CHAPTER IV 



CHALK, LIME, AND THE ALKALIS 



A. CHALK AND LIME 



The burning of chalk to lime. The changes wrought 

 by the action of fire have been described (Chapter I, p. 7) 

 as leading at an early period to the preparation of tin from 

 tinstone, iron from ironstone, and so forth. In addition to 

 these metallic ores, a number of rocks were known which 

 were converted by burning into a caustic substance, known 

 as LIME, which was used in the preparation of mortar. 

 Joseph Black (1728-1799), an Edinburgh physican who 

 afterwards occupied the chairs of chemistry at Glasgow and 

 at Edinburgh, was one of the first to make a careful study of 

 these substances. He regarded chalk as the typical source 

 of lime, and included in the calcareous class of substances 

 " all those that are converted into a perfect quick-lime in 

 a strong fire, such as limestone, marble, chalk " and " those 

 spars and marls which effervesce with aqua fortis n 

 (A.C.R. I. 10). 



The substance formed by heating chalk has remarkable 

 properties. When brought into contact with the skin, it 

 produces blisters and wounds resembling those caused by 

 fire. Hence it was known as a caustic. This property has 

 long been used for removing the hairs from hides in the 

 manufacture of leather. The lime drawn from the kilns is 



