NATURAL HISTORY. 29 



consists of a combination of three other primary 

 rocks quartz, mica, and felspar. Gypsum ala- 

 baster, or plaster of Paris, used for a variety of 

 purposes in the arts, is simple lime combined 

 with sulphuric acid ; and sulphuric acid is sul- 

 phur in combination with oxygen. 



Among the saline substances, muriate of soda, 

 or common salt, is composed of soda and of the 

 muriatic acid. Borax, so much used in the arts, 

 consists of soda and the boracic acid ; and these 

 acids are compounds, of which oxygen forms a 

 part. Nitre, or saltpetre, is a combination of 

 potash and the nitric acid ; sal ammoniac, of 

 the volatile alkali and the muriatic acid ; and 

 alum, of alumen and the sulphuric acid. 



Among the metals, only a few of which are 

 found in the simple state, a great variety of com- 

 binations are to be met with, highly interesting in 

 a chemical sense, and most important to be 

 known in the arts. Iron, the most common and 

 the most useful of all the metals, is found in a 

 state of ore, in combination with sulphur or with 

 oxygen, or it is united to one of the acids. Cop- 

 per and most of the other metals are to be met 

 with in the same compound state; and in this man- 

 ner an almost endless variety of substances are 

 produced in the grand laboratory of nature, the 

 greater part of which, from a knowledge of their 

 combinations in some form or other, are applicable 

 to human purposes, and from an acquaintance 



