130 HOW CROPS GROW. 



The ashes of plants sometimes yield this gas when they are moistened 

 with water. In such cases, a sulphide of potassium or calcium has been 

 formed in small quantity during the incineration. 



Hydrogen Sulphide is set free in the gaseous form by the action of an 

 acid on various sulphides, as those of iron (Exp. 17), antimony, etc., as 

 well as by the action of water on the sulphides of the alkali and alkali- 

 earth metals. It may be also generated by passing hydrogen gas into 

 melted sulphur. 



Sulphuretted hydrogen has a slight acid taste. It is highly poisonous 

 and destructive, both to animals and plants. 



SULPHUK DIOXIDE, commonly called SULPHUROUS ACID, Sym. SO 2 , mo. 

 wt. &4. When sulphur is burned in the air, or in oxygen gas, it forms 

 copious white suffocating fumes, which consist of one atom of sulphur, 

 united to two atoms of oxygen ; SO 2 (Exp. 15). 



Sulphur dioxide is characterized by its power of discharging, for a 

 time at least, most of the reel and blue vegetable colors. It has, how- 

 ever, no action on many yellow colors. Straw and wool are bleached 

 by it in the arts. 



Sulphur dioxide is emitted from volcanoes, and from fissures in the 

 soil of volcanic regions. It is produced when bodies containing sul- 

 phur are burned with imperfect access of air, and is thrown into the 

 atmosphere in large ^quantities from fires which are fed by mineral 

 coal, as well as from the numerous roasting heaps of certain metallic 

 ores (sulphides) which are wrought in mining regions. 



Sulphur dioxide may unite with bases, yielding salts known as sul- 

 phites, some of which, viz., calcium sulphite and sodium sulphite, are 

 employed to check or prevent fermentation, an effect also produced by 

 the acid itself. 



Sulphur-Trioxide, Sym. S0 3 , mo. wt. 80, is known 

 to the chemist as a white, silky solid, which attracts 

 moisture with great avidity, and, when thrown into 

 water, hisses like a hot iron, forming sulphuric acid. 

 Sulphur trioxide was formerly termed sulphuric acid or 

 anhydrous sulphuric acid, and now it is common in 

 statements of analysis to follow this usage. 



Sulphuric Acid, Sym. H 2 S0 4 , mo. wt. 98, is a sub- 

 stance of the highest importance, its manufacture being 

 the basis of the chemical arts. In its concentrated form 

 it is known as oil of vitriol, and is a colorless, heavy 

 liquid, of an oily consistency, and sharp, sour taste. 



It is manufactured on the large scale by mingling sul- 

 phur dioxide gas, nitric acid gas, and steam, in large 

 lead-lined chambers, the floors of which are covered with 

 water. The sulphur dioxide takes up oxygen from the 



