1886.] Conduction and Molecular Composition, Sfc. 285 



It is difficult to over-estimate the importance of the theory of 

 valency: its application has led to an enormous extension of our 

 knowledge of carbon compounds especially, and it has furnished us 

 with a simple and consistent system of classifying the mighty host of 

 these bodies ; but on the other hand, it may be questioned whether it 

 has not led us away from the search into the nature of chemical 

 change, and even if the introduction of the terms saturated and 

 unsaturated has not had a directly pernicious effect. The almost 

 universal disregard of molecular composition as an important factor in 

 chemical change in the case of solids and liquids, and the popular 

 tendency to overlook the fact that our formulae of such bodies are 

 purely empirical expressions, has undoubtedly exercised a prejudicial 

 influence. 



No known compounds are saturated — if any were, such would be 

 incapable, I imagine, of directly taking part in any interaction, and in 

 their case decomposition would necessarily be a precedent change. 

 The paraffins are apparently of all bodies the most inert and the most 

 nearly saturated, and next to them comes hydrogen — the unsaturated 

 character of which is displayed in interactions such as occur at 

 atmospheric temperatures between it and platinum and palladium, and 

 when it displaces silver from silver nitrate or certain of the platinum 

 metals from their salts. One of the most striking instances perhaps 

 of popular error in this respect is water, which is always regarded as. 

 a saturated compound, although its entire behaviour and especially its 

 physical properties characterise the molecule H 2 0, 1 think, as that of an 

 eminently unsaturated compound : I fail to see how otherwise we are 

 to explain the high surface tension and high specific heat of liquid 

 water, its high heat of vaporisation, and its imperfectly gaseous beha- 

 viour up to temperatures considerably above its boiling point, let alone 

 its great solvent power and its tendency to form hydrates with a mul- 

 titude of compounds — especially oxygenated compounds, be it added. 



The theory was brought most prominently under the notice of 

 chemists by Helmholtz in the last Faraday lecture that electricity,, 

 like matter, is as it were atomic, and that each unit of affinity or 

 valency in our compounds is associated with an equivalent of elec- 

 tricity — positive or negative ; that the atoms cling to their electric 

 charges and that these charges cling to each other. Thus barely 

 stated, this theory does not appear to take into account the fact that 

 the fundamental molecules even of so-called atomic compounds are 

 never saturated, but more or less readily unite with other molecules to 

 form molecular compounds — molecular aggregates ; and unless the 

 application of the theory to explain the existence of such compounds 

 can be made clear, chemists must, I think, decline to accept it.* 



* It is noteworthy that Clerk Maxwell (" Electricity and Magnetism," 1873, vol. i, 

 p. 313), when speaking of the theory of molecular charges, says, "This theory of 



