ON THE FOOD OF PLAKTvS. 113 



" was the necessary consequence of the imper- 

 " fection of the chemistry of the age. Calca- 

 " reous matter was considered, by the alche- 

 ** mists, as a pecuUar earth, which in the fire 

 ** became combined with inflammable acid ; and 

 ** Evelyn and Hartlib, and still later, Lisle, in 

 " their works on husbandry, have characterised 

 " it merely as a hot manure, of use in cold 

 ** lands. It is to Dr. Black of Edinburgh, that 

 " our first distinct rudiments of knowledge on 

 " the subject are owing. About the year J 755, 

 *'' this celebrated professor proved, by the most 

 " decisive experiments, that lime-stone and all 

 " its modifications, marbles, chalks, and marles, 

 *' consist principally of a peculiar earth united 

 *' to an aerial acid ; that the acid is given out in 

 " burning, occasioning a loss of more than forty 

 " per cent. ; and that the lime in consequence 

 " becomes caustic. 



*' These important facts, immediately applied 

 " with equal certainty to the explanation of the 

 ** uses of lime, both as a cement, and as a 

 *' manure. As a cement, lime, applied in its 

 *' caustic state, acquires its hardness and dura- 

 " bility by absorbing the aerial (or, as it has 

 " been since called, the carbonic,) acid, which 

 " always exists in small quantities in the atmo- 

 " sphere; it becomes, as it were, again lime-stone.** 



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