Intelligence and Miscellaneous Articles. 483 



the spark springs through the alcohol. The point then remaining 

 fixed, a succession of sparks is set up and persists for a long 

 time. These sparks are lively and sonorous, and can readily be 

 counted. 



The level of the mercury visibly oscillates beneath the point. A 

 possible cause of the oscillation is : — the spark being formed by the 

 vapour of the mercury, the elastic force of this vapour depresses 

 the level of the liquid ; this returns to its former level, passes it in 

 virtue of its acquired velocity, and rejoins the platinum point. Fall- 

 ing again, the mercury produces a fresh interruption, and the same 

 phenomenon is repeated. 



This purely mechanical cause cannot be the only one ; for the 

 circumstances favourable to this new mode of automatic interruption 

 are those which accompany the decomposition of the rupture-spark 

 into a small number of successive bright strokes. This correlation 

 is recognized by changing the extent of the condenser, which alters 

 the number of the divisions of the spark. Thus, on continually 

 diminishing the surface of the condenser, the sparks are seen to 

 follow one another more and more rapidly ; and finally, when the 

 condenser is omitted, there is only a crepitating voltaic arc. It is 

 probable that the oscillation-period of the mercury comprises a de- 

 finite number of intermissions in the discharge of the condenser, 

 and that these two causes are in mutual dependence. 



I believe that the discharge through air, under the form of a 

 voltaic arc, and the discharge through glass, of which experi- 

 ment 1 supplies an example, are comparable; and that the well- 

 known crepitations of the voltaic arc are due to the same cause as 

 the phenomena of which I have just spoken. All these facts may 

 be brought into connexion by a single proposition : the insertion of 

 a suitable resistance in the voltaic circuit determines the intermission of 

 the current. The laws of the intermission will have to be studied 

 with the interposition of a condenser, because the periods are long 

 enough to be observed with facility. The laws found in this man- 

 ner will afterwards be generalized, and must conduct us to the 

 known laws of the currents which are regarded as continuous. 



We cannot omit to derive from the whole of these considerations 

 the important conclusion that the current is a succession of modifica- 

 tions accomplished periodically in the circuit. — Comptes Rendus de 

 VAcad. des Sciences, vol. lxxvii. pp. 1095-1098. 



EXPERIMENTS ON EVAPORATION. BY M. STEFAN. 



Atmometric experiments have hitherto led to no results expres- 

 sible in the form of laws. The conditions under which they were 

 made were not simple enough ; yet they were sufficiently varied. 

 The science of evaporation, especially of the diffusion of vapours, 

 remained an unoccupied field. 



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