248 CHEMISTRY. 



pering process consists in reheating the steel and then let- 

 ting it cool slowly. The character of the effect depends 

 upon the degree of heat to which it is carried, and this is 

 measured by the workmen by the color caused by the heat. 

 You can see the various colors by experimenting with a 

 common knitting-needle. First heat it to redness in the 

 flame of a spirit-lamp, and quench it in cold water. Now 

 hold it again in the flame, and observe the changes of 

 color. It first becomes a pale yellow, then orange, crim- 

 son, violet, blue, and finally dark gray. The explanation 

 is this : A film of oxide forms, which, being at first exceed- 

 ingly thin, is pale yellow, and deepens in its tint as the in- 

 creased heat thickens it. The final dark-gray coating is 

 scales of the oxide of iron. Now there is a definite degree 

 of hardness on the one hand, and of elasticity on the other, 

 corresponding to each one of these colors, the yellow giv- 

 ing the most brittleness and hardness, and the blue the 

 most softness and elasticity, the other colors giving inter- 

 mediate results. Accordingly, tools for cutting metal, 

 which require to be very hard, are heated till they become 

 a pale yellow ; knives and planes to an orange ; chisels, 

 hatchets, etc., to a crimson ; and springs to a violet or blue 

 tint. 



346. Sulphides of Iron. There are three sulphides of 

 iron. One of them, the disulphide FeS 2 , is what is usually 

 called iron pyrites. It received this name among the an- 

 cients because it strikes fire, pur being the Greek for fire. 

 The idea which they had of it may be gathered from what 

 Pliny says, who states that " there was much fire in it." 

 It crystallizes, and has a brilliant yellowish brassy color. 

 It has sometimes been supposed by the ignorant to be gold, 

 and so has received the name of fool's gold. It is of great 

 value in the arts in obtaining several important substances, 

 as sulphur, ferrous sulphate, and sulphuric acid. As more 



