30 SCIENTIFIC AGRICULTURE. 



united by affinity, they must possess different chemical proper- 

 ties: thus acids and alkalies are chemically opposed, and are 

 consequently drawn together, while they rarely, if ever, unite 

 with each other. "We might [says Dr. Fownes] define 

 chemical affinity to be a force by which new substances are 

 generated." 



LAWS OF COMBINATION. 



The elements of chemical compounds are -generally limited 

 to fixed and invariable proportions on both sides. It is this 

 constancy of proportions alone, which gives to chemistry its 

 title to the character of an exact science ; for had all bodies 

 the property of combining in every possible proportion under 

 every variety of circumstances, no definite or certain knowledge 

 could be obtained in relation to the constitution, properties or 

 chemical uses of bodies: experiments would give results so 

 different and variable, at different times, and under various 

 circumstances, that the votaries of this sublime and useful 

 science would long since have abandoned it in despair. 



The elements of a given chemical compound are always in 

 the same proportions : thus, green oxide of iron is composed 

 of 27 parts, by weight, of iron, and 8 parts of oxygen: com- 

 mon salt is a compound of 23 parts sodium and 35 parts 

 chlorine; and these are the smallest proportions in which 

 those elements can be made to unite. When two elements 

 unite in more than one proportion on either side, the additional 

 proportions are just double, triple, quadruple, &c., or 1 to $ 

 that is 2 to 3, 3 to 5, the amount of the first : that is, they 

 increase in exact multiple proportions. To illustrate this 

 principle we may allude to the five compounds of oxygen and 

 nitrogen. 



Protoxide of nitrogen consists of 



Nitrogen, 14.06 Oxygen, 8 



Deutoxide " 14.06 " 16 



Hyponitrous acid " 14.06 " 24 



Nitrous add 14.06 32 



Nitric acid 14.06 40 



