42 CHEMISTR Y FOR A GRICUL TURAL STUDENTS 



substances dealt with are physical^ and (2) those which have 

 reference to their cotnposition are chemical. The systematised 

 knowledge obtained by a study of the properties of matter 

 constitutes the science oi Physics \ that obtained by a study of 

 its composition constitutes the science of Chemistry. The 

 changes observed which have involved alteration in composition, 

 such as the rusting of iron, are chemical changes ; those which 

 have not necessarily involved any alteration in composition, 

 such as the change of state of water to steam, are physical 

 changes. 



The chemical changes are of two kinds : — Those which are 

 concerned with the decompositio7i of compounds into simpler 

 compounds or into elements, such as the change produced by 

 heating oxide of mercury, and those which are concerned with 

 combination and the formation of compounds, such as the 

 combustion of phosphorus. Very often a chemical change 

 involves both chemical decomposition and combination, as in 

 the action of steam on heated iron filings. It has been noticed 

 that chemical change is promoted by heat, but that whereas 

 chemical combination is accompanied by production of heat, 

 it has been necessary to put back heat into a substance to 

 decompose it. It is easily conceivable that if, owing to a force 

 of chemical attraction between two elements, heat is produced 

 when they unite, it would be necessary to employ a force, such 

 as heat, capable of overcoming this attraction, to decompose a 

 chemical compound. 



Two methods are therefore available for investigating the 

 composition of matter: — (i) Analysis^ i.e. splitting up the 

 compound in order to obtain its constituents ; (2) Synthesis, 

 i.e. putting together the constituents in order to obtain the 

 compound. The composition of red oxide of mercury was 

 found by analysis, that of carbonic acid gas by synthesis. 



These investigations have shown that chemical combination 

 takes place in definite proportions by weight. With 8 parts by 

 weight of oxygen, i of hydrogen, 3 of carbon, and too of 



