214 PRINCIPLES OF CHEMISTRY 



the law of multiple proportions, because water contains eight parts and 

 hydrogen peroxide sixteen parts of oxygen to one part of hydrogen, and 

 these figures are commensurable and are in simple proportion as 1 : 2. 



An exactly similar multiple proportion is observed in the composition 

 of all other well-investigated definite chemical compounds, 30 and there- 

 fore the law of multiple proportions is accepted in chemistry as the 

 starting point from which other considerations are judged. 



The law of multiple proportions was discovered at the very 

 beginning of this century by John Dalton, of Manchester, in investigat- 

 ing the compounds of carbon with hydrogen. It appeared that two 

 gaseous compounds of these substances marsh gas, CH 4 , and olefiant 

 gas, C 2 H 4 , contain for one and the same quantity of hydrogen quanti- 

 ties of carbon which stand in multiple proportion ; namely, marsh gas 

 contains relatively half as much carbon as olefiant gas. Although the 

 analysis of that time was not exact, and did not give Dalton results 

 in complete accordance with truth, still the accuracy of this law, 

 recognised by Dalton, was confirmed by further more accurate investiga- 

 tions. On establishing the law of multiple proportions, Dalton gave a 

 hypothetical explanation for it. This explanation is based on the 

 atomic theory of matter. In fact, the law of multiple proportions is 

 understood with unusual ease by admitting the atomic structure of 

 matter. 



50 When, for example, any element forms several oxides, they are subject to the 

 law of multiple proportions. For a given quantity of the non-metal or metal the 

 quantities of oxygen in the different degrees of oxidation will stand as 1 : 2, or as 1 : 3, or 

 as 2 : 3, or as 2 : 7, and so on. Thus, for instance, copper combines with oxygen in at 

 least two proportions, forming the oxides found in nature, and called the suboxide and 

 the oxide of copper, Cu 2 O and CuO ; the oxide contains twice as much oxygen as the sub- 

 oxide. Lead also presents two degrees of oxidation, the oxide and peroxide, and in the 

 latter there is twice as much oxygen as in the former, PbO and PbO.,>. The substance 

 known under the name of minium, and which is somewhat widely used as a red paint, 

 is only a mixture of the mutual compounds of these oxides, which is proved not only by 

 the inconstancy of its composition, but also by the fact that reagents capable of extract- 

 ing the oxide of lead, especially acids, do actually extract it and leave lead peroxide. 

 When a base and an acid are capable of forming several kinds of salts, normal, acid, basic, 

 and anhydro-, it is found that they also clearly exemplify the law of multiple proportions. 

 This was demonstrated by Wollaston soon after the discovery of the law in question. We 

 saw in the first chapter that salts show different degrees of combination with water of 

 crystallisation, and that they obey the law of multiple proportions. And, more than 

 this, the indefinite chemical compounds existing as solutions may, as we saw in the same 

 chapter, be brought under the law of multiple proportions by the hypothesis that solu- 

 tions are unstable hydrates formed according to the law of multiple proportions, but 

 occurring in a state of dissociation. By means of this hypothesis the law of multiple 

 proportions becomes still more general, and all the aspects of chemical compounds are 

 subject to it. The direction of the whole contemporary state of chemistry was deter- 

 mined by the discoveries of Lavoisier and Dalton. By bringing indefinite compounds 

 also under the law of multiple proportions we arrive at that unity of chemical conceptions 



