596 REPORT— 1886. 



detector is used, the amount of it put into the tube ought to be almost 

 infinitesimal, or it will cause disturbance and prevent the full velocity of 

 the ion being observed. 



There is some slight difficulty with a solid precipitate, as detector, 

 when free liquid instead of jelly is employed : for its settling causes 

 currents and convective disturbances — especially in the vertical ends of 

 the tube. Fluid detectors, whose colour the ions change, are therefore 

 in some respects better. 



Roughly examining the results obtained by the methods just described, 

 one may say that they give the velocity of Ba through jelly, for a fall 

 of 1 volt per centimetre, as about -00012 centimetre per second ; the 

 velocity of Sr is about -00015. The speeds of CI, Br, and I are not very 

 different from each other, and about "00024. This rough statement is 

 merely to give a notion of the order of magnitude of the velocities 

 obtained, and by no means represents the full deduction from the tabulated 

 numbers. 



Experiments on Speed of Hydrogen, 



To detect the motion of Hydrogen Mr. Robinson devised the following 

 arrangement : — We happened to have been using phenol-phthallein as a 

 detector of alkali in some other quite distinct experiment, and so it was a 

 handy substance. The jelly tube contains a little phenol-phthallein and 

 a trace of common salt, just made alkaline enough with soda to bring out 

 the colour. The solution in the anode vessel is H2SO4 ; in the cathode 

 vessel the same, or sometimes CUSO4. (For details see below, under 

 date August 17, p. 407.) 



The result is that SO4 travels one way, and Hj the other. As the H 

 travels, it liberates HCl, and decolorises the solution. As the SO4 travels, 

 it also decolorises the solution by forming neutral Na2S04. The velocity 

 of hydrogen, for 40 volts applied to a 40-centimetre tube, came out from 

 the very first observation thus made 



•0029 

 centimetre per second. Kohlrausch's theoretical number, deduced from 

 conductivity and migration data, is 



•003. 

 Later experiments gave respectively '0026 and ^0024. SO4 seems to 

 -travel at about one-third this speed. 



Another experiment was made with I^aHO in the cathode vessel, and 

 ■CUSO4 in the anode, and with NaOl and phenol-phthallein in the tube, as 

 before, but colourless (p. 409) . HO now travels against the current and pro- 

 duces colour as it goes. It seems to travel nearly as quickly as hydrogen. 



The following abbreviated excerpts from Laboratory Note-hooks will sufficiently 

 illustrate the results so far attained. 



Specific Gravity Data. — \1'7'6 grammes of crystallised baric chloride added to 

 88'27 grammes of water give a solution of sp. gr. 1'095, containing about 10 per 

 cent, of the salt itself. One-thirtieth of its bulk of water added reduces it to 1^093. 

 22-68 grammes of crystallised sodic sulphate added to 77'32 grammes of water give 

 a solution of specific gravity 1-093, likewise containing about 10 per cent, of the 

 salt itself. 19-98 grammes of hydrochloric acid added to 80-02 of water make 

 sp. gr. 1-098, and is a 25 per cent, solution. This also was reduced to 1-093. 



Experiment made of inverting a test-tube containing one of these liquors into a 

 jnenmatic trough containing another. In each case pretty rapid mixture resulted. 



