THE VOLATILE PART OF PLANTS. 47 



Chemical decomposition, Water, thus composed or 

 put together by the exercise of affinity, is easily decom- 

 posed or taken to pieces, so to speak, by forces that op- 

 pose affinity e. g., heat and electricity or by the greater 

 affinity of some other body e. g., sodium as already 

 illustrated in the preparation of hydrogen, Exp. 11. 



Definite proportions, A further distinction between 

 chemical union and mere mixture is, that, while two or 

 more bodies may, in general, be mixed in all proportions, 

 bodies combine chemically in comparatively few propor- 

 tions, which are fixed and invariable. Oxygen and hydro- 

 gen, e. g., are found united in nature, principally in the 

 form of water ; and water, if pure, is always composed of 

 exactly one-ninth hydrogen and eight-ninths oxygen by 

 weight, or, since oxygen is sixteen times heavier than 

 hydrogen, bulk for bulk, of one volume or measure of 

 oxygen to two volumes of hydrogen. 



Atomic Weight of Elements, On the hypothesis 

 that chemical union takes place between atoms or indi- 

 visible particles of the elements, the numbers expressing 

 the proportions by weight* in which they combine, are 

 appropriately termed atomic weights. These numbers are 

 only relative, and since hydrogen is the element which 

 unites in the smallest proportion by weight, it is assumed 

 as the standard. From the results of a great number of 

 the most exact experiments, chemists have generally agreed 

 upon the atomic weights given in the subjoined table for 

 the elements already mentioned or described. 



Symbols. For convenience in representing chemical 

 changes, the first letter, (or letters,) of the Latin name of 

 the element is employed instead of the name itself, and is 

 termed its symbol. 



* Unless otherwise stated, parts or proportions by weight arc always to be 

 understood, 



