900 Transactions of the American Institute. 



however, local in its occurence. From the moment the mortar is 

 exposed to the sea-water, carbonate of lime will be formed wherever 

 lime, be it free or combined, comes in contact with this agent. As to 

 the salts of magnesia, they will convert free lime or its carbonate into 

 a sulphate or chloride of calcium. With regard to silicate or alumi- 

 nate of lime, the magnesia probably takes the place of the lime 

 without changing the properties of the respective compounds. As 

 lime is dissolved by all these reactions, the cement becomes porous ; 

 and, if a formation of sulphate of lime in crystals takes place, the 

 solidity of the cement is endangered. From what has been said, it is 

 evident that protection is afforded only by the physical properties of 

 the mortar. If it is fine and of a high specific gravity, a layer of 

 carbonate of lime will be formed on the exterior, and thus the sea- 

 water will be prevented from penetrating further. Exposed to the 

 sea immediately after setting, the induration takes place quite slowly ; 

 but, if once completed, the permanency of the cement, in so far as air 

 and water are concerned, is secured for ever. The cement stone should , 

 therefore, be allowed to harden well before it is immersed in the sea. 

 Moreover, we have in soluble glass the most simple and effective 

 means of securing and increasing the durability of hydraulic cement, 

 as far as these qualities result from its being rendered inaccessible to 

 water and carbonic acid. The effect of soluble glass is here two-fold ; 

 silicate of lime is formed, and silica deposited ; and, if the former 

 should again become decomposed by carbonic acid, it will only help 

 to form a layer on the exterior, upon which air and water have hardly 

 any effect. 



On the Effect of Frost During Tnchiratlon. 



So long as only the lesser portion of the water that was used to 

 mix with the cement has entered into chemical combination, tlie work 

 executed would necessarily be endangered by a frost. After the lapse 

 of a week or so, only one-third or one-fourth of the water remains 

 free ; while, after four weeks' time, the whole amount used is found 

 combined. Although this amount of time is very small, and though 

 the cement greatly surpasses ordinary lime mortar on account of the 

 solidity it attains, builders are often quite unreasonable in regard to 

 the matter. How often, for instance, are extensive works executed 

 in cement, when a frost may occur every night ! And how often, 

 too, is even plastering done at such times ! But what are the conse- 

 quences? Even before the winter has passed, the prematurely under- 

 taken construction tails to pieces on the ground ; or, at the least, 



