1 22 British Association, 



posed of basalt, to sustain thesignal-staft" whicli they had erected on 

 tl)e highest hill, near Belfast ; the effect of that heap of stones on 

 the magnetic needle was so great, that in walking round it the needle 

 would veer round to every point of the compass. 



Prof, de la Rive having read a paper on the interference of electro- 

 magnetic currents, in which, among other facts, it was stated that when 

 wires of platina are employed to transmit the magneto-electric cur- 

 rents into any solution, the abundant evolution of gas which is at first 

 observed diminishes, and after fifteen or twenty minutes disappears j 

 Professor Andrews, of Belfast, observed, that there was one por- 

 tion of the detail upon which he thought he could throw some light, 

 by mentioning a fact with which he had lately become acquainted. 

 If the poles of a galvanic arrangement of low tension, say a single 

 pair of plates charged with weak acid, were made to communicate 

 with two broad slips of platina immersed in water, no action what- 

 ever would take place; but if one of the broad slips, was replaced by 

 a fine wire, the pole which it represented would give off" the appropri- 

 ate gas, whether oxygen or hydrogen; but the broad slip at the other 

 pole would give off none whatever — or whichever pole it might be 

 connected with. Now the appearance of the gas at one pole was a 

 clear proof that water was decomposed ; and therefore the gas which 

 must be developed at the broad slip must be dissolved in the liquid, 

 or otherwise prevented from assuming the gaseous form. Persons 

 not acquainted with this fact might infer, that there was no decom- 

 posing action exerted; when, in fact, as appeared plainly upon a 

 more extended view of the phaenomena, there was. — Mr. Lubbock 

 inquired, from Professor Andrews, whether the hydrogen disap- 

 peared as well as the oxygen ; for, although water can condense oxy- 

 gen in considerable quantities, he was not aware that any consider- 

 able quantity of hydrogen could be so condensed. — Professor An- 

 drews replied that either would be given off at the fine platina wire, 

 according to the pole you made it to represent, and neither would 

 appear at the broad plate, showing that each would in turn be dis- 

 solved, or otherwise detained in the fluid.* 



M. de la Rive read a second paper * On an Optical Phaenomenon 

 observed at Mont Blanc' When the sun has set at Geneva, it is 

 observed that Mont Blanc remains illuminated by its direct rays for 

 a much longer time than the surrounding mountains. This phaeno- 

 menon is owing to the great height of Mont Blanc. But after it has 

 ceased to be illuminated the summit of Mont Blanc sometimes reap- 

 pears at the end of ten or fifteen minutes, less intensely enlightened 

 than at first, but nevertheless in a manner very decided, and often 

 very brilliant. This phaenomenon takes place especially when the at- 

 mosphere is very pure — highly charged with aqueous vapour in an in- 

 visible state — and consequently very transparent. The author has 

 satisfied himself (by the exact observation of the time which elapses 

 between the two successive illuminations of the mountain, combined 



• On this subject see Mr. Faraday*s observations, Lond. & Edinb. Phil. 

 Mag., vol. iv., p. ^1. 



