134 ESSAYS BIOGRAPHICAL AND CHEMICAL 



appearance which accompanies the passage of a high- 

 tension electric current through rarefied gases. Each gas 

 gives out a soft, coloured light, totally different from the 

 lightning-like sparks which pass between the positive and 

 negative poles through gases at ordinary atmospheric 

 pressure. The pressure must be reduced to about one- 

 hundredth of its normal amount before such phenomena 

 begin to appear; but the actual reduction of pressure 

 depends on the particular gas submitted to the dis- 

 charge. Under such conditions hydrogen glows with a 

 red light ; air with a pale violet glow ; and carbonic acid 

 has a steel blue appearance. The resistance of such 

 rarefied gases to the passage of the electric current is 

 much less than of gases at atmospheric pressure. As with 

 solid conductors, it depends on the distance between the 

 poles and the particular kind of matter employed. 

 Hittorf, the eminent electrician, Professor in Miinster, 

 was the first to conduct experiments at still lower 

 pressures, on still more rarefied gases, and he noted an 

 increase in the resistance of the gas to the passage of 

 electricity. Further, he observed that from the negative 

 electrode, or cathode, the glow proceeded in straight lines, 

 so as to cast the shadow of an interposed object on the 

 opposite wall of the tube. He discovered, too, that such 

 rays can be deviated by a magnet, a discovery made for 

 the electric arc by Sir Humphry Davy in 1821. Sir 

 William, then Mr., Crookes took up this subject in 1878, 

 simultaneously with M. Goldstein, and has made it 

 popular, by reason of the ingenious experiments which he 

 devised to exhibit the rectilinear course of these rays. 

 He devised a theory, moreover, to account for the recti- 

 linear path, namely, that the electric current when it 

 leaves the negative pole attaches itself to the molecules of 

 gas, which, projected with great velocity, will pursue a 

 parallel path, if the cathode is a flat piece of metal, or can 



