130 ELEMENTS OF SCIENCE 



are mixed in water), or in an active, kinetic state (as 

 when they rush together in their effervescence), and 

 then as such chemical energy ceases and disappears, 

 it gives rise to, and is replaced by, a definite amount 

 of heat. Chemical combination therefore appears to 

 be an altogether different thing from a mere mixing 

 or blending of substances, each of which retains its 

 own essential characteristics. Different substances may 

 be merely mixed in any proportion, as we may colour 

 water the more by putting in it more indigo, or as 

 we may mix sand, or sulphur, and iron filings in 

 any quantity we like. All the masses thus produced 

 consist of minute separate particles, each of which 

 retains its own properties. But if we mix together 

 sulphur and iron filings and pour some warm water on 

 the mixture, we shall then have a manifestation of 

 chemical energy resulting in the production of some- 

 thing altogether different from the bodies which before 

 existed, more or less of which will be thus made to 

 disappear. The mixed mass becomes hotter, swells and 

 assumes a blackish colour, and the new body which thus 

 makes its appearance is what chemists call sulphide of 

 iron. Such is the difference between a mechanical 

 mixture and a chemical combination. 



A careful study of the process of resolving some sub- 

 stances into others (called analysis), and of the opposite 

 process of producing new substances by the junction of 

 others (called synthesis), shows us that chemical transfor- 

 mations take place in an exactly definite manner as 

 estimated by weight, 



Thus, in forming sulphide of iron, four equivalent 

 units of sulphur and seven of iron will disappear and 

 eleven units of sulphide of iron will be produced. If 

 there be more than this of either material, then such 



