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characterised as the epoch of their circumstantial application to par- 

 ticular cases. The expression of the quantitative (volumetric or gravi- 

 metric) composition of substances now forms the most important pro- 

 blem of chemical research, and therefore the entire further exposition 

 of the subject is subordinate to stoichiometrical laws. All other 

 branches of chemistry are clearly subordinate to this most important 

 portion of chemical knowledge. Even the very signification of re- 

 actions of combination, decomposition, and rearrangement, acquired, as 

 we shall see, a particular and new character under the influence of the 

 progress of exact ideas concerning the quantitative relations of sub- 

 stances entering into chemical changes. Furthermore, in this sense 

 there arose a new and, up to then, unknown --division of compound 

 substances into definite and indefinite compounds. Even at the beginning 

 of this century, Berthollet had not made this distinction. But Prout 

 showed that a number of compounds contain the substances of which 

 they are composed and into which they break up, in exact definite pro- 

 portions by weight, which are unalterable under any conditions. Thus, 

 for example, red mercury oxide contains sixteen parts by weight of 

 oxygen for every 200 parts by weight of mercury, which is expressed 

 by the formula HgO. But in an alloy of copper and silver one or the 

 other metal may be added at will, and in an aqueous solution of sugar, 

 the relative proportion of the sugar and water may be altered and 

 nevertheless a homogeneous whole with the sum of the independent 

 properties will be obtained i.e., in these cases there was indefinite 

 chemical combination. Although in nature and chemical practice the 

 formation of indefinite compounds (such as alloys and solutions) plays 

 as essential a part as the formation of definite chemical compounds, yet, 

 as the stoichiometrical laws at present apply chiefly to the latter, all 

 facts concerning indefinite compounds suffer from inexactitude, and it 

 is only during recent years that the attention of chemists has been 

 directed to this province of chemistry. 



In chemical mechanics it is, from a qualitative point of view, very im- 

 portant to clearly distinguish at the very beginning bet ween reversible and 

 non-reversible reactions. One or several substances capable of reacting on 

 each other at a certain temperature produce substances which at the same 

 temperature either can or cannot give back the original substances. For 

 example, salt dissolves in water at the ordinary temperature, and the 

 solution so obtained is capable of breaking up at the same temperature, 

 leaving salt and separating the water by evaporation. Carbon bisul- 

 phide is formed from sulphur and carbon at the same temperature at 

 which it can be resolved into sulphur and carbon. Iron, at a certain 

 temperature, separates hydrogen from water, forming iron oxide, which, 



