268 SCIENCE PROGRESS 



10-12 hours; while changing the concentration from N/1,000 

 to N/500 reduced the period before the appearance of an 

 opalescence from 3-5 minutes to one second, and the time 

 of separation of the precipitate from within 2-3 hours to \- 1 

 hour. The precipitate obtained with a concentration of 

 N/1,000 appeared, under a magnification of 15,000, to consist 

 of a few parallelepipedal barium sulphate crystals (of which 

 the largest were about 5x3 mm.) and of aggregates of the 

 smallest particles which appeared spherical under the highest 

 magnification. After the expiration of six months, the 

 smaller particles had disappeared, owing to their greater 

 solubility, and the precipitate was seen to consist almost 

 entirely of right-angled crystals, which under a magnifica- 

 tion of 500 appeared to be 2-5 mm. long and 1-3 mm. 

 wide. 



The third stage commences with a concentration of about 

 N/600. As the strength of the reacting solutions increases, 

 the number of perfect crystals quickly falls, until, at a con- 

 centration of N/100, the precipitate consists of compact star- 

 like skeletons and fine needles. At N/10 the star-like growth 

 figures disappear, and the precipitate (from reacting drops) 

 consists entirely of needles of a maximum length of 3 mm. 

 under a magnification of 1,500. When o'75N/i is reached the 

 precipitate has become so fine-grained and mutually entangled 

 that its needle form can hardly be discerned under the higher 

 magnification. This is the commencement of the fourth 

 stage. The formation of so-called amorphous precipitates 

 begins in the neighbourhood of N/i. The precipitate appears 

 to consist of flocks of different sizes as well as a fine-grained 

 precipitate. Under a magnification of 1,500, the flocks are 

 seen to be composed of spherical grains so small as to appear 

 as points. 



The fifth stage begins at about 3N/1 with the appearance of 

 translucent gelatinous flocks. At 4N/1, at a temperature of 

 ioo°, a partially transparent coarse-celled gel is formed which 

 quickly falls in flocks. At 5N/1 the gel is formed at 20 , as 

 well as ioo°, and is more permanent. When the concentration 

 of 7N/1 is reached the single cells of the gel remain transparent 

 for many hours. But after about 24 hours the gel falls into a 

 fine flocky white precipitate, which is very plastic. The results 

 are summarised as below : 



