EXPANSION OF MATTER IN A GASEOUS STATE. 75 



best means in our power ; and in doing so we scarcely reach 

 anything like the precise degree of purity, which we so earnestly 

 desire. 



As the momentous ignition of any of the gases separately 

 fired, when placed in a close vessel, depends on the presence 

 of some one or more of the others, though but in a very minute 

 proportion, perhaps after the usual applications for purifying 

 gases by absorption, &c., the electric spark might afford the 

 best means of producing a greater degree of purity. 



One per cent, of azote has been found in oxygen gas procured 

 from manganese by sulphuric acid. 



The expansion of matter in the gaseous state on the appli- 

 cation of heat, is the same according to Messrs. Dalton and 

 Gay Lusac 100 parts of each species of gas at the freezing 

 point of water is dilated to 137.5 at the boiling point. 



Bodies when heated to a certain temperature become lumi- 

 nous from 752 to 800 of Fahrenheit. The capacity of afford- 

 ing light, evident to the sense of vision, is different : thus, iron 

 wire becomes red hot when immersed in melted lead, which 

 does not at the time afford the smallest appearance of light. 



The simple gases are an exception : they do not become lumi- 

 nous at a much higher temperature, when SEPARATELY 

 treated, because they have not there the necessary requisites 

 is produce light, which can only be obtained by a combination 

 of the matter of its rays in due proportion, and is resplendent 

 in the ratio of their precise arrangement. As to their relative 

 proportion, according to Sir H. Davy, bodies continue united, 

 or resist decomposition, because they are in different electrical 

 states ; that if we bring them into a similar state by making 

 them both positive or both negative they will repel each other, 

 and of course be decomposed ; and that this is effected by the 

 galvanic battery which in his opinion will decompose any com- 

 pound. He has applied this theory with success to the decom- 

 position of the alkalies, showing them to be compounds of 

 oxygen with a metallic basis. Some of the earths submitted 



