314 'Mr. Napikr on Sandstones used/or Building. 



which compose them anrl the circumstances in which they have been 

 deposited. Sometimes they have a laminated structure, and betvceen these 

 lamina? are deposits of mica in very fine scales. Stones of this class are 

 often placed into a wall with their lamina^ in a vertical position. When 

 this is done such stones are found to scale in flakes, and should therefore 

 always be laid upon bed ; but even in stones that have no apparent 

 laminje, but seem one solid mass, still they have a stratified structure, 

 and when not built into the wall in what is termed their natural bed — 

 having the strata in a horizontal position — they are always more liable to 

 decay. 



Freestone being simply sand particles cemented together under 

 pressure, and corrosion or decay being the dissolving or loosening of this 

 cement, my first inquiry was to find out what the cementing material 

 was. I had formed an opinion that this in all probability was either 

 lime or iron, and that the carbonate of lime being soluble in water impreg- 

 nated with carbonic acid, the lime would be liable to be dissolved out by 

 rain water, which generally contains that acid, or the acid might be ab- 

 sorbed from the air by water in the stone. These ideas were strengthened 

 a little by finding that some of the stones contained iron as a carbonate, 

 which by exposure was converted into a peroxide, and which might thus 

 yield its carbonic acid to the lime, and by converting it into a bicarbonate 

 render it soluble in water ; but subsequent inquiry brought out so many 

 circumstances to be looked to as made these suppositions not so promi- 

 nent. 



It often happens that stones most exposed stand best, or what is the 

 same thing, the same stone will decay most where least exposed, as 

 for instance pillars are often found to corrode most rapidly on the inside, 

 or least exposed parts. Sometimes it is observed that a stone wasting 

 away is very red in colour, and a conclusion come to that it is the ii'on, 

 which being oxidised loosens the particles, an idea very tenable, but then 

 probably in the same building are stones equally red, and containing more 

 iron than the other, which has also peroxidised, and no decay taking 

 place ; and there may be also a stone a few feet off of marked whiteness, 

 mouldering so rapidly that if the hand bo drawn over it, a shower of sand 

 particles fall, while the hand is stained as if by clay. These facts 

 render the inquiry interesting, and increases the diflSculty. 



Sandstone, as I have already intimated, is not a chemical compound, 

 such as slate or felspar, but a mechanical mixture, composed of the 

 debris of chemical compounds mixed and cemented together. And to 

 take the mass and analyse them as one compound, would be to give a 

 false data to reason from. I therefore endeavoured fii'st to separate this 

 mixture mechanically. 



In looking at a piece of sandstone through a lens, every particle of 

 silica seems coated over with some white, opaque matter, resembling clay 

 or lime. If this is connected with the binding of these sand particles, 

 then, what under ordinary circumstances will dissolve this away"? I 



