit.) 

 also found that, on dialysing a concentrated ferric chloride solution, 

 it finally coagulated. It would appear as though the dialysis always 

 favoured the formation of larger molecular aggregates. A possible 

 reason for this, in the case of ferric hydroxide, at any rate, is not 

 difficult to see. In the solutions containing hydrochloric acid the 

 hydroxide must be regarded as constantly being acted upon by the 

 acid with formation of ferric chloride, which is at once decomposed 

 again. Consequently the hydroxide never has the opportunity of 

 forming large molecular groups. 



Solutions containing excess of Hydrochloric Acid. 



The addition of a small quantity of hydrochloric acid to a solution 

 containing the hydroxide diminishes the quantity of the latter, and 

 therewith the intensity of the brown colour of the solution, to a 

 marked extent. I found, for example, by filtration through the 

 Chamberland Pasteur candle, that the addition of one-sixth of an 

 equivalent of HC1 to a dilute solution of ferric chloride reduced the 

 quantity of Fe(OH) 3 present by nearly a half. The following are 

 the numbers : 



' Churn. Soc. Journ.,' 1892, p. 142. 



