DECOMPOSED BY THE CARBONIC ACID Of THE AIR. 207 



Silica combines with these alkalies* in various proportions. If it be 

 melted with much potash, the glass obtained will be readily soluble in 

 water* if with little, the silicate which is formed will resist the action 

 of water for any length of time. Window and plate-glass contain 

 much silicate of potash or soda. A large quantity of alkali renders 

 these varieties of glass more fusible and more easily worked, but at the 

 same time makes them more susceptible of corrosion or tarnish by the 

 action of the air. 



The insoluble silicates of potash and soda exist also in many mineral 

 substances. In the felspar and mica, of which granite in a great mea- 

 sure consists, they are present in considerable quantity. The former 

 (felspar) contains one-third of its weight of an insoluble silicate of potash, 

 consisting of nearly equal weights of potash and silica. In the variety 

 called albite or cleavelandite, silicate of soda alone is found, while in 

 some other varieties a mixture of both silicates is present. In mica from 

 12 to 20 per cent, of the same silicate of potash occurs, but soda can 

 rarely be detected in this mineral. The trap-rocks also (whin, basalt, 

 green-stone), so abundant in many parts of our island, consist almost 

 entirely of silicates. Among these, however, the siUcates of potash and 

 Boda rarely exceed 5 or 6 per cent, of the whole weight of the rock, and 

 are often entirely absent. 



Thes^ insoluble silicates also exist in the stems and leaves of nearly 

 all plants. They are abundant in the stems of the grasses, especially 

 in the straw of the cultivated grains, and form a large proportion of the 

 ash which is left when these stems are burned [p. 178.] 



It is important to the agriculturist to understand the relation which 

 the carbonic acid of the atmosj)here bears to these alkaline silicates which 

 occur in the mineral and vegetable kingdom. Insoluble as they are in 

 water, they are slowly decomposed by the united action of the moisture 

 and carbonic acid of the air, the latter taking the potash or soda from the 

 silica", and forming carbonates of these bases. In consequence of this 

 decomposition the rock disintegrates and crumbles down, while the so- 

 luble carbonate is washed down by the rains or mists, and is borne to 

 the lower grounds to enrich the alluvial and other soils, or is carried by 

 the rivers to the sea. 



In some cases, as in the softer felspar of some of the Cornish granites, 

 this decomposition is comparatively rapid, in others, as in the Dartmoor 

 and many of the Scottish granites, it is exceedingly slow, — but in all 

 cases the rock grumbles to powder long before the whole of the silicates 

 are decomposed, so that potash and soda are always present in greater 

 or less quantity in granitic soils, and will continue to be separated from 

 the decaying fragments of rock for an indefinite period of time. 



But the silica of the felspar, or mica, or zeoliticf trap, when thus de- 

 prived of the potash with which it was combined, is in that peculiar state, 

 in which, as above described [p. 206], it is capable of being dissolved 

 in small quantity by pure water, and more largely by a solution of 

 carbonate of potash or soda. Hence the same rains or mists which dis- 



• Potash, soda, and ammonia are called alkalies; lime and magnesia are alkaline earths. 

 See Lecture III., p. 51, note. 



t The trap-rocks always mors or less abound in zeolitic minerals, of which there is a great 

 variety, and in which nearly al Jie alkali present in these (trap) rocks is contained. 



