FACTITIOUS PUZZOLANA, 335 



quires and preferves its tenacity in it, fets but imperfectly m 

 water. This difficulty, of which the Inftitute will perceive 

 the caufe, lias obliged me lo keep two forts of the factitious 

 puzzolana ; on the reciprocal ufe of which a memoir of in- 

 duction will accompany the fa!e. The two forts may be 

 diflinguiflved by their colour. 



The ■ factitious puzzolana proper for works under water, is One fit for 

 of a reddifh-brown. That which is fit for works expofed to vvater - work s. 

 the air, is a dark violet. The latter is ufed for terraces, the Another for ex- 

 embankments of bafons, for the compofition of inclofures, or am j proper f Q * ir * 

 for light roofc. Bridges of a (ingle arch may be formed with terraces, roofs> 

 it; and I have feen it adhere fo ftrongly to glazed tiles, that 

 it was neceifary to break the tiles to detach it. 



The puzzolana proper for conftructions beneath the water, Water puzzola- 

 forms the rtldft folid body in it. Three months after immer- £ p ^tfV^ 

 fion it is an actual ftone capable of receiving a polim. The lift, 

 lime in it is always regenerated into carbonate of lime in ten 

 weeks. 



When it may be thought by any one that he has been de- Nullity of effect 

 reived as to the certainty of thefe effects, it will always be^3 ke °^. y th ^ 

 found, that he either has not obferved the quantities directed operator, 

 of the puzzolana and the lime, or that he has ufed the reverfe 

 of that kind of the cement proper for the work. 



I commonly ufed lime in the ftate of impalpable powder, Lime ufed in ^ 

 flacked in Lafaye's manner, for works expofed to the airj pow r Wlt lt * 

 and employed lime in the itate ofpulty, for works which were ar - d In P utt f« 

 to be covered with water. Sometimes I ufed lime in powder 

 for the fame work. This difference depends on the degree of 

 goodnefs of the lime, on its greater or lefTer richnefs, or its 

 proportional poverty. Outturn gives the advantage of know- 

 ing the different kinds on mere infpection. 



The ufe of lime in powder appeared to me to merit a pre- 

 ference in the preparation of mortars or cements. I prepared 

 my factitious puzzolana in a certain quantity as foon as I knew 

 the proper proportion of the lime; and I had thus the ad- Advantages of 

 vantage of being able to work it in troughs in the fame man- ^liftl^^' 

 ner as fulphate of lime. The whole was well mixed together mixed. 

 and put into facks ; by which means the mafons had nothing 

 to do with the mixture of the articles (which is too often left 

 to unprincipled workmen) ; and being thus matter of the re- 



• fpective 



