144 



carbonate of soda be melted, and if while in this state powdered Hint 

 be added to it, a slight effervescence was caused : the silica united 

 with the soda, discharged the carbonic acid, and formed a silicate of 

 soda. 



What now was the bearing of these facts on the silica which was 

 found dissolved in the water of rivers, springs, and lakes. We are not 

 surprised to find that the water of Loch Katrine, though its tributaries 

 might have run for miles over rocks containing from 60 to 70 per cent, 

 of silica, contained not one part in a million (by weight) of this sub- 

 stance, while, on the other hand, that from Trafalgar-square, coming 

 from the chalk and holding in solution alkaline carbonates and 

 chlorides, contained 13. Bearing in mind that a hot alkaline solution 

 was capable of dissolving large quantities of silica, we saw how it was 

 that the mineral waters of Vichy, highly charged with carbonate of 

 soda, contained 122 parts in a million, that the water of Plombieres 

 after passing over Roman cement deposited in their efflux opal. 

 Having learnt from experience that water charged with carbonic acid 

 could dissolve silicates, we were not surprised to find that Karls- 

 brunnen in Silesia, which contained 406 cubit centimetres in a litre of 

 carbonic acid, should hold dissolved 72 parts in a million of silica. 

 Knowing that pressure and heat were alike favourable to the solution 

 of this substance, we learned the cause of the splendid stalagmites of 

 opal-silica round the Geysers of Iceland, and the terraces of the same 

 substance, 300 feet high, round those of the Colorado district. Before 

 them was a fine specimen of this hydrated silica from the great Ice- 

 landic Geyser. 



Although as a rule an analysis of sea-water discovered to them but 

 a very small proportion of silica among its constituents — seldom 

 amounting to more than a trace, though in one case (from the English 

 Channel) it rose to 16 parts in a million — yet the existence of diatoms 

 in enormous quantities in some parts of the ocean was an important 

 fact with regard to its presence. It had been found in even large pro- 

 portions in the asihes of sea-weeds. Those of the common Delesseria 

 Sanguinca yielding no less than 12 per cent, of this substance. The 

 fact, however, of siUceous sponges, such as the beautiful Euplectella, 

 composed wholly of pure silica, growing at the bottom of the sea, 

 spoke louder than any analysis, as to its being, at any rate in some 

 places, more perhaps than in others, an important constituent of sea- 



