42 ESSAYS AND OBSERVATIONS 



EX PER. XXV. 



Four parts of chalk were properly- 

 kneaded with one part of clay, and dried. 

 This mixture, when calcined and piit in- 

 to water, fuffered no perceptible change. 

 The event was the fame, whatever pro- 

 portion of clay was ufed ; provided this 

 was not Icfs than the proportion employ- 

 ed in the lafl compofition. 



EX PER. xxyi. 



Four parts of chalk, one part of fand, 

 and one of clay, were well kneaded to- 

 gether, and dried. When this mixture 

 was put into water after calcination, it 

 cracked in feveral places, but did not 

 fall down into powder. From this we 

 fee, that the addition of a little fand to 

 the proportions ufed in the lafl Experi' 

 ment, makes a compofition unable, when 

 fufficiently burned, to rehft the influence 

 of water. And this happens, becaufe 

 fuch an addition augments the furfaces 



to 



