Electroluminescence 275 



light appears only when the mercury is falling. He held that lumi- 

 nescence was due to the " sulphur " of the mercury in movement, 

 but that a material, different from common air, remained in the 

 vacuum and arrested the movement of the " sulphur " and conse- 

 quently the light. When the mercury descends this " sulphur-arrest- 

 ing material cannot follow the mercury rapidly enough and the 

 " sulphur " can escape, with luminescence, but when the mercury 

 rises the material prevents escape of " sulphur " and no light appears. 



In 1723 a final paper on the mercury barometer was published in 

 the Memoires of the French Academy by C. F. Dufay (1698-1739) , 

 whose later researches in electricity (1733-1734) were to make him 

 famous. Dufay also studied the luminescence of minerals and 

 precious stones in three later papers (1726, 1732, 1738) , again pub- 

 lished in the Memoires of the French Academy. It would seem that 

 an interest in luminescence and electricity would be a happy com- 

 bination and the explanation of the barometer light might naturally 

 follow. However, such was not the case. Les Barometres Lumineux 

 (1723) , after an historical introduction, is largely concerned with 

 Dufay's attempts to find a fool-proof method of making a luminous 

 barometer. He came to the conclusion that an excess of air pre- 

 vented luminescence and that water was absolutely prohibitive. He 

 suspected that something in the mercury must be involved in lumi- 

 nescence or non-luminescence and postulated two principles— one 

 was common air which he proved to be present, "as it is in all 

 liquids," and the second a " matiere subtile," responsible for the 

 light. He believed that ordinarily the air in the mercury enveloped 

 and prevented the subtile matter from getting out, but if there was 

 not much air present, on shaking in a vacuum the subtile material 

 could escape. 



It is squeezed out particularly as the column of mercury of a 

 barometer moves do^vnward. When the mercury moves up, the 

 pores of the mercury are so dispersed as to receive the subtile ma- 

 terial and no light appears. The subtile material is thus not lost or 

 gained during a long rest of the barometer, but appears again when- 

 ever it is agitated. Dufay's theory is little more than a restatement 

 of previous views. 



Despite the many observations of Hauksbee from 1705 to 1711, 

 and despite Dufay's later interest in electricity, there is no mention 

 in Dufay's Memoire of a possible electrical origin of the light and no 

 identification of the subtile material with electrical effluvia. Dufay 

 did mention Hauksbee's experiment of 1708 in which light appeared 

 when he brought his hand near a rotating globe from which the air 

 had been exhausted and also Hauksbee's belief that the barometer 



