96 ANNUAL OF SCIENTIFIC DISCOVERY. 



of the surface, and often a rapid removal of the particles beneath the sur- 

 face, especially on the planes of bedding. When stones are not placed in a 

 building as they were in the quarry, the surface peels off in natural films, 

 and is more rapidly acted on than it need be; but not unfrequently, even 

 when well placed, the surface gets hardened by exposure more rapidly than 

 the substance of the stone, and a scaling still takes place. The more ex- 

 posed parts, those subject to drip and constant damp, and the more deli- 

 cately sculptured portions, are among the first to decay; and, owing proba- 

 bly to differences in the mode or rate of deposit of the mud of which the 

 limestone was formed, or some partial change that has since taken place, 

 there is great irregularity in the rate of decay. Magnesian limestones, or 

 dolomites, when quite crystalline, behave like marble; but when, as is usual, 

 only half crystalline, they are very apt to become reduced to powder in 

 parts, and the decay thence proceeds with extreme rapidity. The professor 

 next proceeded to consider the remedies for decay. He alluded to paint as 

 at once unsightly and not permanently beneficial, and included the large 

 class of preservatives that have been suggested, in Avhich any animal or 

 vegetable oil or fatty matter was contained, as equally valueless, either peel- 

 ing off, or rotting in the stone, and leaving it soon exposed to ordinary 

 decay. The mineral bitumens, he stated, had not been much tried, owing 

 to their dark, unsightly color. What is required is some mineral prepara- 

 tion. He then alluded to the water-glass, a soluble silicate of potash, origi- 

 nally described by Dr. Fuchs, and applied to indurate stone by M. Kuhl- 

 mann. He explained the principle of this process as depending on slow 

 decomposition by exposure to the air, and stated that, as meanwhile the 

 influences of the weather continued to act, the method could not be adopted 

 with advantage in the open air in a damp climate, where preservation is 

 chiefly required. The only plan that, as far as he was aware, met the 

 requirements of the case, he stated to be that adopted by Mr. Ransome, 

 according to which the absorbent surface, whether of stone or terra-cotta, 

 was saturated with the diluted solution of soluble silicate of soda, and then 

 treated with a solution of chloride of calcium. By the mutual action of 

 these solutions, a double decomposition is induced, the silicie acid parting 

 with its soda to the chlorine, producing chloride of sodium, or common 

 salt, and combining with the lime to form silicate of lime. The salt being 

 washed away, only the silicate of lime remains. The silicate of lime, thus 

 thrown down, he next explained to be a salt, which was not only itself non- 

 absorbent, and singularly powerful in resisting the action of ordinary at- 

 mospheric influences, but as having the property of adhering readily to the 

 surface of the minute particles of which stone was formed. He illustrated 

 this by the case of mortar and concrete, which owe their adhesive properties 

 to this habit of silicate of lime, which is the mineral formed by the mutual 

 action of the cement on the substances in contact with it. The stone hav- 

 ing its particles thus coated with silicate of lime, and all the absorbent sur- 

 face being thus protected, the result is an immediate and great hardening of 

 the stone, so far within its substance as the solutions have been absorbed, 

 and a complete immunity to that extent from the action of atmospheric 

 influences. The stone does not necessarily become non-absorbent, though 

 it can be made so; but it absorbs much less rapidly than before, and 

 appears to resist decay much in the way that some of the best natural 

 sandstones are known to do. 



