ON ELECTROLTSIS IN ITS PHYSICAL AND CHEMICAL BEARINGS. 393 



1. Proportion of ingredients in original solution. 



2. Initial weight of cathode vessel. 



3. Final weight of ditto. 



4 and 5. Any two of the following three things : the amount of Cu, or 

 of SO4, or of free H2SO4, in the final solution. 



The object of the long tube is to be sure that the current shall have 

 io travel through the unaltered original solution in some part of its 

 •course ; also to preserve uniform some liquid near the constriction. 



I was under the impression that this scheme of experiment was more 

 complete than any that had been previously attempted, but I have since 

 looked up Hittorf's papers,' and found them very admirable. I do not 

 say that nothing better can be done, but I have done nothing better yet, 

 and therefore shall not at present rehearse the mode of treating the above 

 -data in order to extract from them their meaning, especially as it is long 

 -enough for a separate paper. 



DiEECT Experimental Determination op Absolute Ionic Velocities. 



Let us pass now from this rather laborious method of determining 

 T^'jMi, N,v,, &c., to a simple and direct mode of experimenting on the 

 velocitiestti, ^1, &c., themselves. It is manifest that if one can determine for 

 any substance, Nu by one method of experiment, and u by another, its dis- 

 sociation-ratio N fm, which must be a very important chemical constant, 

 is known instantly. 



One of the early forms of experiment for observing ii and v was 

 arranged thus : — 



Two vessels, containing fairly strong sulphate of soda and baric 

 chloride, respectively, adjusted to equal density, were joined by a longish 

 tube full of dilute hydrochloric acid p „ 



■of small specific gravity (fig. 2). 



A current from some twenty stor- p^ jpj 



age batteries was then applied, and 

 the tube examined from time to time 

 for the first appearance of a precipi- 

 tate of baric sulphate. 



When everything went well the W^ 



precipitate appeared as a fine ring 



inside the tube, which rapidly filled up into a beautifully sharp thin 

 disc or complete partition, and then grew in thickness, spreading out 

 slowly both ways till it formed a solid plug and rather obstructed the 

 current. 



The locality of the disc and the time of its appearance after starting 

 the current were noted ; but I do not here record these first results, be- 

 -cause they were very variable, owing to disturbing causes. 



One obvious disturbing cause is that of slight differences of level, 

 produced either by earth- war pings and local pressures, or by evaporation. 

 To diminish mechanical changes of level the two vessels were next 

 arranged side by side, and the tube was bent double so as to have its ends 

 close together and yet to afibrd a good length for observation. 



The level of the vessels was accurately adjusted, and the tube intro- 

 •duced with as little disturbance as possible, various devices, such as super- 



' Pogff. Ann., vols. Ixxxix., xcviii., ciii., cvi. 



