4 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



their power of effervescing when mixed with acids. This 

 property served to distinguish the Egyptian nitre or soda 

 from the common nitre or saltpetre described in the next 

 paragraph. It is referred to in the proverb : " As he that 

 taketh away a garment in cold weather, and as vinegar upon 

 nitre, so is he that singeth songs to an heavy heart" 

 (Prov., xxv. 20). In reference to this passage Robert Boyle, 



FIG. 2 CRYSTAL OF 



SALTPETRE. 

 British Museum 

 (Natural History). 



FIG. 3 SAL-AMMONIAC. 



who had received a specimen of Egyptian " nitre " from 

 Constantinople, wrote in 1680: 



" And here .... give me leave to take notice of a text 

 of the holy Scripture, that has sometimes puzzled not only 

 me, but far better Critics in the Hebrew tongue than I, .... 

 where to illustrate Things very incongruous to one another 



