iNTHonrrnox 8 



change or reaction, 4 as it is called, can only take place under a condi- 

 tion of most intimate and close contact of the reacting substances, 5 and 

 is determined by the forces proper to the smallest invisible particles 

 (molecules) of matter. We must distinguish three chief classes of 

 chemical transformations. 



1. Combination is a reaction in which the union of two substances 

 yields a new one, or in general terms, from a given number of sub- 

 stances a lesser number is produced. Thus, by heating a mixture of 

 iron and sulphur 6 a single new substance is produced, iron sulphide, in 

 which the constituent substances cannot be distinguished even by the 

 highest magnifying power. Before the reaction, the iron could be 

 separated from the mixture by a magnet, and the sulphur by dissolving 

 it in certain oily liquids ; 7 in general, before combination they might 

 be mechanically separated from each other, but after combination both 

 substances penetrate into each other, and are then neither mechanically 

 separable nor individually distinguishable. As a rule, reactions of 

 direct combination are accompanied by an evolution of heat, and the 

 common case of combustion, evolving heat, consists in the combination 

 of combustible substances with a portion (oxygen) of the atmosphere, 



ment. Therefore, mechanics, which treats of movement, forms the fundamental science 

 of natural philosophy, and all other sciences endeavour to reduce the phenomena with 

 which they are concerned to mechanical principles. Astronomy was the first to take 

 to this path of reasoning, and succeeded in many cases in reducing astronomical to 

 purely mechanical phenomena. Chemistry and physics, physiology and biology are 

 proceeding in the same direction. 



4 The verb ' to react ' means to act or change chemically. 



5 If a phenomenon proceeds at visible or measurable distances (as, for instance, 

 magnetic attraction or gravity) it cannot be ascribed to chemical phenomena, which are 

 only accomplished at distances immeasurably small and undistinguishable to the eye or 

 the microscope ; that is to say, which belong to the number of purely molecular pheno- 

 mena. When a change of material is accomplished within a substance without visible 

 motion or the interference of foreign matters (for instance, when new wine ' ages ' by 

 keeping, and acquires a peculiar aroma), it may be classed as a chemical phenomenon ; but 

 the ordinary cases of chemical reaction are accomplished by the mutual action of different 

 substances which, previously free, on reaction mutually permeate each other. 



f> For this purpose a piece of iron may be made red hot in a smith's furnace, and then 

 placed in contact with a lump of sulphur, when iron sulphide will be obtained as a 

 molten liquid, the combination being accompanied by a visible increase in the glow of 

 the iron. Or else iron filings are mixed with powdered sulphur in the proportion of 

 5 parts of iron to 3 parts of sulphur, and the mixture placed in a glass tube, which is 

 then partially heated. Combination does not commence without the aid of external 

 heat, but when once started in any portion of the mixture it extends throughout the 

 entire mass, because the portion first heated evolves sufficient heat in forming iron 

 sulphide to raise the adjacent parts of the mixture to the temperature required for 

 starting the reaction. The rise in temperature thus obtained is so high as to soften the 

 glass tube. 



7 Sulphur is slightly soluble in many thin oils; it is very soluble in carbon bisulphide 

 and in some other liquids. Iron is insoluble in carbon bisulphide, and therefore the 

 sulphur can be dissolved away from the iron. 



B 2 



