1852.) OF THE ROYAL INSTITUTION. 201 
Portraits of Sir B. C. Brodie and the Rey. T. Eagles, and Sketch of 
a Poor Irish Girl, by J. Z. Bell, Esq. 
15 Ammonites from the Secondary Formations, Exhibited by Mr. 
Tennant ; and an Engraving of the ‘‘ Ammonites Heterophyllus,” 
presented by him to the Royal Institution Library. 
Mr. Varley showed by the Microscope the beating of the heart in 
the Monoculi and Wheel-animalcules. 
WEEKLY EVENING MEBTING, 
Friday, May 21. 
W. R. Grove, Ese., M.A., F.R.S., Vice-President, 
in the Chair. 
B. C. Broniz, Ese. 
On the Allotropic Changes of Certain Elements. 
Tue earliest conception of the nature of a chemical substance was 
limited to the knowledge of the ultimate or elemental particles into 
which it could be broken up. To this after a time was added that 
of the proportion in which these elements were combined. But this 
too proved inadequate to explain the chemical differences of bodies, 
especially their dynamic differences, that is, the different modes of 
change of which they are susceptible—why for example, from 
certain bodies containing many atoms of hydrogen, one of these 
atoms can readily be removed and replaced by a metal, while no 
skill has yet effected a similar exchange witha second. The progress 
of discovery, moreover, established beyond a doubt the existence of a 
class of bodies consisting of the same elements, combined in the 
very same proportions, which yet differed in their chemical and 
physical properties. To meet these and other difficulties, gradually 
arose an idea new to chemical science, the idea of structure or 
chemical form, in the elaboration of which, chemists of late years 
have been principally engaged. The way in which this conception 
has been applied to explain the relation of isomeric bodies may be 
seen by the following illustration. Representing water as : O, alcohol 
is represented as C,H, O= C, H, O, and methylic ether as 
p H q 146 y: 
Be sg, 20 tO) This lost. substanca sia idancioel Awan 
C, H, 
alcohol in its elemental constitution, but differs from it in its 
