1865.] Williamson On the Atomicity of Aluminium. 75 



as anomalous Deville's vapour-densities, which correspond to the higher 

 formulae APCl,Fe 2 Cr 5 . It is well known that Laurent and Gerhardt, 

 whose penetrating minds raised so many vital questions of chemical philo- 

 sophy, laid down a preliminary rule that every molecule must contain an 

 even sum of the atoms of chlorine, hydrogen, nitrogen, and metals. Accord- 

 ing to this rule, the formulae AP Cl a and Fe 2 Cl would have no greater pro- 

 bability than the formulae Fed 3 , A1C1 3 ; and judging by that rule, Dr. 

 Odling naturally preferred the simpler formulae. 



Since Gerhardt's time chemists have, however, extended to the greater 

 number of metals the arguments which proved oxygen to be biatomic ; and 

 we now know that the alkali-metals, the nitrogen series, silver, gold, and 

 boron, may count with the atoms of chlorine, hydrogen, &c. to make up an 

 even number in each molecule, but that the greater number of metals 

 must not be so counted ; for that in each molecule in which they are con- 

 tained the sum of the atoms of chlorine, hydrogen, nitrogen, potassium, &c. 

 must be even, just as much as if the atom of the diatomic or tetratomic 

 metal were not in the compound. In a paper " On the Classification of the 

 Elements in relation to their Atomicities," I had occasion to point out that 

 inasmuch as iron and aluminium belong, partly by their own properties, 

 partly by their analogies, to the class of metals which do not join with 

 chlorine, &c. in making up an even number of atoms, the number of those 

 other atoms in each molecule must be even in itself, just as if iron or alumi- 

 nium were not there ; and that accordingly the formulae Fe 2 Cl 8 , Al 2 01 

 are really quite normal. In like manner I showed that the vapour-density 

 of calomel, HgCl = 2 vols., is anomalous, as containing in a molecular 

 volume a single atom of chlorine, although, in accordance with Gerhardt's 

 rule, Dr. Odling had classed it as normal. I certainly understood that rny 

 able friend accepted my suggestion in this case at least, for he speedily 

 brought forward theoretical and experimental facts in confirmation of it. 



These examples serve to show that it was to be expected that the ethyle 

 and methyle compounds of aluminium would contain an even number of 

 atoms of ethyle and methyle in each molecule, and that their formulae 

 would accordingly be Al 2 Me 8 , AP Et 6 . 



It remains for us to consider how the deviation from our theoretical anti- 

 cipations in the case of aluminium ethyle and the partial deviation in the 

 case of aluminium methyle ought to be treated. 



Fortunately we have the benefit of some experience to guide us in 

 this matter, for a considerable number of other compounds have been 

 found to occupy in the state of vapour nearly double the volume which 

 corresponds to one molecule ; but, with very few exceptions, all of them 

 have already been proved to have undergone decomposition, so as to con- 

 sist of two uncombined molecules. Thus sal-ammoniac is admitted to have 

 the molecular formula NH 4 Cl ; yet in the state of vapour this quantity 

 occupies the volume of nearly two molecules, viz. four volumes. Has the 

 anomaly led us to doubt the atomic weight of chlorine, nitrogen, or hy- 



