14 CHEMISTRY OF FARM PRACTICE 



is composed always of the same elements combined in 

 the same proportion by weight. A compound possesses 

 properties which differ from those of the elements of which 

 it is composed. Chemists have recognized many thousands 

 of compounds, each of which has a characteristic set of 

 properties which is constant and differs from that of other 

 compounds. 



The element carbon, a black solid, will combine chem- 

 ically under certain circumstances with the yellow solid, 

 sulphur, and form a compound, carbon disulphide, which 

 is a transparent liquid with a characteristic odor, and 

 very volatile at ordinary temperatures. The formation 

 of compounds is due to the chemical attraction existing 

 between the atoms of the elements which causes them 

 to combine to form the molecules of the compound. 



15. Mixtures. Two or more substances may be ever 

 so intimately blended together, but, if no chemical com- 

 bination takes place, there is no compound produced. 

 Such a mingling is termed a mixture. A mixture differs 

 from a compound in two ways: First, it possesses the 

 characteristic properties of all of its ingredients; second, 

 the proportion of its constituents may vary each time 

 the mixture is made. In this respect a mixture differs 

 from a compound in which the percentage content of its 

 constituents does not vary. 



16. Valence. . When hydrogen gas is mixed with chlorine 

 gas and exposed to diffused light, it is found that hydrogen 

 chloride, a compound whose molecules consist of one atom 

 of each element, is produced. While the reason for the 

 affinity between atoms of hydrogen and of chlorine is not 

 known, yet there is some kind of attraction, possibly elec- 

 trical, which draws and binds these atoms together. We 

 may imagine that each hydrogen atom has a string attached 

 to it by means of which it unites itself to other atoms. The 

 hydrogen atom seems to have but one of these strings 

 which may be called bonds or valences, and it is there- 



