74 Williamson On the Atomicity of Aluminium. [Feb. 9, 



mentary to that of the corpus callosum. This is, moreover, a special 

 characteristic of the lowest group of the mammalia, most remarkable 

 because it is entirely lost in the next step of descent in the vertebrated 

 classes. 



After a description of the brain of a bird, the conclusion is arrived at 

 that, great as is the difference between the placental and implacental 

 mammal in the nature and extent of the connexion between the two 

 lateral hemispheres of the cerebrum, it is not to be compared with that 

 which obtains between the latter and the oviparous vertebrate. 



IV. "Note on tie Atomicity of Aluminium." By Professor A. \Y. 

 WILLIAMSON, F.R.S., President of the Chemical Society. Re- 

 ceived February 6, 1865. 



In the " Preliminary Note on some Aluminium Compounds," by Messrs. 

 Buckton and Odling, published in the last Number of the Society's ' Pro- 

 ceedings,' some questions of considerable theoretical importance are raised 

 in connexion with the anomalous vapour-densities of aluminium ethyle and 

 aluminium methyle. The authors have discovered that the vapour of 

 aluminium methide (Al 2 Me 6 ) occupies rather more than two volumes 

 (H=l vol.) at 163, when examined by Gay-Lussac's process, under less 

 than atmospheric pressure. The boiling-point of the compound under atmo- 

 spheric pressure is given at 130, and the compound accordingly boiled a 

 good deal below 130 at the reduced pressure at which the determination 

 was made. The vapour was therefore considerably superheated when found 

 to occupy a little more than two volumes. When still further superheated 

 up to 220 to 240, it was found to possess a density equivalent to rather 

 less than four volumes at the normal temperature and pressure. 



The aluminium ethyle was found to have a density decidedly in excess of 

 the formula Al 2 Et 6 = 4 vols., but far too small for Al 3 Me 6 = 2 vols. From 

 their analogy to aluminic chloride, Al 2 C1 G = 2 vols., the methide and ethide 

 might be expected to have vapour-volumes corresponding to AP Me 8 

 =2 vols., AP Et 6 = 2 vols. The authors seem, however, more inclined to 

 doubt the truth of the general principles 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 AP Me 6 , and a vapour-density corresponding to AP Me 6 = 2 vols. ; 

 for the close analogy of aluminic and ferric salts is perfectly notorious, and 

 the constitution Fe 2 O 3 for ferric oxide settles AP O 3 as the formula for alu- 

 mina. 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, pub- 

 lished in 1864, expressed an opinion in favour of these formulae by classing 



