70 



THE GEOLOGIST. 



limpid, and not in the least viscous. It may be boiled in a flask for a con- 

 siderable time, and concentrated considerably without change. When 

 heated in an open vessel, a ring of insoluble silica is apt to form around 

 the margin of the liquid, and this may soon. cause the whole to gelatinize. 

 The solution is, as I said, durable in proportion to its purity. It is not 

 easily preserved beyond a few days, unless considerably diluted. It 

 becomes opalescent after a short time, and then the jelly separates ; and 

 once separated it cannot be redissolved in water. When the jelly is 

 formed suddenly it is always more or less opalescent. Formed slowly, it 

 is a jelly, perfectly colourless and limpid, like rock-crystal. If touched 

 slightly it gives rise to a vibratory tremor. It contracts, after a few 

 days, even in a close vessel, and then pure water separates from it. It is 

 a very curious fact that coagulation, or the separation of silica in the jelly- 

 like state, is effected in the course of a few minutes by a solution con- 

 taining one ten-thousandth part of any alkaline or earthy carbonate, but 

 not by caustic ammonia or neutral or acid salts, nor by sulphuric, nitric, or 

 acetic acid. Coagulation occurs in a short time after passing carbonic acid 

 through the solution. A little carbonate of soda to it will make it so solid 

 that it may be inverted without spilling. When it is suddenly made so, it 

 is always opalescent, and not transparent. Dried by the air-pump in 

 vacuo, at the ordinary temperature, it forms a beautiful, transparent, 

 glassy mass of great lustre, no longer soluble in water, and which reminds 

 one greatly of that beautiful variety of opal termed "hyalite." 



Ordinary silicate of soda is not at all what is termed " colloidal ; " 

 if silicate of soda were put into the hoop-vessel, and left there floating 

 upon the water, it would pass through, to a certain extent, to the water, 

 but there would be no separation of its constituents. When hydrochloric 

 acid is added and the constituents eliminated, then this action is set up. 

 This soluble form of silica unites with various organic matters, as, for 

 example, with common gelatine, or with skin. In fact you may tan by 

 means of silica, and produce leather containing as much as 70 per cent, of 

 silica. 



We will now inquire whether there is any reason to suppose that a 

 similar process may play any part in the operations of nature. The con- 

 dition required is a soluble silicate dissolved in water, and the decomposi- 

 tion of that silicate by some agent, such as hydrochloric acid. Does 

 nature present us with any apparatus which can take the place of this 

 so-called dialyser? All that we want is the porous bed of some rock like 

 sandstone, in some convenient position, and that sandstone will act exactly 

 as the dialysing apparatus. 



In the separation of the silica every bubble of the fluoride of silicon 

 as it passes up through the water becomes immediately decomposed, and 

 a portion of the gas escapes, not beiug thoroughly in contact with the 

 water everywhere, and produces a slight smoke. We have reason to 

 believe that this solution may play an important part in the phenomena of 

 nature ; for there is no difficulty in explaining how such a solution may be 

 obtained as is requisite to exhibit the phenomena of dialysis ; and very 

 probably in nature we may find conditions exactly suitable for dialysis. 

 If this be the case, we shall be at no loss to understand how in many 

 instances silicification has occurred. We know that it has occurred, and 

 to an enormous extent, in nature. The mineral termed " opal " is nothing 

 more than amorphous silica containing a little water. The proportion of 

 water is not definite ; it is variable, the extremes being somewhere about 

 3 per cent, and 13 per cent of water. Sometimes this opal exhibits most 

 beautiful colours, and then it acquires the name of " precious opal." 



