114 Mv. Grove on the Effect of 



of Darfur, drawn at Cairo in 1841 by Dr. Perron, under the 

 dictation of Sheikh Mohammed el Tunisi, there is a large 

 river in the west of that country named Bare ; and that from 

 the evidence collected by M. Jomard in his Preface to the 

 Voi/age an Darfour of the Sheikh, it may be concluded that 

 both that country and Kordofan lie within the basin of the 

 Nile, and that this river Bare is an affluent of the Bahr el 

 Ghazal of the Egyptian expeditions. 



We have thus brought the limits of the hydrographical 

 system of the Nile round again to the confines of Nubia and 

 Egypt ; in which countries, as it is well known, the basin of 

 that river, on the western as on the eastern side, consists of 

 little more than the bed of the stream itself. 



St. Mildred's Court, 

 Dec. 30, 1848. 



XV. On the Effect of mrrounding Media on Voltaic Ignition. 

 By W. R. Grove, Esq., M.A., V.P.R.S.* 



IN the Philosophical Magazine for December 1845, I 

 pointed out a striking difference between the heat gene- 

 rated in a platinum wire by a voltaic current, according as 

 the wire is immersed in atmospheric air or in hydrogen gas, 

 and in the Bakerian Lecture for 1847 I have given some fur- 

 ther experiments on this subject, in which the wire was ignited 

 in atmospheres of various gases, while a voltameter enclosed 

 in the circuit yielded an amount of gas in some inverse ratio 

 to the heat developed in the wire. It was also shown, by a 

 thermometer placed at a given distance, that the radiated heat 

 was in a direct ratio with the visible heat. 



Although the phaenomenon was apparently abnormal, there 

 were many known physical agencies by which it might pos- 

 sibly be explained, such as the different specific heats of the 

 surrounding media, their different conducting powers for elec- 

 tricity, or the varying fluency or mobility of their particles 

 which would carry off the heat by molecular currents with 

 different degrees of rapidity. 



The investigation of these questions will form the subject 

 of this paper. 



An apparatus was arranged, see fig. 1. Two glass tubes A 

 and B, of 0-3 inch internal diameter and 1-5 inch length, were 

 closed with corks at each extremity; through the corks the ends 

 of copper wires penetrated, and joining these were coils of fine 



* From the Philosophical Transactions for 1849, part i.; having been 

 received by the Royal Society August 10, and read December 14, 1848. 



