210 DISCOURSE ON THE STUDY 



with each other only in fixed or definite proportions 

 by weight, and not arbitrarily ; so that when any 

 two substances are put together with a view to 

 unite them, if their weights are not in some certain 

 determinate proportion, a complete combination will 

 not take place, but some part of one or the other 

 ingredient will remain over and above, and uncom- 

 bined. Suppose, now, we have found a substance 

 having all the outward characters of a homoge- 

 neous or unmixed body, but which, on analysis, we 

 discover to consist of sulphur, and lead in the 

 proportion of 20 parts of the former to 130 of the 

 latter ingredient ; and we would know whether this 

 is to be regarded as a verification of the law of 

 definite proportions or an exception to it. The 

 question is reduced to this, whether the proportion 

 20 to 130 be or be not that fixed and definite pro- 

 portion, (or one of them, if there be more than one 

 proportion possible,) in which, according to the law 

 in question, sulphur and lead can combine ; now, 

 this can never be decided by merely looking at the 

 law in all its generality. It is clear, that when par- 

 ticularized by restricting its expression to sulphur 

 and lead, the law should state what are those par- 

 ticular fixed proportions in which these bodies can 

 combine. That is to say, there must be certain data 

 or numbers, by which these are distinguished from 

 all other bodies in nature, and which require to be 

 known before we can apply the general law to the 

 particular case. To determine such data, observ- 

 ation must be consulted ; and if we were to have 

 recourse to that of the combination of the two sub- 

 stances in question with each other, no doubt there 



