1813.] On the Cause of Chemical Proportions. 451 



could be assigned for the existence of chemical proportions. 

 Here, as well as in the theory of atoms, it is absolutely necessary 

 that in each compound one of the constituents should be consi- 

 dered as a single volume. 



It is evident that if the weight of the volumes of the elemen- 

 tary bodies be known, and expressed in numbers, we have 

 nothing more to do in every case of analysis but to count the 

 relative number of volumes of the constituent parts, whatever 

 the form of their aggregation may be : but in order to obtain the 

 relative weights of the elementary volumes expressed in num- 

 bers, that is to say, to obtain their specific gravity in the form of 

 gas, we must have a general measure with which we may com- 

 pare them. We may choose among the elementary bodies one, 

 the weight of a volume of which must be denoted by unity; 

 just as water has been chosen for unity in determining the speci- 

 fic gravity of liquids and solids. 



There are only two elementary bodies possessed of the requisite 

 qualities to serve as our unit. These are oxygen and hydrogen. 

 But hydrogen has disadvantages from which oxygen is free. The 

 weight of a volume of hydrogen is so small, that if we employ 

 it as our unit, the number representing a volume of some of 

 the metals becomes inconveniently great. Besides, hydrogen 

 enters much less frequently into compounds than oxygen; and of 

 course the number 100, when applied to hydrogen, does not 

 nearly so much facilitate calculation as when it is applied to 

 oxygen. Add that oxygen constitutes among elementary bodies 

 a particular class, and, as it were, the centre round which che- 

 mistry turns. It exists in the greater number of unorganic 

 bodies, and without exception in all the products of organic 

 nature. I think, then, that it is at once most convenient, and 

 most agreeable to the scientific views of chemistry, to take 

 oxygen as our unit. I shall represent its. volume by the number 

 100. 



The question which we have now to resolve is this, IV hat is 

 the specific ^xuity of all other elementary lodies in the form of 

 gas, compared with that of oxygen ? This question is not easily 

 answered : for at present there are no other bodies except oxygen 

 and hydrogen which we are capable of weighing in the state of 

 gas. All other bodies arc converted into gas at such hitih ■•tem- 

 peratures that it is not in our power to ascertain their weight. 

 We must therefore endeavour to discover the weight of their 

 volumes by other means. Our results will be, doubtless, very 

 uncertain ; but not altogether unsuccessful ; as 1 hope to be able 

 !iow in the sequel. 



In the fir t place, it appears reasonable to suppose tlmt bodies 

 ought to combine moit generally in etjual volumes : but in e.\:i 

 mining the greatest number of the combinations of elementary 



