LAWS AND THEORIES OF CHEMISTRY. 95 



exact analysis since his time, and which is one of the most important 

 foundation-stones upon which the structure of chemical science is 

 reared. The law of multiple proportions maybe stated thus: If 

 tiro dements, A and B, are capable of uniting in several proportions, 

 the quantities of B which combine with a fixed quantity of A bear a 

 xitnple ratio to each other. 



Besides the above illustrations of the law, several other examples 

 may be mentioned. Sulphur and oxygen unite in two proportions to 

 form two distinct compounds, one a gas known as sulphur dioxide, 

 the other a solid known as sulphur trioxide. In these the quantities 

 of oxygen, united with a fixed weight of sulphur, are in the ratio of 



1 : 1 J or 2 : 3. There are two sulphides of iron, known respectively 

 as ferrous sulphide and pyrite ; in these the weights of sulphur 

 united with a fixed weight of iron are in the ratio of 1 : 2. The 

 phrase simple ratio, in the law, means in the ratio of small whole 

 numbers. This feature of the law is strikingly illustrated in the 

 case of the four compounds containing potassium, chlorine, and 

 oxygen, in which the variable weights of oxygen united with fixed 

 weights of potassium and chlorine are in the ratio of 1:2:3:4; 

 also in the case of the five compounds of nitrogen and oxygen, in 

 which the weights of oxygen united with a fixed weight of nitrogen 

 bear the ratio of 1 : 2 : 3 : 4 : 5. 



Combining- weights of elements. The proportions in which any 

 two elements unite may be expressed by any two figures which stand 

 in the proper ratio. Thus we may say that water is made up of 

 hydrogen and oxygen in the proportions by weight of 1 to 8, or 



2 to 16, or 5 to 40. Similarly we may state the composition of 

 ferrous sulphide as 7 parts of iron to 4 parts of sulphur, or 56 parts 

 of iron to 32 parts of sulphur. Expressing the composition of com- 

 pounds thus in a random manner, there seems to be no relationship 

 between the relative quantities with which the different elements 

 unite with one another. But upon closer inspection of the propor- 

 tions by weight in which the elements unite, it is found chat they can 

 be reduced to a system of figures which show a remarkable relation- 

 ship. It is found, purely from the results of analysis and inde- 

 pendently of any theory, that a certain figure can be assigned to each 

 element, which has the remarkable property that it or a simple mul- 

 tiple of it expresses the relative proportion by weight in which that 

 element unites with the other elements. Chemists soon became 

 aware of the fact that hydrogen is the lightest of all known matter 



