282 Mr. Exleys Application of 



case can occur : therefore, the resulting volume will be 

 always exactly one or two, however, many volumes com- 

 bine. 



Cor. This proposition embraced the theory of volumes, 

 which it explains, simplifies, and extends. 



The important law contained in this proposition, in its 

 whole extent, has not been before determined, although 

 approaches towards it have been made in the theory of 

 volumes : and after Gay Lussac had discovered that theory, 

 a striking relation between the atomic weights of bodies, 

 in the gaseous form, and their specific gravities could not 

 long remain unobserved ; and this was particularly noticed 

 by Dr. Prout; but the exact and remarkable law just de- 

 monstrated, has, I believe, never been made clearly out in 

 its generality, but many particular cases have been esta- 

 blished. As far as experiments go they serve to confirm 

 the above mathematical conclusion ; and this demonstra- 

 tion would not have been the less valuable, had the law 

 been established in its full extent by observation of facts : 

 for even in that case, it could havebeen received only as 

 an ultimate fact utterly inexplicable. 



From this proposition and the new theory, we have the 

 farther advantage of knowing in what manner the atoms 

 may be combined : and that frequently the combinations 

 of the same atoms admit of different forms, producing 

 isomeric bodies: also the manner of combining being 

 given, the theory foretells perfectly the resulting volume. 



Having deduced this exact law from my two simple prin- 

 ciples, it became to me exceedingly important to ascertain 

 if this result is really true in fact. To determine this 

 point, I have calculated from this law the specific gravities 

 of a great number of compounds supposed to be in the 

 gaseous form, having regard to the combinations as to their 

 being only cohesive, or in single or double groups, and in 

 doubtful cases, calculating the specific gravity both on the 

 supposition of single, and that of double groups. I then 

 collected all the substances I could find of which the specific 

 gravities had been determined by experiment, and have 

 put them to the amount of 57, in a tabular form, from 

 which it appears that there is a complete agreement within 

 the allowable errors of such experiments, except in boro- 



