The Eighteenth Century 155 



noctiluca aeria and fulgurante (from urine) and the phosphorus 

 Balduinus. The second section deals with pyrophori fecali, the pyro- 

 phoro sacro machabaeorum, pyrophori and phosphori of vegetable 

 origin and the mercurial phosphor, together witli their uses. 



In the third part of the book a great deal of attention is devoted 

 to the hermetic phosphorus of Baldouin, which was the pride of the 

 Germans. There is an excellent index. 



Throughout the work, Cohausen attempted not only to describe 

 the himinescence but to explain the light itself. However, there is 

 little but metaphysical speculation, based on the chemistry of the 

 day. A few excerpts will illustrate the treatment of the subject: 



There is in all things a certain salt, which is nothing other than poten- 

 tial fire or, as the philosophers say, earthly water impregnated with fire. 



Therefore, just as light concentrated and rather vigorously set in 

 motion produces fire, so fire, rarified, produces light. Particles of light, 

 however, confined at least, though not compressed, generate heat or 

 invisible fire. 



For indeed this stone [the Bononian], by calcination, and especially by 

 being freed from heavier sulphur, takes on such a texture that it gives 

 off a peculiarly modified air (aether) , which easily acquires from the 

 luminous air circumfused around it such an impulse and motion as this 

 air acquires from light itself, or from material of the primary "^ element, 

 even when that extraneous source shall have been removed.^ 



It would be futile to continue the presentation of such ideas, 

 which seem far more obscure than those of Lemery. The Lumen 

 Novum Phosphoris Accensum was Cohausen's only work on lumi- 

 nescence, btit he was a learned man of wide reading and reflection, 

 who published a number of medical works. He ridiculed the use 

 of snuflF in a book in 1716 and held that the secret of attaining a 

 vigorous old age was the company of young people, whose insensible 

 perspiration was important, particularly the breath of virgins. His 

 views on this subject will be found in Hermippus Redivivus, etc. 

 (Frankfurt, 1742) . It was translated into English and issued as a 

 privately printed limited edition from Edinburgh in 1885. 



Dujay and Reaumur 



Charles Francois de Cisternay Dufay (1698-1739) was another 

 important investigator of the early seventeen-hundreds. He not only 

 wrote a paper giving the history of the luminescence of the barome- 

 ter in 1723 but also carried out many other experiments on lumi- 

 nescent substances, partictdarly minerals and precious stones, which 



* Probably the " materia coelestis " of Descartes. 



Trom Cohansen, Lumen novum, etc. (1717) , translated by Dr. Hannah Croasdale. 



