DISPERSOIDOLOGY 269 



Concentration of P Form of precipitate, 



reacting substances. L 



N/2o,ooo-N/7,ooo 0-2 I. Macro-crystals appear after many years. 



N/7,ooo-N/6oo 2-32 II. The precipitate appears as complete 



crystals within a short time. 



N/600-075N/1 32-14,000 III. The precipitate occurs in the form of 



growth figures (skeletons and needles). 



075N/1-3N/1 14,000-60,000 IV. The precipitate appears in different forms 



—ball-like, curdy, flocky, and gelatinous. 

 These are so-called amorphous pre- 

 cipitates, which appear to consist of 

 spherical grains under the microscope. 

 3N/1-7N/1 60,000-140,000 V. The precipitate separates in the gelatinous 



form, and, during the first part of its 

 existence, cannot be differentiated under 

 the microscope. 



By the inoculation of supersaturated solutions, v. Weimarn 

 demonstrated that even the amorphous precipitates are essenti- 

 ally crystalline in nature. 



The most permanent sol, or colloid solution, of barium 

 sulphate results at a concentration of N/3,000. But its 

 stability is not great, and most of the precipitate is deposited 

 within 24 hours. Ultra-microscopic examination showed that 

 the number of sub-microns was small compared to those in 

 a typical sol of the same concentration. Consequently their 

 size increased rapidly, owing to the greater amount of nutrient 

 material, with a corresponding loss of Brownian movement and 

 eventual precipitation. To prepare more permanent sols of 

 barium sulphate, it was evidently necessary to increase the 

 number of crystallisation centres in relation to the nutrient 

 material, and so reduce their rate of growth. This was done by 

 further reducing the solubility by mixing solutions containing 

 50 per cent, of alcohol. The reaction between CoS0 4 .7H 2 

 and Ba(CNS) 2 . 3H 2 was employed, with concentrations vary- 

 ing from N/10 to N/500. The resulting sols were extra- 

 ordinarily permanent. One containing 0*25 per cent. BaS0 4 

 had not deposited any precipitate after seven years. By 

 mixing solutions containing progressively more alcohol from 

 1 to 50 per cent, it was shown that the ultra-microscopic particles 

 must consist of the finest crystalline needles of BaS0 4 . It 

 appears, therefore, that from the first stage to the last the 

 disperse phase is crystalline. 



The number of crystallisation centres may also be increased 

 18 



