Prof. Williamson on the Atomicity of Aluminium. 397 



number of atoms of ethyle and methyle in each molecule, and that 

 their formulae would accordingly be AP Me 6 , Al 2 Et 6 . 



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

 tical anticipations 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 consist of two uncombined molecules. Thus 

 sal-ammoniac is admitted to have the molecular formula NH 4 CI ; 

 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 hydrogen, or 

 to doubt any other of the results of our comparison of their com- 

 pounds? or has it led chemists to diffusion experiments with its va- 

 pour, proving it to contain uncombined HC1 and NH 3 , each occupy- 

 ing its own natural volume ? Has it not been proved that at the 

 temperature at which sal-ammoniac vapour was measured, its con- 

 stituents mix either without evolving heat (that invariable function 

 of chemical action), or, according to another experimentalist, with 

 evolution of far less heat than of the whole quantity of hydrochloric 

 acid and ammonia combined, on coming together at that high tem- 

 perature ? 



Again, SO 4 H 2 is known to represent the formula of one molecule 

 of hydric sulphate, yet the vapour formed from it occupies nearly the 

 bulk of two molecules. Has this fact cast any doubt on the atomic 

 weights of the elements S, O, or H ? Or has it led to the discovery 

 of peculiarities in the constitution of the vapour which would pro- 

 bably have escaped notice had they not been anticipated by theory, 

 peculiarities which go a long way towards bringing the apparent 

 anomalies within the law ? 



Nitric peroxide, N 2 O 4 , was considered, from our knowledge of 

 other volatile compounds of nitrogen, to be anomalous in its vapour- 

 volume being N 2 4 =4 vols.; and we have been shown by the 

 experiment of Messrs. Playfair and Wanklyn, that the anomaly almost 

 disappears when the compound is evaporated by the aid of a per- 

 manent gas at a temperature considerably below its boiling-point, as 

 its theoretical molecule N 2 O 4 is then found to occupy the two volumes 

 which every undecomposed molecule occupies. This explanation 

 seems to me to be the more entitled to grave consideration on the 

 part of the discoverers of the new aluminium compounds, from the 

 fact that the evidence in favour of it has been admitted to be con- 

 clusive by Dr. Odling, who classes nitric peroxide by the formula 

 N 2 O 4 = 2 vols, among compounds with normal vapour-densities, in 

 virtue of the fact that at low temperatures it can be obtained with 

 that density, though having half that density at higher temperatures. 



The arguments for admitting that the low vapour-densities of the 

 aluminium compounds are anomalous are even stronger than those 

 which are admitted in the case of nitric peroxide ; for it did require 



