ON ELECTROLTSIS IN ITS PHYSICAL AND CHEMICAL BEAEINGS. 225 



hydration both occur outside the true circuit of chang-e, they only afl'ect the tem- 



r nature of the cell, and have but an indirect and slight influence on the E.M.F. 

 therefore repeat that ' I venture to think that up to the present no experimental 

 disproof of Sir William Thomson's generalisation has been given.' In other words, 

 we may calculate the E.M.F. of a cell from a knowledge of the heat disturbances 

 corresponding to the various changes which occur in it, providing only that we know 

 accurately what are the changes which occur in the circuit ; in cases in which the 

 found and calculated value do not agree, we may fairly conclude that we have not 

 accurately realised what the changes are, and in this way the determination of E.M.F. 

 may be expected to afford most important aid in the study of chemical change. 



A discovery in electrolysis of much interest to chemists is that made by 

 Warburg and Tegetmeier, that rock crystal conducts electrolytically, but in the 

 direction of the principal axis alone; they conclude that conductivity is con- 

 ditioned by the presence of sodium in the form of sodium silicate, and they 

 consider that, according to the Clausius theory of electrolysis, this fact of electro- 

 lytic conduction only taking place in the direction of the principal axis would 

 tend to the inference that in the case of rock crystal not traversed by an electric 

 current the interchange of atoms between the molecules can only take place, at 

 any rate to a sensible extent, in the direction of the principal axis. It may, how- 

 ever, be suggested that probably in the original formation of the crystal the 

 impurity became included only in planes in the direction of the principal axis. 



APPENDIX. 



On the Discharge of Electrification by Flames. Bij A. M. WORTHINGTON, 

 Boyal Naval Engineering College, Devonport. 



1. It is well known that an ebonite or glass rod that has been electrified by 

 rubbing is immediately and completely discharged by passing it either through or 

 over a flame, for example, of a Bunsen burner. It is, I think, less well known, 

 though the observation seems to have been made by Priestley ', that the discharge 

 takes place with apparently equal rapidity if the rod be held at the side of, or even 

 below, the flame at a distance of, say, 5 cm. This is the case whether the Bunsen 

 burner be insulated or not, and it is equally true of a candle flame. 



A red-hot piece of iron or copper discharges a rubbed rod that is held very 

 near it, whether above, below, or at the side. If the rubbed rod be a very thick one, 

 say of 3 cm. diameter, it is not at once discharged on the side remote from the flame. 

 If held at a distance of, say, 8 cm. from the flame, a rubbed rod of 8 or 10 mm. in 

 diameter is less rapidly discharged than at the nearer distance, but is still dis- 

 charged in a few seconds ; but the rate of discharge diminishes very rapidly as the 

 distance from the flame is increased. 



2. If a plate of metal, insulated or uninsulated, or a piece of wire gauze of even 

 coarse textui'e be held with one side close to the flame and then a charged rod be 

 brought up to the other side of the plate, no discharge takes place. A plate of 

 glass or ebonite held in the same way between the flame and the rod equally pre- 

 vents discharge. 



But when the ch.arged rod and glass plate are withdrawn together from the 

 neighbourhood of the flame, then the glass plate is found to be charged on the side 

 nearest the flame with a charge of opposite sign to that of the charge on the rod. 



8. The above experimental facts, which will be more or less familiar to 

 physicists, are here cited in order that the significance may be perceived of the 

 experiment that will next be described. 



Being under the impression, which I fancy is pretty general, that the flame and 

 the surrounding air constituted a sort of conductor which removed the charge of 



' An historical resiivic of early observations on the electrical properties of flames 

 is given in the paper of Peter Riess, ' Ueber die elektrischen Eigenschaften bren- 

 nender Kiirper,' Pogg. Ann., vol. Ixi., 1844. 



1889. Q 



