THE VOLATILE PART OF PLANTS. 31 



ume ; 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 bpdies 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 arid 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- 



