380 



RECENT PROGRESS IN PHYSICS. 



" When tlie phenomena of currents are observed in dense insulating 

 dielectrics they present us with extraordinary degrees of mechanical 

 force. Thus, if a pint of well rectified and filtered oil of turpentine | 

 be put into a glass vessel and two wires be dipped into it in different 

 places, one leading to the electrical machine and the other to the dis- 

 charging train, on working the machine, the fluid will be thrown into ■ 

 violent motion, whilst, at the same time, it will rise 2, 3, or 4 inches up » 

 the machine wire, and dart off in jets from it into the air." — (1595.) 



'' A drop of mercury being suspended from an amalgamated brass i 

 ball preserved its form almost unchanged in air, but when immersed . 

 in the oil of turpentine it became very pointed and even particles of i" 

 the metal could be spun out and carried off. The form of the liquid 

 metal was just like that of syrup in air." — (1597.) 



"If the mercury at tlie bottom of the fluid be connected with the 

 electrical machine, whilst a rod is held in the iiand terminating, in a 

 ball three quarters of an inch in diameter, and the ball be dipped into 

 the electrified fluid, very striking appearances ensue. When the ball 

 is raised again so as to be at a level nearly out of the fluid, large por- 

 tions of the latter will seem to cling to it, (fig. 76.) If it be raised 



Fig. 76. 



Fig. 77. 



higher a column of the oil of turpentine will still connect it with 

 that in the basin below, (fig. 77.) If the machine be excited into 

 * more powerful action this will become more bulky, and may then also 

 be raised higher, assuming ths form, (fig. 78.) 



'' A very remarkable effect is produced on these phenomena, con- 

 nected with positive and negative charge and discharge, namely, that 

 a ball charged positively raises a much higher and larger column 

 of the oil of turpentine than when charged negatively." — (Faraday 

 Eesearches, series XIII, 1600.) 



§ 89. Laws of the brightness of the electrical spark. — Masson pub- 

 lished in the 14th volume of the Armeies de Chimie et de Physique^ 

 page 129, (1845, 3d part,) his researches upon the brightness of the 

 electrical spark, under the title : " Etudes de Photometric Electrique." 



The ordinary photometre can be used only for permanent and not for 

 momentary sources of light ; for measuring the brightness of the elec- 

 trical spark, which gives only a momentary illumination, Masson was 



