TRANSACTIONS OF THE SECTIONS. G5 



and vertically), yet the convexity of the sea exercises an influence on the outline of 

 very distant mountains which are seen beyond the horizon ; for these do not exhibit 

 as tliey approach the horizontal line any kind of break or change in the direction of 

 their slopes, as is usually observed in the forms of mountains which fall down to the 

 visible edge of the water, but their characteristic curves are cut off, as it were, mid- 

 way by the line of the horizon which conceals the sea-worn base of each mountain. 



CHEMISTRY. 



Address by Dr. Lyon Playfair, F.R.S., President of the Section. 



My predecessor in this chair, Sir John Herschel, drew our attention to the great im- 

 portance of studying, with increased accuracy, the combining proportions of bodies in 

 the hope of determining the exact numerical relations which prevail between the ele- 

 ments. He justly regarded it as a subject worthy of the most accurate experiment, 

 to ascertain whether the combining proportion of the Elements are multiples of the 

 combining number of hydrogen, as suggested by Prout; cautioning chemists at the 

 same time not to accept mere approximative accordances as evidence of this relation. 

 I have now to congratulate the Section on the publication of the laborious investigations 

 of Dumas on this important inquiry. 



It required a chemist of great manipulative skill, as well as of fertile experiment, to 

 obtain combining numbers for the elements upon which a greater reliance could be 

 placed than upon those determined with such admirable precision by Berzelius, that 

 great master of analysis. The atomic weights found by that chemist did not, for 

 many of the simple bodies, confirm the suggestion of Prout as to the multiple rela- 

 tions of these numbers to the equivalent of hydrogen. At the same time the more re- 

 cent determinations for the atomic weights of Carbon, Silver, and some other elements, 

 so closely coincided with this view, that it was very desirable to extend new experi- 

 ments to the bodies which had fractional atomic weights assigned to them. In 

 M. Dumas' memoirs there are the results, though not the details, of a large series of 

 experiments on many of the elements. He obtained numbers of precisely the same 

 value as those of the Swedish philosopher when he followed his methods of analysis — 

 numbers which are not the multiple of the equivalent of hydrogen. But when he pur- 

 sued his experiments upon these same elements, with the new methods of discovery 

 and his own inventiveness, then atomic weights were obtained which corrected them- 

 selves from the error inherent in former methods of analysis, and resulted in being 

 multiples of the combining proportions of hydrogen, or in standing in a very simple 

 relation to that number. There is on this point evidence so clear that there is scarcely 

 a chance of deception. 



The labours of Dumas, Schneider, Marignac, Pierre, Peligot, and others, have esta- 

 blished the relation by recent determinations of chlorine, iodine, bromine, silver, tita- 

 nium, &c, -elements differing so much in chemical character as well as in atomic weight, 

 that it is difficult to conceive any fortuitous combinations which could have produced 

 such uniformities in the results of analysis. Hence the general view of Prout, that the 

 equivalents of the elements, compared with certain unities, are represented by whole 

 numbers, seems to be established by recent experiment, although it would be premature 

 to declare that there are no exceptions to the law. We are familiar with many inge- 

 nious discussions on the natural grouping of the elements, and the relations of their 

 equivalent numbers to each other. I allude to the papers of Gladstone, Odling, and 

 Mercer, and to the views of Cook, in America. Although these efforts point to im- 

 portant dependences of the elements on each other, we cannot yet adopt them as parts 

 of our scientific system. Another question of a different character, as regards equiva- 

 lents, has recently received attention. I refer to the proposal to double the equiva- 

 lents of carbon and oxygen, that is, to raise them from 6 and 8, to 12 and 1G respect- 

 ively. As these two elements are essentially connected with the whole system of 

 chemistry, the right determination of their equivalents is a matter of extreme import- 

 ance. Undoubtedly there are cogent reasons which induce many of our able chemists 



1859. yf 5 



