DISPERSOIDOLOGY 267 



IV. In which the precipitate appears in the form of 

 plastic masses (so-called amorphous precipitates, 

 curdy and gelatinous masses) which are composed 

 of particles so small as to appear spherical under 

 the microscope and ultra-microscope. 

 V. The state corresponding to the separation of a pre- 

 cipitate of which the individual grains cannot 

 be differentiated with the aid of our present-day 

 optical instruments. (Homogeneous gels and 

 the first products of their decomposition). 



The influence of concentration upon the form of the 

 precipitate may be explained by the consideration of some 

 examples which, with many others, have been fully worked 

 out by v. Weimarn and his pupils. 



The solubility, L, of barium sulphate corresponds to about 

 N/20,000. In order to obtain, at need, a large value of P, the 

 barium sulphate is produced by the double decomposition, 

 MnS0 4 + Ba(CNS) 2 = BaS0 4 + Mn(CNS) 2 , from the mixing of a 

 litre each of solutions of equal molar strength. By changing 

 the concentration from N/20,000 to N/ 10,000, no separation of 

 barium sulphate is observed during many months. From 

 N/5,000 to N/1,000, a precipitate falls in a short time in the 

 form of macro-crystals. The progress of the reaction in the 

 neighbourhood of the lower limit of concentration is thus 

 described. In the course of the first two weeks an opalescence 

 appeared in the liquid, though no separation of precipitate on 

 the base of the vessel could be observed. After about a month 

 an extraordinary fine deposit of precipitate appeared which 

 might easily have been taken for dust. A month later the 

 mass of the precipitate on the base of the flask was somewhat 

 greater. After six months from the commencement of the 

 experiment the liquid was poured off, the bottom of the flask 

 cut out and examined under the microscope. With a mag- 

 nification of 500, the particles were seen to consist of practically 

 perfect crystals of apparently 2 — 2-5 cm. length and a width of 

 from 0*5-1 cm. 



With a concentration of N/3,000, the opalescence appeared 

 in from 6-8 hours, and the precipitate had mostly separated 

 within 24 hours. A concentration of about N/2,000 showed 

 an opalescence in from 2-3 hours, and gave a precipitate within 



