THE CORROSION OF BUILDING STORES. 311 



lain, I am unable to make any forecast of its probable dura- 

 bility. 



The experiments I made at the time named with this 

 and other building materials justified the conclusion that 

 the worst of all material for exposure to acid atmospheres 

 is a sandstone, the particles of which are held together by 

 limestone, or are otherwise surrounded by or intermingled 

 with limestone; and that the best of ordinary material is a 

 pure sandstone quite free from lime. I do not here con- 

 sider such luxurious material as granite or porphyries. 



Compact limestone, such as good homogeneous marble, 

 stands fairly well, although itis slowly corroded. The corro- 

 sion, however, in this case, is purely superficial and tolerably 

 uniform. .It is a very slow washing away of the surface, 

 without any disintegration such as occurs where a small 

 quantity of limestone acts as binding material to hold to- 

 gether a large quantity of siliceous or sandy material, and 

 where the agglomeration is porous, and the stone is so laid 

 that a downward infiltration of water can take place; for it 

 must be remembered that although the acid originally ex- 

 ists as vapor in the ah', it is taken up by the falling rain, 

 and the mischief is directly done to the stone by the acidi- 

 fied water. This, of course, is very weak acid indeed. 

 That which I used for testing the stone was many thousand 

 times stronger, but then I exposed the stone for only a few 

 days instead of many thousand days. 



As above stated, my experiments were but rude, but I 

 think it would be quite worth while to construct crushing 

 apparatus capable of registering accurately the pressure 

 used, and to operate with standard solutions of acid upon 

 carefully squared blocks of standard size, and thus to make 

 comparative tests of various samples of stone when competi- 

 tions for building materials are offered. In the case of the 

 Birmingham and Midland Institute building there was no 

 such competition, the choice was left entirely to the archi- 

 tect, and my examination was unofficially conducted upon 

 the material already chosen with the .intent of protesting if 

 it failed. As it stood the test I merely reported the results 

 informally to the architect, the late Sir Edward Barry, no 

 further action being demanded. 



