392 



visionally be called clay, of which it consists in small part ; but till it 

 is analysed, it would be premature to discuss its nature. Mr Napier's 

 observation, however, that sandstones contain clay, is an important 

 one, especially in reference to their power to retain moisture, and 

 continue long damp in the walls of buildings. 



The iron which occurs so generally in sandstones, and is so im- 

 portant an ingredient, from its tendency to stain the stone after it is 

 quarried and exposed to the air, is certainly present in different 

 chemical conditions. It has been generally assumed, I think, to occur 

 in carboniferous sandstones as bisulphuret ; but it appears to be 

 chiefly present in the Craigleith rock as carbonate of the protoxide, 

 the form in which it has always been recognised as prevailing in the 

 shales accompanying such sandstones. As already mentioned, the 

 protocarbonate of iron occurs in detached portions, coating and di- 

 viding certain strata of the stone from each other ; but it is not on 

 this circumstance that I found the conclusion stated above, but on the 

 following results: — 1000 grains of the stone, finely powdered, were 

 suspended in cold distilled water, and a stream of washed carbonic 

 acid gas sent through the liquid for an hour. The water passed 

 through a filter quite transparent ; but upon boiling became troubled, 

 and deposited carbonates of lime and magnesia, peroxide of iron, 

 and a little silica. Of these substances the peroxide of iron was the 

 most abundant, and it had plainly been dissolved as protocarbonate. 

 The probability, accordingly, is that the metal existed as carbonate 

 in the sandstone ; but it may have been present as metal or as black 

 oxide, though scarcely as bisulphuret, and certainly not as peroxide. 

 The point of most practical interest, however, is, that rain-water, 

 containing, as it always does, carbonic acid, is able to dissolve iron 

 as well as lime and magnesia from exposed sandstones ; so that we 

 may always expect to find them colour more or less from the solution 

 and subsequent peroxidation of the iron which they contain. 



It was not found possible to remove the whole of the iron from the 

 powdered sandstone by the action of carbonic acid water, for after it 

 had exerted its full effect, hydrochloric acid, if boiled on the powder, 

 extracted iron as peroxide, unaccompanied by protoxide. 



The action of other solvents on the stone is as follows : — Distilled 

 water boiled upon it in fine powder acquired a notable quantity of 

 lime, a small quantity of sulphuric acid, and a trace of iron. Mi- 

 nute quantities of the alkalies, of magnesja, and of silica were doubt- 



