206 Thickness^ Cement and Materials of Walts* 



way, I have rough casting, free from, any defects, com- 

 posed oifive parts sand (river sand from my island) and 

 one of hme. The first coat should never have less than 

 four parts sand, and one of lime. And the second, 

 should not be much richer than three to one. On a 

 north wallof my house, rough casting, thus composed, 

 IS now perfect, after a lapse of fifty years. ^ 



^ Bullock'' s blood ^Yid smith^s^ or furnace^ cinders pulverized^ 

 mixed with the mortar for rough casting or pointings in a 

 small proportion, are knowni to be highly beneficial. The 

 reason may, probably, be ; that they contain oxygen^ and car- 

 ban y these are also found, plenteously, in our common air ; 

 which, though it consists chiefly of the former and nitrogen^ 

 in a gaseous state ; yet in it are found all the substances ca- 

 pable of existing in an aeriform state, at the common tempe- 

 rature of our globe. Oxygen and carbon, form the carboiiic 

 acidy ox fx able air ; which immediatelv operates, to crystal- 

 lize,, or harden the cement, by its affinity for lime. This acid 

 probably exists, ready combined by combustion, in the cin- 

 ^dT*/ and produces, at once, its effects on the lime; which 

 must otherwise harden, or crystallizCy by the more tedious 

 process of collecting the acid, or its component parts, from 

 the atmosphere. Whether this theory be or not chj-mically 

 correct, the effects of these additions to the cement, are prac- 

 tically known ; and it requires practice, to ascertain the pro- 

 poi-tions. Pointing, or rough cast, j orced, to harden too quick, 

 cracks, and will not adhere. They should never be laid on 

 in very hot weather; which dries away the moisture neces- 

 sary to ciystallization. Frost injures mortar of any compo- 

 sition ; if it occurs before induration. Ignorant or conceited 

 workmen, think liming high is a safeguard. Those who (too 

 commonly) have more work than they can faithful!}' perform 

 in the season, put off the pointing till it is too far advanced ; 

 and vainly endeavour to repair the inconveniencies of delay, 

 by over liming, or forcing. 



