1S13.) On the Cause ef Chemical Proportions. 453 



of an electro-negative body, were it not for the numerous 

 exceptions which exist to the rule, some very remarkable in- 

 stances of which I shall have occasion to notice in the sequel. 

 Hence it is always necessary, in order to discover those excep- 

 tions, and to verify the weight of the volume sought, to compare 

 the result of the preceding calculation with the known degrees 

 of oxidation of the substance whose volume is wanted. If, for 

 example, we find that an acid or electro-negative oxide is 

 neutralized by a quantity of base or electro- positive oxide which 

 contains £ of the oxygen in the acid, this acid will appear to 

 contain 3 volumes of oxygen. But if among the oxides of the 

 radicle of this acid we find one which contains half the oxygen 

 of the acid, it is clear that the saline combination in question is 

 an exception, and that the acid must contain 6 instead of 3 

 volumes of oxygen. I refer, for a farther explanation of this, 

 to what I shall say in the sequel concerning arsenic and chro- 

 mium. 



The preceding observations explain why, when a salifiable base 

 lias combined with more oxygen, it requires always an additional 

 volume of acid for every volume of oxygen which it has ab- 

 sorbed. It is for the same reason that oxygen appears to deter- 

 mine exclusively the composition of bodies; though there can be 

 no doubt that every element contributes equally to that compo- 

 sition. 



While treating in the sequel of each particular substance, I 

 shall explain the way in which I determine the weight of a 

 volume of it, and likewise state the experiments on which the 

 calculation is founded. As none of our experiments, except 

 from accident, can be perfectly correct, and as a small error in 

 the result often increases in the calculation, it is not possible 

 that my determinations can be perfectly exact : but I hope to 

 approach within very near limits, at least, of the truth. The 

 difference in the analytical results will point out to us the limits 

 of error, and show us degrees beyond which our determinations 

 cannot be incorrect. I shall give an account of these minima 

 and maxima indicated by experiment, as well as of the experi- 

 ments themselves, which point them out. We have, for ex- 

 ample, every reason to believe that a volume of sulphur weighs 

 201 : but some experiments raise it as high as 210, while others 

 sink it as low as 200. As we cannot determine at present which 

 of these numbers is most exact, it is good to know within what 

 limits our knowledge is uncertain. 



As far as 1 know, the English chemists Dalton, Davy, ami 

 Young, arc the only persons who have yet attempted to make 

 these determinations ; and they have proceeded in a manner 

 somewhat different. Mr. Dalton, to whom the honour of the 

 fust attempt is due, has endeavoured to determine the relative 



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