CHEMICAL DECOMPOSITION. 33 



permanent change been produced, upon any of the substances. The two 

 former were merely mixtures. 



In all cases of chemical action a permanent change iahes place in some 

 of the substances employed ; and this change is the result either of a chem- 

 ical combination^ or of a chemical decomposition. 



Tlius when sulphur is burned in tiie air, it is converted into white va- 

 pours possessed of a powerful and very un[)leasanl odOur, and which 

 continue to be given off until the whole of the sulphur is dissipated. 



Here a solid substance is permanently changed into noxious vapours 

 ■which disappear in the air, and this change is caused by the combination 

 of the siilphur with the oxygen of the atmosphere. 



In like manner when lim^tone is put into a kiln and strongly heated 

 or burned, it is changed or converted into quicklime — a substance very 

 different in its properties from the natural limestone employed. But 

 Jiis is a case of chemical decomposition. The limestone consists of 

 lime and carbonic acid. By the lieat these are separated, the latter is 

 driven off and the former remains in the kiln. 



Again, when a jet of hydrogen gas is kindled in the air or in oxygen 

 gas, it burns with a pa^e yellow flame. If a cold vessel be held over 

 this flame, it speedily becomes bedewed with moisture, and drops of wa- 

 ter collect upon it. How remarkable the change which hydrogen un- 

 dergoes during this combustion ! It unites with the oxygen of the 

 atmosphere and forms water. How different in its properties is this 

 water from either the oxygen or the hydrogen by the union of which it is 

 formed! The former a liquid, the latter gases; the former an enemy 

 to all combustion, while of the latter, the one (hydrogen) burns readily, 

 the other (oxygen) is the very life and support of combustion in all oth- 

 er bodies. 



1°. It appears, therefore, that chemical combination or decomposition 

 is always attended by a permanent change. 



2°. That when combination takes place, a new substance is formed 

 differing in its properties from any of those from which it was produced, 

 or of which it consists. 



When two or more elementary bodies thus unite together to form a 

 new substance, this new substance is called a chemical compound. 

 Thus water is a compound (not a mixture) of the two elementary bodies 

 oxygen and hydrogen. • 



Now when such combination takes place, it is found to do so always 

 in accordance with certain fixed laws. Thus : 



I. Bodies unite together only in constant and defmte proportions. We 

 can 7nix together oxygen and hydrogen gases, for example, in any pro- 

 portion, a gallon of the one with any number of gallons of tlie other, but 

 if we burn two gallons of hydrogen gas in any greater number of gallons 

 of oxygen, they will only consume or unite with one gallon of the oxy- 

 gen, the rest of this gas remaining unchanged. A quantity of water will 

 be formed by this union, in which the whole of the hydrogen will be 

 contained, combined with all the oxygen that has disappeared. Under 

 no circumstances caji we burn hydrogen so as to cause it to consume 

 more oxygen, or from a given weight ot hydrogen to produce more than 

 a known weight of water. And as oxygen is nearly sixteen times 

 heavier than uitrogen, it is obvious that one gallon of the former is about 



