294 History of Luminescence 



prismatic colors are not present in the electric light. As early as 

 1748, Stephen Hales (1677-1761), better known for his Vegetable 

 Staticks (1727) and Hemastaticks (1733) , published a short paper, 

 " Concerning some Electrical Experiments," in which he related 

 when he was in London he saw green, yellow and white electric 

 sparks depending on the material from which they arose: 



The active electric Fluid seems to be a great Agent in conjunction with 

 the Air, in the Production of Fire. ... A warm thick Piece of Iron 

 being suspended by two silk Lines, had a warm very thick Piece of Brass 

 laid on it, on which was placed a common Hen's Egg: When electrified, 

 the Flashes from the Iron were of a bright silver light colour; from the 

 Brass (especially near it) the Flashes were green; and from the Egg 

 of a yellowish flame colour: which seemed to argue, that some Particles 

 of those different Bodies were carried off in the Flashes, whence these 

 different colours were exhibited. 



Hales was quite correct in his conclusions. 



One whole section of Priestley's book is specifically devoted to 

 " Experiments and Observations concerning Electric Light " in 

 vacuo. The observations of Hauksbee, Watson, Smeaton (trans- 

 mitted to Wilson) , Canton, Beccaria, Wilke, and others are de- 

 scribed in detail. Priestley noted particularly the effect of the elec- 

 tric light in exciting the luminescence of such substances as Canton's 

 phosphor, or the surface of marble as described by Mr. Lane (see 

 Chap. VIII) , a phenomenon indicating " that electric light is more 

 subtle and penetrating, if one may say so, than light produced in any 

 other way. . . ." Here we see the realization that electric discharges 

 are quite different from ordinary light, a difference now known to 

 be due to the ultraviolet light they contain, so important in modern 

 methods of illumination by the fluorescent lamp. The electrical 

 machine of Priestley's day had grown greatly in size, as illustrated 

 in figure 26, taken from B. Wilson's (1778) book on conductors. 



Toward the end of the century, a number of papers appeared on 

 the electric light in a vacuum, but little was added to general knowl- 

 edge. Edward Nairne (1726-1806), one of the first foreign mem- 

 bers of the American Philosophical Society, noticed that the electric 

 light is very faint in a moist vacuum, and he also described (1777) 

 the striae which often appear when electricity is conducted through 

 evacuated tubes. These striae are particularly marked if mixed 

 gases are present. They attracted much attention in the middle of 

 the next century when the conduction of electricity was a popular 

 and rewarding field of experimentation. Starting at the negative 

 pole (cathode) there is a thin layer of luminosity, the cathode 

 glow, then a dark region, the " Crookes dark space " or " Hittorf's 



