894 



SCIENCE. 



IN. S. Vol. XV. No. 



is inverted by acetic acid alone, and with 

 varying quantities of sodium acetate added 

 to it. But as such an experiment cannot 

 be made visible to a large number of specta- 

 tors at once, I thought of a way of showing 

 the same thing, which, while not capable 

 of the same degree of accuracy, would 

 prove the principle qualitatively. I have 

 here a solution of ferrous acetate to which 

 I have added enough acetic acid to prevent 

 the precipitation of ferrous sulpliide on the 

 addition of sulphiiretted hydrogen. I add 

 sulphuretted hydrogen; of course no pre- 

 cipitate is formed. I now add a solution 

 of sodium acetate mixed with rather more 

 than three equivalents of acetic acid, so as 

 to make it plain that the effect is not due to 

 the formation of an acid acetate, and you 

 see that we have at once a precipitate of 

 ferrous sulphide. To show that the addi- 

 tion of the water has not produced the re- 

 sult, I add to another portion of the same 

 solution as much water, and you see that no 

 precipitation takes place. 



I have not spoken of non-aqueous solu- 

 tions. At the rise of the dissociation the- 

 ory, these were generally supposed to be 

 non-conductors, but many of them have 

 now been examined both by scientific work- 

 ers in the old world, and very specially by 

 our colleagues on the other side of the 

 Atlantic, and have been found to conduct 

 electrolytically. It seems likely that these 

 investigations will throw much light on 

 the influence of the solvent on the conduc- 

 tivity of the dissolved salt. Particularly 

 interesting is the relation, indicated in some 

 cases, between the specific inductive capa- 

 city of a solvent and the dissociation of the 

 dissolved salt. But this is one of the ques- 

 tions not yet ripe for treatment in a dis- 

 course such as this. 



I had also thought of saying something 

 as to the atomic character of electricity, 

 and the compounds of electricity with what 

 we may venture to call the other chemical 



elements, and had even some idea of poach- 

 ing on Lord Kelvin's domain of 'Aepinus 

 atomized, ' but time has saved me from this. 



I have been describing the history of the 

 theory of electrolysis from the time of Para- 

 day, in such a way as is possible within the 

 limits of an hour. I have necessarily omit- 

 ted mention of many active, able and suc- 

 cessful workers, and I cannot in every case 

 justify the omission except by referring to 

 the time limit. I have as far as I could 

 explained the evidence which we have for 

 the theories described, but I have not in- 

 tended to argue for or against the essen- 

 tial truth of them. I have sometimes been 

 asked in reference to the theory of electro- 

 lytic dissociation, Do you really believe it 

 to be true ? My answer to that question is,. 

 I believe it to be an eminently useful theory. 

 It has led to a great deal of most valuable 

 experimental work. It has enabled us to 

 group together things that without its help 

 seemed very little connected. It has led 

 to the discussion of problems that could 

 scarcely, without its suggestion, have oc- 

 curred to any one. It does not seem to be 

 exhausted, and I look forward to much 

 good to be got from it yet, and therefore 

 I am willing to take it as a guide. But 

 I do not look on it as an infallible guide; 

 we cannot expect, we do not need, an in- 

 fallible guide in physical science. A long 

 life may be anticipated for this theory; if 

 that be so, we may be sure that it will un- 

 dergo modifications, for if it is to act, it 

 Avill be acted on. 



Nothing but good can come from the 

 fullest discussion, either of the theoretical 

 basis or of the experimental evidence for 

 or against a theory. No great principle 

 in science or in law can be satisfactorily 

 settled without full argument by competent 

 advocates on both sides, and the eager hunt 

 for evidence by those who attack and by 

 those who defend will lead to a more com- 

 plete investigation of the whole field than 



