Quantities of Foreign Matter on Crystallization. 395 



substances, and may therefore allow their crystallizing 

 tendency to gain ground and develope itself, particularly 

 where the crystal once formed is completely insoluble, as 

 with quartz." Prof. Graham found that a liquid silicic acid 

 of 10 or 12 per cent, coagulates or pectizes spontaneously in 

 three hours at the ordinary temperature, and immediately 

 when heated. A liquid of 5 per cent, may be preserved for 

 five or six days ; a liquid of 2 per cent, for two or three 

 months ; and a liquid of 1 per cent, had not pectized after two 

 years. Dilute solutions of 0*1 per cent, or less are probably 

 unalterable by time, and hence the possibility of soluble silicic 

 acid existing in nature. It was not found that any solution, 

 weak or strong, of silicic acid in water showed any disposition 

 to deposit crystals ; but it always appeared on drying as a 

 colloidal glassy hyalite. 



In connexion with this subject, we may refer to Professor 

 Bischoff in his Chemical and Physical Geology (Cavendish 

 Society). He states that crystals of gypsum, thirteen lines in 

 diameter, have been formed within four or six years on the 

 thorn walls of salt-works, from drops of brine containing 

 sulphate of lime. Gypsum dissolves in 460 parts of water : 

 the water of some springs contains 1-10, 000th of silica, so 

 that this solution is about twenty-two times more dilute than 

 the former. Supposing a crystal of quartz, one inch in 

 diameter, to form under circumstances similar to those that 

 produce a gypsum crystal of the same dimensions, rather 

 more than a century would be required ; the water perco- 

 lating through the thorn faggots, and depositing nothing but 

 silica, would in that time form a crystal of quartz consisting 

 of 4,7b'6,652 concentric layers of silica, each about one 

 millionth of a line in thickness ; but as the water of springs 

 usually contains much less than l-10,000th of silica, the 

 time would probably have to be doubled, or even further 

 increased ; but such periods are by no means remarkable in 

 geological phenomena. 



In the early proceedings of the Geological Society, Mr. 

 Bowerbank described some moss agates from Oberstein, con- 

 taining remains of sponges. In 1848 Mr. Hamilton stated 

 that the real agates of Oberstein are found in igneous rocks, 

 so that they could not contain the remains of organic bodies. 

 Whereupon Mr. Bowerbank produced sections of moss-agates, 

 and placed them under a powerful microscope, when Mr. 

 Hamilton admitted that they certainly did contain remains of 

 spongeous structure*, but how far they are really found at 



* The chalk flints best known to me are those found near Salisbury, 

 and they commonly have a spono C ous nucleus. 



