CHAP, iv.] ELECTRICITY AND MAGNETISM. 247 



and by placing solutions of quinine or other fluorescent 

 liquids in outer tubes of glass. 



293. Phenomena in High Vacua. Crookes has 

 found that when exhaustion is carried to a very high 

 degree, the dark space separating the negative glow 

 from the negative pole increases in width ; and that 

 across this space electrified molecules are projected in 

 parallel paths normally to the surface of the electrode. 

 The chief point relied upon for this theory is, that if 

 exhaustion be carried to such a high degree that the 

 dark space fills the entire tube or bulb, and bodies 

 (whether opaque or transparent) be then interposed in 

 front of the electrode, sharply defined shadows l of these 

 bodies are projected upon the opposite wall of the vessel, 

 as if they stopped the way for some of the flying mole- 

 cules, and prevented them from striking the opposite 

 wall. Lightly-poised vanes are also driven round if 

 placed in the path of the discharge. 



294. Striae. The strice or stratifications have been 

 examined very carefully by Gassiot, by Spottiswoode, and 

 by De la Rue. The principal facts hitherto gleaned 

 are as follow : The striae originate at the positive 

 electrode at a certain pressure, and become more 

 numerous, as the exhaustion proceeds, up to a certain 

 point, when they become thicker and diminish in number, 

 until exhaustion is carried to such a point that no dis- 

 charge will pass. The striae are hotter than the spaces 

 between them. The number and position of the striae 

 vary, not only with the exhaustion but with the difference 

 of potentials of the electrodes. When striae are pro- 

 duced by the intermittent discharges of the induction 

 coil, examination of them in a rotating mirror reveals 

 that they move forward from the positive electrode 

 towards the negative. 



1 Holtz has more recently produced " electric shadows/' by means of 

 discharges in air at ordinary pressure, between the poles of his well-known 

 machine (Fig. 29), the discharge taking place between a point and a disc 

 covered with silk, on which the shadows are thrown. 



