274 History of Luminescence 



experimenter, Francis Hauksbee, whose studies of the light from 

 mercury and other materials in a vacuum during the first decade of 

 the eighteenth century are so important that they will be considered 

 in a separate (the next) section. 



During the years 1710-1719 a number of publications on the 

 barometer light appeared; by N. Hartsoeker (1710), who denied 

 Bernoulli's facts and theory without adding anything himself; by 

 T. Negotius (1715), an inconsequential pamphlet of eight pages; 

 by J. F. Weidler (1715), who also combatted Bernoulli's views, 

 holding that the pellicle on the mercury does not interfere with the 

 light, which comes from rebounding of the rays of luminous ma- 

 terial; by J. M. Heusinger (1716), an excellent forty-eight-page 

 dissertation (see title page in figure 22) under the auspices of J. G. 

 Liebknecht (1679-1749) professor of mathematics and theology at 

 the University of Giessen; then the views of J. J. D. de Mairan 



(1678-1771), expressed in his prize essay on phosphores (1717); 

 finally another dissertation of seventy-four pages by W. B. Nebel 



(1719), under the auspices of J. Bernoulli^" at Basel. The Nebel 

 thesis gave Bernoulli an opportunity to sum up the knowledge and 

 to answer his critics. The pamphlet ends with a discussion of the 

 uses of the light and dedicatory poems. 



It is not worth while to recount the detailed experiments of all 

 these men. The most surprising result of the extended investiga- 

 tions is the totally opposite conclusions drawn from the experiments. 

 For example, Bernoulli stressed removal of air, while Heusinger 



(1716) found luminous barometers in which he could observe bub- 

 bles of air, and pointed out that small air pockets sometimes ob- 

 served along the sides of a barometer tubes could emit flashes of 

 light. Nevertheless, he agreed that air should be removed and that 

 water was particularly harmful, if light were to be observed. Pure 

 mercury did not appear to be necessary, as he had made a lumi- 

 nescent barometer with an amalgam in which five parts of lead 

 were added to twenty-three parts of mercury. Heusinger called at- 

 tention to the ready divisibility of mercury into the finest globules 

 and, like Bernoulli, held that a subtile luminous material must be 

 present in the interstices between the minute globules, a material 

 which is expressed when the mercury is agitated and produces the 

 light. 



De Mairan (1717) had an ingenious explanation for the fact that 



^^This thesis was included in the Opera omnia of J. Bernoulli, 2: 319-392, Lausanne 

 and Geneva, 1742. Bernoulli's work and the experiments of others excited considerable 

 general interest and were discussed in Elementa phy sices (Jena, 1727) of Georg Erhardt 

 Hamburger (1697-1755) in Chapter X, dealing with fire. 



