The Eighteenth Century 181 



luminescence (1753, 1754). The most important student of bio- 

 luminescence was Lazzaro Spallanzani (1729-1799) , who carried out 

 experiments on medusae (1794) , and on the glowworm and lumi- 

 nous wood (1796) , designed to determine the nature of the light. 



Priestley and his Contemporaries 



It is disappointing to find that Joseph Priestley (1733-1804) , for- 

 eign member of the American Philosophical Society since 1785, 

 whose studies on the gaseous exchange of plants and animals were 

 so important, carried out practically no original experiments on 

 luminescence, although he mentioned the subject many times. He 

 did write two very readable scientific books, histories or summaries 

 of what was then known concerning electricity, in 1769, and con- 

 cerning light, in 1772. They are both full of facts regarding lumi- 

 nescence. His third scientific book, on Different Kinds of Air 

 (London, 3 v., 1774, 1776, 1777) contains most of his experimental 

 work, but little on luminescence. 



The book, History and Present State of Electricity (London, 

 1767) , went into many editions, a fifth appearing in 1794. Priestley 

 describes most of the many experiments of various investigators on 

 the " electric light," a subject which will be fully considered in 

 Chapter VH on electroluminescence. 



The second book. History and Present State of Discoveries relat- 

 ing to Vision, Light and Colours (London, 1772) , is the nearest 

 approach to a general treatise on luminescence written during the 

 latter part of the century. It is exactly what its title (see fig. 17) 

 implies and contains a long section (p. 360-382) on " The property 

 of some substances to imbibe and emit light, especially the Bolognian 

 Phosphorus " and another (pp. 563-588) on " Light proceeding from 

 Putrescent Substances, etc. and Phosphorus." The two would make 

 up a pamphlet of forty-eight pages. No mention is made of the 

 firefly or glowworm but studies on Pholas dactylus and luminous 

 worms are considered, as well as the phosphorescence of the sea, 

 shining flesh fish and wood, corposants and ignes fatui, luminous 

 sweat, and the luminous woman of Milan. 



Priestley's approach is mostly descriptive. As far as luminescence 

 is concerned he is a narrator rather than a theorist, presenting the 

 hypotheses of the authors whose observations he describes. The 

 nearest recital of his own views is to be found in a paragraph inter- 

 posed between discussion of the light of putrescent substances and 

 that of phosphorus. Priestley wrote: 



A light in some respects similar to that of putrescent matter has been 



