in highly Rarefied Elastic Fluids, 245 



thus known within a tenth of a millimetre. These two points 

 lead off an almost inappreciable proportion of the current which 

 traverses the distilled water, but a quantity which is nevertheless 

 sufficient to act very distinctly on the magnetic needle. For a 

 constant strength of the principal current, the intensity of the 

 derived current depends on the distance between the two points; 

 and hence, if the intensity of the principal current varies, the dis- 

 tance to which it is at any time needful to separate the two points 

 in order that the indication of the galvanometer may remain con- 

 stant is a measure of the proportionate strength of the corre- 

 sponding derived current, and therefore gives, by a relation 

 which is easily found, the intensity of the total current. 



The apparatus that I have employed to study the propagation 

 of electricity by metallic vapours consists of a large glass globe 

 provided with four tubulures and supported on a foot. The two 

 tubulures which are at the extremities of the horizontal diameter 

 are provided with stuffing-boxes, through which pass metallic 

 rods fitted with points of metal or of carbon, between which the 

 voltaic arc can be produced by means of a battery of sixty or 

 eighty Bunscir's cells. The two tubulures situated at the extre- 

 mities of the vertical diameter allow of the passage of two brass 

 rods terminated by metallic balls, between which the discharge 

 of the RuhmkorfPs apparatus passes at the same time. After 

 exhausting the globe, it is filled with well-dried nitrogen, and 

 this is rarefied till its pressure becomes from 2 to 3 millims. ; 

 the electric discharge is then allowed to pass, and its intensity is 

 measured by the process of derivation that I have just described. 



After having ascertained that this intensity is constant, the 

 horizontal metallic points are brought near each other, so as to 

 set up the voltaic arc, which acts here simply as a source of heat, 

 and is kept going for some minutes. At a certain moment the 

 intensity of the electric discharge, w T hich is in action at the same 

 time as the voltaic arc, is observed to increase very considerably. 

 At the same instant the colour of the discharge, which in the 

 nitrogen was a dark rose-colour, changes completely, and it 

 assumes a hue which depends upon the nature of the points 

 between which the voltaic arc is passing. This new appearance 

 lasts for some instants after the arc has ceased; and in fact it 

 is then that it is most remarkable, for it is no longer affected by 

 its contrast w T ith the light of the arc. 



The voltaic arc was produced successively between points of 

 silver, copper, aluminium, zinc, cadmium, and magnesium, and be- 

 tween points of gas-carbon, all these substances being capable of 

 being volatilized at the high temperature to which they were 

 subjected. 



With points of silver and zinc, the electric discharge assumes 



