33 REPORT 1864. 



into our daily life." Hence the great difficulty of effecting a speedy change in 

 chemical usages alike so time-honoured and intimately ramified with the affairs of 

 our everyday existence. I propose, by your permission, to make a few remarks 

 upon the history of this chemical reformation, more especially in connexion with 

 certain points which one or two of its acknowledged leaders have scarcely, I 

 think, correctly estimated. 



From the time when Dalton first introduced the expression " atomic weight," up 

 to the year 1842, when Gerhardt annoimced his views upon the molecular con- 

 stitution of water, there does not appear to have been any marked difference of 

 opinion among chemists as to the combining proportions of the principal elements. 

 That 1 part by weight of hydrogen united with 36 parts by weight of chlorine to 

 form a single molecule of hydrochloric acid, and with 8 parts by weight of oxygen 

 to form a single molecvde of water, was the notion both of Berzelius and Gmelin, 

 who may be taken as representatives of the two chief Continental schools of theo- 

 retic chemistry. Indeed, no doubts seem to have been entertained in their time 

 as to the combining proportions of the three elements. Using the hydrogen scale 

 of numbers, both chemists represented the combining proportion of hydrogen as 1, 

 that of chlorine as 36, and that of oxygen as 8. Both, moreover, represented the 

 molecular weight of hydrochloric acid as 37, and the molecidar weight of water 

 as 9. It is true that Berzelius professedly regarded the single combining propor- 

 tions of hydrogen and chlorine as consisting each of two physical atoms ; but, since 

 the two atoms of hydrogen, for instance, which constitvited the one combining 

 proportion of hydrogen, were chemically inseparable from one another, they were 

 really tantamount to one atom only of hydrogen, and, in point of fact, were always 

 employed by Berzelius as representing the single chemical atom of hydrogen, or its 

 smallest actual combining proportion. Distinguishing thus between the physical 

 atom and the combiningproportion,Berzelius'srecognitiouof the truth, that equal vo- 

 lumes of the elementai-y gases contain an equal number of atoms, was utterly barren. 

 But, identifying the physical atom -with the combining proportion, Gerhardt's re- 

 cognition, or rather establishment, of the broader truth, that equal volumes of fill 

 gases, elementary and compound, contain the same number of atoms, has been in 

 the highest degree prolific. From Gerhardt's division of volatile bodies into a 

 majority whose recognized molecules corresponded respectively with four volumes 

 of vapour, and a minority whose recog-nized molecules corresponded respectively 

 with but two volumes, kud from his proposal, in conjunction with Laurent, to 

 double the molecular weights of these last, so as to make the molecules of all 

 volatile bodies, simple and compound, correspond each with four volumes of vapour, 

 must, I conceive, be traced the development by himself and others of the matured 

 views on chemical philosophy which now prevail. With evei-y respect for my 

 predecessor in this chair, and for the accomplished author of the ' Lemons de Philo- 

 sophic Chimique,' from neither of whom do I ever venture to difier without fear 

 and trembling, I cannot join with them in regarding the initiation of Gerhardt's 

 system as an imperfect retm-n, and its remarkable maturation in these recent days 

 as a more complete return to the notions of Berzelius. Although, indeed, the ele- 

 mentary weights now employed, with the exception of those for some half-dozen 

 metals,' are identical with the atomic weights of Berzelius, yet so unlike are they 

 to his combining weights that fully four-fifths of all known compounds have to be 

 expressed by formula entirely different from his — namely, all those bodies, with 

 but very few exceptions, into which hydi'ogen, fluorine, chlorine, bromine, iodine, ni- 

 trogen, phosphorus, arsenic, boron, and the metals lithium, sodium, potassium, silver, 

 and gold, enter as constituents. Fully adiiiitting that the new system of atomic 

 weights, as it now exists, is the joint product of many minds — fully admitting 

 that it owes its present general acceptance chiefly to the introduction of the water 

 type by Williamson dming Gerhardt's lifetime, and the recognition of diatomic 

 metals by Wurtz and Cannizzaro, after his decease — and fully admitting, moreover, 

 that some of Gerhardt's steps in the development of his unitary system were de- 

 cidedly, though perhaps excusably, retrograde, I yet look upon him, not I trust 

 with the fond admiration of the pupil, but with the calm judgment of the chemist, 

 as being the gi-eat founder of that modern chemical philosophy in the general spread 

 of which I have already ventured to congratulate the Members of the Section. 



