PRESENT PROBLEMS 297 



in the case of a mercury atom which is uncombined, or that the mo- 

 tions may be reversed. In accordance with the hypothesis outlined 

 above, we must assume that when two atoms separate either one 

 may become positive; dependent partly on their nature, partly on 

 the nature of the reacting substance. The conception here proposed 

 is that of something very similar to the action of the pole of a magnet, 

 which may attract another pole of the opposite kind, or induce the 

 formation of a pole of the opposite kind, or it may reverse the polarity 

 of another magnet. 1 This is, perhaps, simpler than to suppose the 

 transfer of an electron from one atom to another in those cases where 

 the electrical charges of the atoms are reversed in the ionization. 

 A very accurate determination of the atomic weight of cupric copper 

 as compared with that of cuprous copper might possibly decide be- 

 tween the two hypotheses. 



It should be noted that the hypothesis that the electrical charges 

 associated with the atoms are of a kinetic nature, and that these 

 charges may be transferred without gain or loss of matter, is quite 

 independent of the first hypothesis, which is that the atoms are 

 ionized when they separate from each other and that the same atom 

 may become either positive or negative. 



In following farther the thought of the attraction between elec- 

 trons as the cause of chemical combination, we must suppose that 

 in addition to the effect of this attraction in holding together the 

 atoms which are immediately attached, there is a residual effect 

 upon other atoms within the molecule. This gives a rational explana- 

 tion of the very great difference in the stability of the union between 

 carbon atoms in different compounds as, for instance, the instability 

 of acetic acid in comparison with butyric acid, occasioned by the sub- 

 stitution of an oxygen atom for two hydrogen atoms of the latter. 

 The study of organic compounds has given us a knowledge of a large 

 number of cases of .this sort, and our text-books contain many em- 

 pirical rules about them, but there have been few, if any, attempts 

 to give for such facts any rational explanation. 



In considering double unions three explanations suggest themselves : 

 (1) We may suppose with Pfeifferthat such unions are in reality single 

 unions and free valences. In this case the presence in adjacent car- 

 bon atoms of positive and negative electrons which are uncombined 

 would reduce the attraction of each for the electrons of another 

 molecule, thus explaining why two free valences are so much less 

 active than a single one. (2) We may suppose that the carbon atoms 

 are in reality doubly united, but that, owing to the localization of 



1 This is, of course, only an analogy and must not be pressed too far; just as 

 the electrical charges of atoms or ions conduct themselves very differently from 

 those of masses. The latter divide themselves between two bodies in contact; the 

 former may be transferred completely from one ion to another. 



