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 are always to be 
understood. 
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