Electroluminescence 293 



mica when two pieces were rubbed together as triboluminescence 

 but this light can actually be shown to be (in part at least) an 

 electroluminescence. Some years ago (Harvey, Science 89, 461, 1939) 

 the author split mica plates in an atmosphere of low pressure neon 

 gas and noted that the light was definitely red, the color of electrical 

 discharges passing through neon. 



Another example was described by Dumas (1826) —a flash of light 

 when fused boric acid cools in a crucible. The luminescence occurs 

 at the time the melt separates from the container. Dumas said the 

 light was electrical and compared it to the flash observed when mica 

 sheets separate. 



FROM JOSEPH PRIESTLEY TO MICHAEL FARADAY 



It was in 1767 that the first edition of Joseph Priestley's (1733- 

 1804) book, The History and Present State of Electricity, ivith Origi- 

 nal Experiments, London, was published. A second edition fol- 

 lowed in 1769 and a third of two volumes in 1775. As the name 

 indicates, his treatise is a chronological account of all electrical 

 experiments and ideas up to the date of publication. In describing 

 a few experiments of his own, Priestley (1767) stated that several 

 previous writers had alleged that the electric light contained no 

 prismatic colors. He decided to test this for himself by observing 

 electric sparks taken from the prime conductor through a prism, 

 and found the colors as distinct as those from the sun. He also noted 

 that the spark in inflammable air (hydrogen) is red in color, and 

 that " when the light was a little diffused, as in those red or purple 

 parts of a long spark, as it is called, the colours were not so vivid, and 

 less easily distinguished from one another; and when the light was 

 still more diffused, through a vacuum, the prism made no sensible 

 alteration in the appearance of it." Priestley then remarked that 

 " the flames of different bodies yield very different proportions of 

 prismatic colours, I have often thought of attempting to ascertain 

 the proportion of these colours in the electric light . . . but have had 

 no leisure to pursue the inquiry." This was in 1766, before it was 

 realized that a slit is necessary for proper spectroscopic observation. 

 Priestley emphasized that " the greatest quantity of electric light is 

 produced in vacuo," and cited Mr. Canton's aurora borealis experi- 

 ment as an example of " entertaining philosophical experiments 

 made by a combination of philosophical instruments," i. e. by an 

 evacuated glass tube, three feet long, which will glow and flash when 

 held in the hand and presented to the conductor from the electrical 

 machine. 



It is rather surprising that there should be a general belief that 



