416 Prof. Masson on the Boiling-points, Molecular 



The figures in the last two columns of the foregoing Table 

 afford, I think it may be claimed, sufficient proof of the truth 

 of the law under discussion, so far as the elements of the 

 seventh natural group are concerned. They show that 

 the different halides of the same radical give an almost 



y 



constant value for -^ , the deviation from perfect constancy 



in only two cases amounting to 2 per cent, of the total 

 value. But an inspection of the figures also proves, I think, 

 that the deviations, such as they are, are real : for in every 

 case but one the bromide has the lowest value, and in every 

 case but two the iodide has a value intermediate between those 

 of the chloride and the bromide. If we assume, as seems fair, 

 that the fluoride possesses the highest value in each group, 

 as it does among the phenyl compounds, then the magnitude 



of 7p- appears to undergo a small periodic variation within the 



group, falling from RF to RBr and rising again at RI to 

 almost the value of RC1. 



Other points are suggested ; but before entering into a dis- 

 cussion of these, it will be best to bring forward the evidence 

 which, as it appears to me, justifies the extension of the law 

 beyond the elements of the seventh group. Unfortunately 

 direct experimental evidence is not as yet available ; for 

 though the densities of some of the alkyl oxides and nitrides 

 at their boiling-points have been recorded, there is a total 

 absence of data with regard to the sulphides, phosphides, &c. 

 The boiling-points of many of these compounds are known, 

 however ; and a fair approximation to their molecular volumes 

 at the boiling-point can be almost certainly arrived at by 

 making use of the average atomic-volume values of Kopp or 

 Thorpe. Those here employed are C = ll, H = 5*5, S = 22*6, 

 P = 25'3, As = 26*3, Sb = 28'6, Si = 30'3, Sn = 40'8 (Thorpe, 

 Trans. Chem. Soc. xxxvii. p. 393). 



Table II. gives the results of such calculation. It will be 

 seen that, if the method may be trusted at all, proof is afforded 



V 



of the close approximation to a constant value for -^ in the 



compounds of the sixth, fifth, and fourth groups of elements. 

 It does not, of course, enable us to compare the extent of the 

 deviations from absolute constancy with that exhibited by the 

 compounds of the seventh group. The successive columns of 

 Table II. give (1) the formula of the compound, (2) the mole- 



