PHYSICS. 503 



tar or petroleum, mixed with lampblack, and of Napoli, made of gas 

 coke and tar. 2d. Voltaic arc lamps, monophotal, like those of Fou- 

 cault, Serrin, Jaspar, &c. 3d. Polyphotal lamps, as the Gramme, which 

 is a shunt lamp; the Siemens and the Brush, which aie differential, 

 and the Brockie, which is periodic in its action. 4th. The distribution 

 of lights (1) all in multiple arc, as in the Giilcher system; (2) all in 

 series, as in the Brush, and (3) upon the different circuits of a multiple 

 machine, as in Mersanne's system. 5th. Electric candles, as the Jabloch- 

 kofif, Jamin, and Soleil. Gth. Incandescent lamps, divided into (1) those 

 in air or an inert gas, as the Reynier, Werdermann and Sawyer, and (2) 

 those in a vacuum, like tlie Edison, Swan, Maxim, and Lane Fox. 7th. 

 Intensity of the light and its distribution in space. {J. Phys., February, 

 March, II, i, pp. 72, 125.) 



Ayrton and Perry have presented to the London Physical Society the 

 results of their experiments on the resistance and counter-electromotive 

 force of the arc. The latter was measured by a voltmeter placed be- 

 tween the terminals of the lamp. When the distance between the car- 

 bons was constant the electromotive force diminished as the current 

 increased. With a constant current, the electromotive force increased 

 rapidly at first with an increasing length of arc, afterward more slowly. 

 To produce an arc one third of an inch long, 80 volts are required. For 

 further increase, the electromotive force is proportional to it. {Nature, 

 December, xxvii, p. 215.) 



Tommasi has shown that when the arc is made to pass between two 

 metallic tubes, of copper for example, so arranged that a rapid current 

 of cold water may flow through them, and placed horizontally, the illumi- 

 nating power is very much weakened, the arc is very unstable, it does 

 not set fire to paper, it appears to be formed of a luminous globule 

 moving up and down between the rheophores, it is extinguished by 

 the presence of a magnet, being attracted or repelled according to the 

 pole presented, and a large amount of ozone is produced. {Compies 

 Rendus, xciii, p. 716; Phil. Mag., January, Y, xiii, p. 75.) 



Jamin has studied the effects produced when the alternating current 

 of a Gramme machine passes between carbon points in a vacuum. 

 Gramme machines with alternating currents resemble both batteries and 

 induction coils, but they differ from batteries by the great intensity of 

 their currents. Hence, the effects will be those of batteries, with an in- 

 tensification due to the high tension, and those of the induction coil, with 

 the advantage of increased quantity. In place, therefore, of a single arc 

 in air several can be maintained; the author has maintained 60 from a 

 machine which originally supported only 8. When, now, this alternating 

 current is used to maintain an arc in an electric egg, as soon as the ex- 

 haustion reaches 12™™ the light begins to spurt out spontaneously from 

 the entire surface of the carbons, both of them being enveloped with the 

 blue aureole noticed in Geissler tubes around the negative pole. The 

 carbons become heated to bright ledness throughout, and are rapidly 



