THE VOLATILE PART OF PLANTS. 51 
Such a formula merely informs us what elements and 
how many atoms of each element enter into the composi- 
tion of the substance. It is an empirical formula, being 
the simplest expression of the facts obtained by analysis 
of the substance. 
Rational formulas, on the other hand, are intended to 
convey some notion as to the constitution, formation, or 
modes of decomposition of the body. For ‘example, the 
fact that carbonate of ammonia results from the union 
of one molecule each of carbonic acid, water, and ammonia, 
is expressed by the formula 
N H,, H, O, C O,. 
A substance may have as many rational formulas-as 
there are rational modes of viewing its constitution. 
Equations of Formulas serve to explain the results of 
chemical reactions and changes. Thus the breaking up 
by heat of chlorate of potash into chloride of potassium 
and oxygen, is expressed by the following statement. 
Chlorate of potash. Chloride of potassium. Oxygen. 
KOLO, = K Cl +} O, 
The sign of equality, —) shows that what is written be- 
fore it supplies, and is resolved into what follows it. The 
sign + indicates and distinguishes separate compounds. 
The employment of this kind of short-hand for exhibit- 
ing chemical changes will find frequent illustration as we 
proceed with our subject. 
Modes of Stating Composition of Chemical Compounds. 
—These are two, viz., atomic or molecular statements and 
centesimal statements, or proportions in one hundred parts, 
(per cent, p. c. or "|,.) These modes of expressing com- 
position are very useful for comparing together different 
compounds of the same elements, and, while usually the 
atomic statement answers for substances which are com- 
paratively simple in their composition, the statement per 
cent is more useful for complex bodies. The composition 
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