396 Royal Society : — ■ 



the methide and ethide might be expected to have vapour-volumes 

 corresponding to AP Me 6 = 2 vols., AP Et 6 == 2 vols. The authors 

 seem, however, more inclined to doubt the truth of the general prin- 

 ciples which lead us to consider these hexatomic formulae the correct 

 ones, than to doubt their own interpretation of the observations 

 already made upon the new compounds. 



Even if the vapour-volume of aluminic chloride had been unknown 

 to us, there were ample grounds for assigning to aluminium methide 

 a molecular formula Al 2 Me 6 , and a vapour-density corresponding to 

 Al 2 Me 6 = 2 vols. ; forthe close analogy of aluminic and ferric salts is 

 perfectly notorious, and the constitution Fe 2 O 3 for ferric oxide settles 

 Al 2 O 3 as the formula for alumina. With regard, however, to the 

 chlorides of these metals, it might be supposed that the formula 

 Fe CP and Al CP would be the most probable molecular formulae ; 

 and Dr. Odling, in his useful Tables of Formulae, published in 1864, 

 expressed an opinion in favour of these formulae by classing as ano- 

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

 formulae AP CP, Fe 2 CP. It is well known that Laurent and Ger- 

 hardt, whose penetrating minds raised so many vital questions of 

 chemical philosophy, laid down a preliminary rule that every molecule 

 must contain an even sum of the atoms of chlorine, hydrogen, nitro- 

 gen, and metals. According to this rule, the formulae AP CP and 

 Fe 2 CP would have no greater probability than the formulae Fe CP, 

 A1CP; 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 

 Diatomic ; 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 contained 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 pro- 

 perties, 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 aluminium were not there ; and that accord- 

 ingly the formulae Fe 2 CP, AP CP 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 my 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 



