PRESIDENTS ADDRESS—SECTION A. 31 
the idea of compounds being formed by the union of ultimate 
particles of their elements was indicated by Higgins (a). In 
1804, having firmly established, so far as the state of chemical 
analysis would then permit, the law of multiple proportions, 
Dalton (6) definitely erected thereupon his chemical atomic 
theory, viz., that (a) every element is made up of homogeneous 
atoms whose weight is constant, and (6) chemical compounds are 
formed by the union of atoms of the different elements in the 
simplest numerical proportion (c). This view reformed and 
vitalised chemistry. 
In 1660 Boyle had shown that gaseous volumes vary sensibly 
in the inverse ratio of the pressure (d). About 1786 Charles (e) 
discovered the relation connecting pressure or volume and tem- 
perature, often called Gay-Lussac’s law because first published 
by him in 1802 (f). The gaseous laws having thus been suffi- 
ciently determined (7), Gay Lussac (/), in 1808, defined the law 
of gaseous combination by volume (z), but did not definitely 
connect this with the atomic theory. This signal office was ful- 
filled by Avogadro (7) in 1811 in his affirmation that, the tem- 
perature and pressure being constant, equal volumes of gases 
contain equal numbers of molecules (#.e., moléeules intégrantes 
ou molécules constituantes), these, however, being themselves 
composed of atoms (molécules élémentaires) united by some form 
of mutual attraction (%). He showed that so far as was then 
known, each compound gas, formed by the union of one volume 
of one gas with one or more volumes of another, occupied two 
volumes (2). Although cases of combination are known where 
the volume is not doubled, this observation remains generally 
true, excepting at temperatures at which dissociation takes place 
27. Molecules, Atoms, and Micro-atoms.—In 1812 Davy (mi, 
declared his adherence to the doctrine that atoms unite to form 
groups of regular constitution, and in 1814 Ampére (7) endea- 
voured to establish definite conceptions regarding the arrange- 
(a) A comparative view of the phlogistic and antiphlogistic theories. 2nd Edit. 1791. 
(b) [1766 —1844.] 
(c) Communicated by Dalton to Dr. Thomson; first published in Thomson’s System of 
Chemistry, 1807. 
(d) New Experiments touching the Spring of the Air. 
(e) Jacques Alexandre César Charles. [1746—1823.] He did not publish his results; 
they became known to Gay-Lussac by accident. 
(7) Annal. de Chim., t. 43, pp. 1837—175. 
(g) An interesting fact in this connection is that the idea of the absolute zero was 
reached as early probably as 1702 by Guillaume Amontons [1662—1705], whose work gave 
—239.5 deg. In 1779, Lambert {[Pyrometrie, Berlin, p. 29], repeating Amonton’s experi- 
ments with greater accuracy, obtained—270.3 deg. 
(kh) [1778—1850]. 
(i) Mém. de la Soc. d’Arceuil, t. 2, p. 207. 
(j) [1776—1856.] 
(%) Journ. de Phys., 73. Juill. 1811. Pp.58—76. Also Févr. 1814. 
(2) Thus:—1 vol. H.H + 1 vol. Cl . Cl = 2 vols. H.Cl. 
2vols. H.H +1vol.O0 : O =2vols. H O.H. 
H 
3 vols. H.H +1vol.N : N=2vols. H.N 
‘-H 
