482 REPORT— 1894'. 



opaque wire give exactly the same phenomenon. This is the simple and 

 useful result called sometimes Babioiefs Principle, and, in any case, the 

 experiment just described gives immediately the means of verifying 

 whether the conditions required for the coriect application of this principle 

 are sufficiently fulfilled. 



All these results can be collected under a pretty form which illustrates 

 one of the finest natural phenomena. It is well known that when the 

 sun is hidden behind the top of a mountain, but near the crest, all objects 

 in its neighbourhood are surrounded by luminous borders, and minute 

 objects, branches, leaves of trees, flying birds, &c., appear on the sky as 

 if incandescent. 



To imitate this appearance it suffices to cut out in cardboard an 

 irregular edge representing the crest of the mountain, and to border it 

 with some blades of grass or moss representing trees. This object placed 

 on the lens reproduces an image of this beautiful phenomenon, which 

 seems to have been observed many centuries ago in deep valleys when the 

 sun rises ; for Shakespeare says {Richard II., act iii. scene 2) : ' He fires 

 the proud tops of the eastern pines.' 



The Connection between Chemical Comhination and' the Discharge of 

 Electricity through Gases. By J. J.^Thomson, Professor of Experi- 

 mental Physics, Cambridge. 



[Ordered by the General Committee to be printed in extenso.^ 



Thk intimate connection between chemical change and the passage of 

 electricity through liquids has been universally recognised ever since 

 Faraday discovered the laws of electrolysis which bear his name. These 

 laws state that, whenever electricity passes through an electrolyte, che- 

 mical changes take place in the electrolyte, and that the quantity of electri- 

 city which passes through the electi'olyte is connected in the most intimate 

 and simple manner with the amount of chemical change which has taken 

 place during its passage. For each unit of electricity which passes through 

 the electrolyte a definite amount of chemical change takes place, and the 

 chemical changes which take place when equal quantities of electricity 

 pass through different electrolytes are chemically equivalent. But althougli 

 chemists have largely availed themselves of the light thrown by electrolysis 

 on chemical phenomena, the subject of the passage of electricity through 

 gases does not seem to have attracted their attention. We have strong 

 evidence, however, that the connection between the discharge of electricity 

 through gases and chemical change is not lees intimate than that between 

 electrolysis and chemical change. Thus, for example, when the electric dis- 

 charge passes through steam, the steam is decomposed, an excess of hydro- 

 gen appearing at one electrode and an excess of oxygen at the other ; these 

 excesses of hydrogen and oxygen are proportional to the quantity of elec- 

 tricity which has passed through the steam, and are equal to the amounts 

 of hydrogen and oxygen which would be liberated if the same quantity of 

 electricity passed through a water voltameter. AVe have here evidence 

 for connecting chemical action with the discharge of electricity through 

 gases of precisely the same kind as that wJiich has connected it with 

 electrolysis. Again, as I hope to show later in this paper, the passage of 



