TRANSACTIONS OF .SECTION A. G41 



owing to au increase in the pressure of the contained gas. We see, therefore, that 

 in the first place, the bright points of light upon the surface of the cathode are 

 due to the emission of fine jets of gas ; also that, such jets, if negatively charged, 

 may he made to cast shadows of objects suitably placed in their paths. It is at 

 the same time clear that the green flecks referred to above are due, during the 

 passage of a discharge, to these same jets of gas impinging upon the inner side of 

 the glass walls, and that, under the conditions existing immediately subsequent 

 to the passage of the discharge, streams of gas continue to be emitted from 

 the pores of the electrodes. In spite of the fact that (with the electrodes con- 

 nected to earth) these streams consist of unelectrified particles, they assume 

 that property of ionisation during their passage across the space within the 

 bulb which appears to be essential to the production of local fluorescence in 

 the glass upon which they impinge. 



This latter eSect, while explaining the process by which a cathode emission 

 may appear to originate at an electrode with no electrical charge upon it, is one 

 to which I wish to draw especial attention, for there is reason to believe that the 

 speed at which the individual particles, constituting the jets, move cannot exceed 

 the rate at which sound is propagated in the rarefied medium. 



It is therefore interesting to find that, under such circumstances, fluorescence 

 was produced in the glass upon which the streams impinged. 



As many of the observations referred to here were made during the course o£ 

 an investigation into the diselectrifying action of magnetism now being carried 

 out by myself at the Davy-Faraday Laboratory of the Royal Institution, I desire 

 to express my indebtedness to the managers of that institution for the facilities 

 which they have kindly placed at my disposal. 



6. On VoUa-electromotive Force of Alloi/s^ and a Test for Chemical Union. 



By Dr. G. Gore, F.R.S. 



The question has been asked, ' How far does change in physical properties, 

 such as electromotive force, &c., enable us to detect the existence of a compound 

 in an alloy ? ' * 



In reply to this I beg leave to say that whilst a (/ain of mean electromotive 

 force of an alloy when used as a positive plate in a voltaic cell indicates that the 

 constituents of the alloy are simply mixed together or dissolved in each other a 

 loss of 7nean electromotive force shows that they are chemically combined. 



It is well known that simple dilution of an electrolyte usually dimimshes the 

 apparent electromotive force of a simple voltaic couple immersed in it. In a 

 research, however, on 'The Relations of Volta-electromotive Force to Latent 

 Heat, &c., of Electrolytes,' - I found that in eighteen out of nineteen cases of 

 mere dilution of electrolytes with water, on measuring the apparent electromotive 

 forces of a simple voltaic couple immersed — first, in water alone ; secondly, in an 

 undiluted electrolyte ; and thirdly, in the same electrolyte diluted — a gain of mean 

 electromotive force was produced by the diluted liquid above that of the tnean 

 amount as calculated from the separate amounts excited by the water alone and 

 by the undiluted liquid. And I further found by a similar process in a subsequent 

 research on 'A Method of Measuring Loss of Energy due to Chemical Union '^ 

 that when the ingredients of two electrolytes (such as an acid and au alkali) 

 mixed together with strong chemical union, a loss instead oi&gain oimean amount 

 of electromotive force occurred ; and that when two electrolytes mixed together 

 without any degree of such union, as they are considered to do in cases of more 

 dilution, a gain of tneati amount of such force nearly always took place. Similarly 

 with metals dissolved in mercury and with solid alloys used as positive electrodes, 

 the admixture of one metal with another when unattended by chemical union 



1900. 



Nature, August 16, 1900, p. 369. 

 2 Phil. Mag., August 1891, p. 165. 

 ^ Phil. Mag., Januarv 1892, p. 28. 



* T T 



