592 REPORT — 1881. 



7. On Moleadar Attraction} By F. D. Brown, B.Sc. 



It is generally asserted by chemists that, when two or more elements combine 

 together to form a compound body, the forces of chemical affinity brought into play 

 are entirely neutralised by the act of combination. In this important respect, there- 

 fore, affinity is assumed to differ from gravitation where no neutralisation or 

 diminution of the attractive force results from the contiguity of two masses of 

 matter. The mere fact that chemical reactions take place, involving as they do the 

 interaction of atoms forming part of dissimilar molecules, shows that this assump- 

 tion of complete neutralisation is inconsistent with fact. 



If we go to the other extreme, and say that the act of combination produces no 

 change whatever in the chemical forces, but that the same attraction is excited 

 between any given pair of atoms, without regard to the state of combination of one 

 or both of the atoms, we are not only able to give a reasonable account of the 

 occurrence of chemical reactions, and to assert that there is nothing very remarkable 

 in the existence of molecular combinations, but we are also provided with a more or 

 less effective explanation of the relative volatility of substances. 



Let (»• s) equal the attraction at unit distance exerted between any two atoms 

 II and S ; it is clear that the attraction between two contiguous molecules, each 

 containing carbon, oxygen, and hydrogen, will be expressed as follows: — 



A(co) + B(cA) + C(ct') + D(o/0 + E(AA) + Yipo), 



whence A, B, 0, &c. are functions of the number and position of the corresponding 

 atoms. Applying this general equation, as best we can, to actual cases, and espe- 

 cially to those cases which admit of a tolerably general statement, we find that the 

 inter-molecular attraction should be greater in an acid than in the corresponding 

 alcohol, greater in an alcohol of high molecular weight than in a homologous one of 

 which the molecule is less complex, greater in a primary alcohol than in its secondary 

 or tertiary isomer, generally greater in a chlorinated compound than in the corre- 

 sponding substance containing hydrogen, and so on. If we allow that the Tolatility 

 of a substance is in some sense a measure of the forces of attraction between the 

 molecules, we must admit that the boiling points of the compounds of organic 

 chemistry would lead us to infer with much reason that the above expression rightly 

 represents the value of the inter-molecular attraction. The study of the latent heat 

 of vaporisation of many of the carbon compounds would seem, from this point of 

 view, to offer considerable chances of advancing the solution of the problem of 

 chemical affinity. 



8. Note on a netv method of Measuring certain Cliemical Affinities. 

 By Alfred Tribe, F.O.S. 



When a metal is immersed in an electrolytic field — i.e. in an electrolyte in the 

 act of electrolysis — and the electromotive force set up on any part of its surface 

 suffices to overcome the affinities of the radicals of the medium, the positive ion 

 separates on that part of the surface which may be supposed to have received 

 — electrification, and the negative ion on that part which receives + electrification. 



When the metal is in the form of a rectangular plate, and placed so that the 

 lines of force are perpendicular to its surface, the maximum electro-motive force 

 set up is on the central parts of such a plate, becoming less towards the edges, 

 where, and for some little distance from which, it is insufficient to initiate electro- 

 chemical action. 



"\^Tien the plate is placed in the electrolytic field so that the lines of force are 

 parallel with one of its edges, the maximum electro-motive force is on the end of 

 the plate, becoming less and less towards the central parts until it no longer suffices 

 to bring about electro-chemical action. This is denoted by the boundaries of the 

 deposits, which in many cases are very sharply defined. A plate, in fact, in the 

 position just named, may be regai'ded, in so far as electrical power is concerned, as 

 a series of pah's of electrodes, the limits of the electro-deposits representing a pair, 



' Phil. Mag. (5th series), 1881, vol. xii. p. 253. 



