206 Thickness, Cement and Materials of Walls, 



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

 posed of Jive parts sand (river sand from my island) and 

 ofie of Ume. The first coat should never have less than 

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

 should not be much richer than thi'ee to one. On a 

 north wall of my house, rough casting, thus composed, 

 is now perfect, after a lapse of fifty years.* 



* Bullock's blood and smithes, ox furnace^ cinders pulverized^ 

 mixed widi the mortar lor rough casting or pointing, in a 

 small proportion, are known to be highly beneficial. The 

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

 bon ; these are also i'ound, plenteously, in our common air ; 

 which, though it consists chiefly oi 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. Oxijgen and carbon, form the carbonic 

 acid, or Jjxuble air ; which immediately operates, to crystal- 

 lize, or hai-den the cement, by its affinity for lime. This acid 

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

 ders ; and produces, at once, its effects on the lime ; which 

 must otherwise harden, or crystallize, by the more tedious 

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

 the atmosphere. Whether this theoiy be or not chjonically 

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

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

 portions. Pointing, or rough cast, forced, 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 crystallization. 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 the}^ can faichlully perform 

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

 and vainlv endeavour to repair the inconvtniencies oi delay, 

 by over liining, or forcing. 



