16 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



the chalk was usually dissolved, but oil of vitriol gave a 

 white solid which was proved by Margraaf, in 1750, to be 

 identical with the mineral GYPSUM or SELENITE (Fig. 10, 

 p. 19), from which "Plaster of Paris" is made by gentle 

 burning. The origin and the nature of the "air" which is 

 set free in these actions are discussed in Chapters IV. 

 and VI. 



Action of acids on metals. The acids also had the 

 property of corroding or dissolving metals. The poisonous 

 green powder known as VERDIGRIS was prepared at a very 

 early date by the action of vinegar on copper, and was used 

 as a paint during the classical period. The mineral acids 

 were found -to act on metals in a much more powerful 

 way ; by the time of Geber (circ. 800 A.D.) methods had" 

 been discovered for dissolving all the metals that were 

 known. The action of acids on metals was often accompanied 

 (as in the case of soda and chalk) by the breaking out of a 

 gas, but for many centuries no method was known by which 

 these fugitive products could be collected. The important 

 discoveries which were made when at last it was found 

 possible to isolate and examine them are described in 

 Chapter V. 



C. PREPARATION OF NEW SALTS AND NOMENCLATURE 

 OF SALTS 



New salts. One result of the discovery of the acids 

 was to add very greatly to the number of "salts" which 

 were known. When an acid acts on a metal, on chalk, or 

 on an alkali, a solution is produced which no longer has the 

 sour taste of the acid. These solutions contain a variety 

 of salt-like substances, which can be isolated by evaporating 

 either to dryness or until crystals begin to separate. In this 

 way many beautiful and useful salts were obtained. At 

 first a special name was given to each salt ; but later a 



