1853.) OF THE ROYAL INSTITUTION. 327 
general rule, and that the molecule of ethyl only occupied half the 
space of the molecule of other bodies, so that the same space which 
contained one molecule of water truly contained two molecules of 
ethyl. This however is evidently but an arbitrary assumption to 
meet the case. Others said that, after all, the true ethyl remained 
yet to be discovered, and that this body was not it, but a hydrocarbon 
isomeric with it, for that the real ethyl would have only half the 
density of this body. 
There is, however, a third view, on which the ethyl of theory is 
also the ethyl of fact. Ona former-occasion I shewed reasons for 
believing that the elements are in a certain sense compound molecu- 
lar groupes, consisting of two or more atoms, which (in the present 
state of our knowledge) we must regard as similar, united to form 
a compound molecule. On this idea the gas hydrogen is re- 
presented, not by the symbol H, but as H H: and ethyl the analogue 
of hydrogen would also consist of a double atom, and be repre- 
sented not as C, H,, but as C, H, C, H,. 
The old view, however, had always a certain advantage over this, 
in the clear and consistent account which it gave of the mode of 
formation of hydrogen. How is it, it may be asked, and by what 
process, that this compound atom of hydrogen is formed? The 
answer is by no means obvious. Indeed the investigation of the 
nature of the process by which ethyl was formed, alone gave the 
key to its solution. 
Ethy] is not, in truth, made by the direct action of zinc upon the 
iodide of ethyl; but by the intervention of another body, which 
belongs to the class of, what I may term, fugitive or evanescent combi- 
nations, and which is made and decomposed again in the course of the 
experiment. This body is zinc-ethyl. The molecule of zinc, con- 
sisting of two atoms Zn Zn, splits into two parts. One atom, 
Zn, combines with the iodine of the iodide of ethyl, C, H, I, to 
form iodide of zinc, Zn I, while the other atom at the same moment 
combines with the ethyl, forming zinc-ethyl, Zn C, H,. 
The mode of action of zinc-ethyl upon iodide of ethyl is perfectly 
analogous to its action upon water. In contact with water, 
H H O, it immediately decomposes, forming hydrated oxide 
of zinc, Zn H O, and hydride of ethyl C, H, H. This hydride of 
ethyl has hardly more than half the density of the ethyl gas. 
In the same space in which, in the other case, are contained 
two heavy atoms of ethyl are here contained one heavy atom of 
ethyl and one light atom of hydrogen. Now the zinc-ethyl with 
the iodide of ethyl decomposes in a perfectly similar manner, 
forming iodide of zinc, Zn IJ, and ethyl gas C,H, C,H,. That 
this is truly the mode of the formation of the ethyl is proved by the 
fact, that by careful modification of the experiment, it is possible to 
break up this process of the formation of the ethyl into the two 
factors (so to say) of which it consists.* At a low temperature the 
* See Quarterly Journal of the Chemical Society, Vol. III. p. 405. 
