105 



On Fungoid Growths in Aqueous Solutions of Silica, 

 and their Artificial Fossilization. By William 

 Chandler Roberts, F.C.S., Associate Royal School of 

 Mines, and Henry J. Slack, F.G.S., Sec. R.M.S. 



(Read May 13th, 1868.) 



By kind permission of the Master of the Mint (Professor 

 Graham) the following experiments and observations were 

 made in his laboratory by Mr. Roberts. 



By bringing together 112 grammes of silicate of soda, 67'2 

 grammes of dry hydrochloric acid, and 1 litre of water, and 

 dialysing for four days, a solution of colloid silica, containing 

 4*9 per cent, of silicic anhydride, remains upon the dialyser, 

 the chloride of sodium and excess of hydrochloric acid having 

 diffused away. This solution becomes pectous somewhat 

 rapidly, forming a sold jelly, which may be dried into a 

 lustrous hydrate by two days' exposure to vacuum over sul- 

 phuric acid, or by a more protracted evaporation in air. This 

 solid is remarkably like the opal from Zimapan, but contains 

 21'4 per cent, of water. There does not appear to be any 

 further loss of water by exposure to air ; a specimen dried in 

 vacuo, that had been in air for three years, still retained 

 21'35 per cent, of water. Natural opals contain from 3 to 12 

 per cent, of water. 



In a specimen of hydrate of silica prepared as above, and 

 allowed to consolidate slowly into a compact mineral mass, 

 Mr. Roberts observed arborescent forms, which, when viewed 

 with the naked eye, bore considerable resemblance to certain 

 formations in moss agates. Examination with a microscope 

 showed that the structure had a vegetable appearance ; and 

 on being shown to Mr. Slack, he suggested that it might be an 

 artificial fossil of one of the various forms of mould. In many 

 cases the vegetation appeared in the form of bundles of radiat- 

 ing and branched fibres, such as are shown in Pl.XII, fig. 1. 

 In other instances the fibres were branched, but the radiating 

 character was imperfectly shown. With a magnification of 

 100 a beaded structure was apparent in most of the threads, 

 and this character was strikingly brought out by higher 

 powers. In many cases the terminal cells were surrounded 

 by spaces, as shown in fig. 2, as if the silica had been eaten 

 away, or reduced in bulk by removal of a portion of its water. 

 These spaces did not exert a refractive power materially 

 differing fi-om that of the adjacent parts. 



Mr. Roberts and Mr. Slack determined to investigate the 



