164 CHEMISTRY. 



boils, renders it unfit for measuring high tempera- 

 tures. Quicksilver will indicate 500 of heat, but 

 freezes at 40 below the freezing point. 



Wedgervood's Pyrometer. 



In Wedgewood's Pyrometer, the zero corresponds 

 with 1077 of Fahrenheit, each degree being equal 

 to 130 Fahrenheit. 



Combustion. 



Combustion is the result of intense chemical ac- 

 tion, the cause of which has not yet been discovered. 



Combustion, in common cases, is the solution of a 

 body in oxygen or the fixation of oxygen in a solid 

 form, or the production of a fluid when oxygen and 

 hydrogen unite. 



Inflammable and Combustible. 



These are not synonymous terms. All metals are 

 combustible, that is, capable of uniting with oxygen ; 

 but they will not burn in atmospheric air, and are 

 therefore not called inflammable. 



Pure Flame. 



Hydrogen gas furnishes the purest flame that can 

 be exhibited ; for the flame of bodies that emit much 

 light derives that power from solid bodies intensely 

 ignited, and diffused through them, and which in or- 

 dinary flames, as of gas, tallow, wax, oil, &c. con- 

 sists of finely divided charcoal. 



Oil and Coal Gas. 



Gas from oil or coal, freed from all impurities, is 

 a mixture of olefiant gas, carburetted hydrogen, hy- 

 drogen, carbonic oxide, and azote, with an essential 

 oil held in solution. Oil gas is free from sulphuretted - 

 hydrogen, which renders the coal gas so injurious to 

 silver and plated goods. 



