THE VOLATILE PAET OP PLANTS. 31 



time ; but if a flame be brought into the mixture they in- 

 stantly unite with a loud explosion, and, in place of the 

 light and bulky gases, we find a few drops of water, which 

 is a liquid at ordinary temperatures, and in winter 

 weather becomes solid, which does not sustain combus- 

 tion like oxygen, nor itself burn as does hydrogen ; but 

 is a substance having its own peculiar properties, differ- 

 ing from those of all other bodies with which we are ac- 

 quainted. 



In the atmosphere we have oxygen and nitrogen in a 

 state of mere mixture, each of these gases exhibiting its 

 own characteristic properties. When brought into chem- 

 ical combination, they are capable of yielding a series of 

 no less than five distinct compounds, one of which is the 

 so-called laughing-gas, while the others form suffocating 

 and corrosive vapors that are totally irrespirable. 



Chemical Decomposition. Water, thus composed 

 or put together by the exercise of affinity, is easily de- 

 composed or taken to pieces, so to speak, by forces that 

 oppose affinity e. g., heat and electricity or by the 

 greater affinity of some other body e. g., sodium as al- 

 ready illustrated in the preparation of hydrogen, Exp. 11. 



Definite Proportions. A further distinction be- 

 tween chemical union and mere mixture is, that, while 

 two or more bodies may, in general, be mixed in all pro- 

 portions, bodies combine chemically in comparatively 

 few proportions which are fixed and invariable. Oxygen 

 and hydrogen, e. g., are found united in nature, princi- 

 pally in the form of water ; and water, if pure, is always 

 composed of one-ninth hydrogen and eight-ninths oxy- 

 gen by weight, or, since oxygen is, bulk for bulk, sixteen 

 times heavier than hydrogen, of one volume or measure 

 of oxygen to two volumes of hydrogen. 



Atoms. It is now believed that matter of all kinds 

 consists of indivisible and unchangeable particles called 

 atoms, which are united to each other by chemical at- 



