272 History of Luminescence 



Royale des Sciences covering 1666-1691, published in 1730. The dis- 

 covery was reported in the publications of other societies, and the 

 public exhorted to carry out research on so singular a phenomenon. 

 One of the remarkable characteristics of the light was its appearance 

 when the mercury moved downward but not when it was moving 

 upwards. A number of barometers behaved like the one of Picard 

 but others did not. After the first enthusiastic report the eerie light 

 was neglected for some years. 



In 1700 Johan Bernoulli the first (1667-1748) , Swiss mathema- 

 tician and son of Nicolas, founder of the famous family, again called 

 attention to the barometer light and considerable discussion ensued 

 in the French Academy at that time. The argument, in which 

 Cassini and de la Hire joined, chiefly centered around the precau- 

 tions necessary to produce the effect. Bernoulli's three letters (1700- 

 1701) were published by the French Academy and translated by 

 Martyn and Chambers (1742). In the first letter, June 19, 1700, 

 he suggested that pure mercury and complete absence of air were 

 essential, because in passing through air, the mercury became 

 covered with an ash gray pellicle which prevented the material of 

 " the first element " from passing out of the mercury into the 

 vacuum. ^^ Consequently, Bernoulli devised some ingenious methods 

 of filling the barometer without allowing the mercury to come in 

 contact with air, but the French Academy was not always successful 

 in obtaining a luminous barometer by following his methods. Ber- 

 noulli reasoned that light appears when the mercury descends be- 

 cause a subtle material must go out of it (call it matter of the first 

 element) and meet another matter that enters through the pores of 

 the glass (matter of the second element or celestial globules) . 



He went on to say that while particles of the first element are in 

 the mercury they cannot make light, because they are " oppressed " 

 by the mercury but when they get out by descent of the mercury, 

 they 



take that rapid course [out of the mercury] . . . and by the effect which 

 they make on the celestial globules which meet them, they produce this 

 light; from whence the reason is seen why this light is only observed in 

 the descent of the quicksilver; for when it reascends the matter of the 

 first element is so far from going out, that there rather enters again, a 

 part of that which went out in the preceeding fall; and the rest is driven 

 away with the celestial globules, out of the tube through the pores of 

 the glass. 



^'' Bernoulli accepted the Cartesian conception of light, " making it consist in the 

 most rapid motion of the matter of the first element, assembled only in some space, 

 and in the effort which it makes on the celestial globules." 



