from Faraday to J . J . Thomson. 391 



equilibrium in the machine " a conception which follows 

 naturally from the combination of Watson's one-fluid theory 

 with the prevalent doctrine of electrical atmospheres.* 



A different explanation was put forward by Nollet, who 

 performed electrical experiments in rarefied air at about the 

 same time as Watson,f and saw in them a striking confirmation 

 of his own hypothesis of efflux and afflux of electric matter.J 

 According to Nollet, the particles of the effluent stream collide 

 with those of the affluent stream which is moving in the 

 opposite direction ; and being thus violently shaken, are excited 

 to the point of emitting light. 



Almost a century elapsed before anything more was dis- 

 covered regarding the discharge in vacuous spaces. But in 

 1838 Faraday, while passing a current from the electrical 

 machine between two brass rods in rarefied air, noticed that 

 the purple haze or stream of light which proceeded from the 

 positive pole stopped short before it arrived at the negative 

 rod. The negative rod, which was itself covered with a con- 

 tinuous glow, was thus separated from the purple column by 

 a narrow dark space: to this, in honour of its discoverer, 

 the name Faraday's dark space has generally been given by 

 subsequent writers. 



That vitreous and resinous electricity give rise to different 

 types of discharge had long been known; and indeed, as we 

 have seen,) | it was the study of these differences that led 

 Franklin to identify the electricity of glass with the superfluity of 

 fluid, and the electricity of amber with the deficiency of it. But 

 phenomena of this class are in general much more complex 

 than might be supposed from the appearance which they 

 present at a first examination ; and the value of Faraday's 

 discovery of the negative glow and dark space lay chiefly in 

 the simple and definite character of these features of the 

 discharge, which indicated them as promising subjects for 

 further research. Faraday himself felt the importance of 



* Cf. ch. ii. f Nollet, Recherches sur FElectricite, 1749, troisiemediscours. 

 t Cf. p. 40. Phil. Trans., 1838 ; Exper. Res. i, 1526. || Cf. p. 44. 



