THE VOLATILE PAET 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 



NH3,H,0, CO,. 



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. 

 KCIO, = KCl + 0, 



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 Cliemical 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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