Mr. Napiek oh Sandstones used for Buildimj. 319 



m 

 Were we warranted in drawing any conclusion from these few analyses 

 there is but one to make, namely, that the presence of a great quantity of 

 clay with little lime prevents the proper adhesion of the sand particles.* 

 As regards the probable cause of stones decaying more rapidly in the 

 shade than exposed to a free current of air and sunshine, the general 

 opinion is that it is caused by want of evaporation. A good stone in a 

 few days' exposure takes upon it a sort of skin, the iron on the suiface 

 peroxidises, and the face of the stone is clean and apparently washed-like ; 

 but it is otherwise with a bSd stone, if the hand be drawn over it in dry 

 weather it seems, and really is, dusty, the particles falling off by the 

 friction of the hand, and this often in exposed parts where evaporation 

 is the same as with other good stones. Nevertheless, the constant state 

 of moisture that stones are kept in, when placed under a cornice or pro- 

 jection, and the absence of sunshine, have a bad effect upon our sandstones. 

 And the fact is sufficiently apparent to anyone who will give the slightest 

 attention in passing along the streets ; and I may mention in connection 

 with this that sandstones having much clay in them absorb moisture 

 rapidly from the atmosphere, so that such stones are not only longer in 

 drying, but a damp atmosphere will moisten them nearly as much as if 

 they were in contact with water. Two thin pieces, one having 20 per 

 cent, of clay, the other none, exposed to a damp atmosphere over night, 

 the former had absorbed 5 per cent, of its weight of water, the other 

 only 1. This tendency to absorb and retain water may facilitate their 

 decay. 



I do not as yet offer any definite solution of the chemical cause of the 

 rapid corrosion of some sandstones. My experiments, however, excite 

 suspicions that the clay which is only mechanically diffused through the 

 stone, has something to do with preventing the adhesion of the siliceous 



* Since making these experiments, I have tried the following specimens. Two 

 pieces of stone from Blair Logic Old Church, which had decayed regularly, but not 

 rapidly, gave — 



No. 1. Silica and mica, 80-a No. 2. Silica and mica, 82 3 



Clay 15-5 Clay 1 1 -0 



Peroxide iron 1-6 Peroxide iron, 2 7 



Carbonate lime, 2-4 Lime 20 



100-0 



Magnesia, trace. 



980 



No. 2 was finer in the grain than No. 1. 

 A piece from an old castle near Stirling that had stood well ; the surfiico of the 

 stone from which the sample was taken seemed little worn, gave 



Silica and mica, 90-6 



Cay, 40 



Iron oxide 20 



Lime 2-6 



o7<> 

 Looking at (his specimen through a lens the silica was transparent and washed like. 



