Chemistry and Physics. 57 



feet in breadth and height, a large part of tlie light which enters 

 passes through the water. The spectroscope show's that the red 

 is wholly absorbed by the water, and the yellow is so far en- 

 feebled that the D line was scarcely visible. The green, bine and 

 indigo on the other hand shone out brightly, with the V and b lines 

 united in a well-marked absorption line. — Pogg. ^inn., clvi, 326. 



11. Aetlon of Magtietism on an Electric Sjyarh.—M. H. Bkc- 

 QUEREL has shown that the action of a powerful electromagnet 

 w^hen its current is broken between its poles, is purely mechanical. 

 The spark in this case is accompanied by an explosion, and takes 

 the form of a small ilanie, which seems to be projected by the 

 action of the magnet as it would be by a current of air. ' The 

 same effects may be produced by a bellows or even by the mouth. 

 A little explosion is produced, increasing in strength with the 

 force of the air current. It is generally admitted that the sound 

 accompanying the induction spark is due to a sudden cx])ansif>n 

 of the air and of the volatilized portions of the electrode, fol- 

 lowed by a sudden return of the particles to their former position. 

 According to this view the shorter the time of discharge the 

 louder should be the noise. 



The substitution of the purely mechanical action of a current 

 of air seems to show that under the influence of the air current as 

 with the magnet, the effect is due to a sudden rupture of the 

 chain of molecules which transmit the electric current of only 

 short duration, forming the induction current. The time of dis- 

 charge being thus notably diminished the sound accompanying it 

 assumes a remarkable intensity. The idea of making an air-cur- 

 rent act in this way is due to M. de Moncel, who has em])loyed 

 it to separate the spark from the aureole. — Jonrn. de /*A '/■".," iv, 

 206. M. (■. r. 



12. Interference Fringes. — M, Nodot suggests the sub-^titution 

 of rhombs of Iceland spar for Nicol's prisms in forTuinix the inter- 

 ference fringes of Fizeau and Foucault. The sunlight is admitted 

 through a narrow slit, and focnssed on the slit of the spectroscope 

 by a lens. Two rhombs of spar are then interposed, and between 

 them is placed a plate of quartz, cut parallel to the axis, and in- 

 clined 45° to the planes of polarization of the spars. Three im- 

 ages of the rhomb are thus formed, the central one of double the 

 brilliancy of the others, since it is formed by the superposition of 

 two images. Allowing each image in turn to fall on the slit of 

 the spectroscope, it will be seen that fringes are present in all 

 three, those of the t ' ' ^ * ' '* ' " 

 fringes of the centn 

 effect will be double that usually attained, since a XicoVs prism 

 necessarily cuts off at least half the light. 



Tlie experiment may be varied by forming the three images one 

 aV)ove the other, and allowing all to fall on the slit of the Tpectro- 

 seojte, when a spectrum is formed in which the upper and lower 

 parts are fainter than the central portion, and the fringes of the 



