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hardiiefs, even under water, and is therefore ufed in the conftrudion 

 of water-works, and in other buildings where more than ordinary 

 ftrength and foUdity are required. 



This fubftance called Tras, is brought to us down the Rhine, and 

 is ground to powder in windmills conll:rucled for that purpofe, and 

 being mixed with common Lime, it is beaten or worked together a 

 very long time," even for feveral days ; for it is faid, that the longer 

 it is beaten the better it is : and, in order to make work firm and 

 llrong, it is cullomary to ufe three parts of this Cement or Tras and 

 four of Lime. 



A piece of this mixture of Lime and Cement, taken from an an- 

 cient building, I heated red hot, and when it was grown cold I found 

 it fo foft, that it was eafily reduced to powder. This powder I 

 mixed up with water, and perceived a great quantity of bubbles 

 arife from it, the fame as we obferve in frefli Lime ; after this had 

 flood for an hour, it was grown ftifF. I then beat it, or mixed it up 

 again, and in a Ihort time it becanie exceeding hard. 



This repeated beating, or working up of the Tras or Cement, is 

 of very great ufe, becaufe, by this means, the air bubbles formed in 

 it are expelled; for want of which being done the Cement cannot 

 acquire its proper firmnefs. This appeared in the fmall portion of 

 Cement which I have jull faid that I mixed or worked up only twice ; 

 for, after it was become dry, I broke it in pieces, and found in the 

 middle of it many round cavities, which I concluded to have been 

 caufed only by the bubbles of air in it. But, when the Cement is 

 beaten or worked together for feveral days, not only the air bubbles 

 are expelled, but, after, a certain time, no new ones are formed in it; 

 and I am certain that if our common Lime was to be beaten or 

 worked up for as long a time as the Tras or Cement, the bubbles 

 of air which are formed in it would be all beat out, and the labour 

 more than twice repaid by the goodnefs of the Mortar, efpecially 

 in cafes where folid and durable work is required. 



