The Nineteenth Century 195 



noulli, dealt almost exclusively with marine luminous organisms. 

 Biology as a science was rapidly becoming a reality. In contrast to 

 the publications of the previous century, an increasing number of 

 books and general articles were devoted solely to bioluminescences. 



Division into the organic and the inorganic is particularly notice- 

 able after the publication of the prize essays on the nature of light 

 by Link, Heinrich, and Dessaignes, and after the appearance of Bio- 

 logie oder Philosophie der lehenden Natur, by G. R. Treviranus. 

 The fifth volume (1818) of this great work contained a long section 

 on " Phosphorische Erscheinungen der organische Natur." 



Such a change in outlook justifies the arrangement adopted in 

 this chapter following the consideration of Heinrich's book, Die 

 Phosphorescenz der Korper (1811-1820). Discoveries concerning 

 the light of luminous organisms and the light of inorganic lumines- 

 cences will be treated in separate sections. The study of lumines- 

 cences became so popular in the nineteenth century that only the 

 more general works and the trends of thought can be considered. 

 Publication was substantial, but an attempt will be made to relate 

 luminescence discoveries to the advancing front of knowledge in 

 chemistry, in physics, and in biology, and to present the views of 

 the leaders in these fields. 



Theories of Light 



During the previous century the corpuscular theory of light, sug- 

 gested by Pythagoras and championed by Newton, made a far more 

 lasting impression on science than did the wave theory of Hooke, 

 Huygens, and Euler. The particle emission hypothesis held almost 

 uncontested sway until the early part of the nineteenth century, 

 when Thomas Young (1773-1829) and Augustin Fresnel (1788- 

 1827) reestablished faith in the wave theory through logical expla- 

 nations of interference and diffraction. At this time, light, together 

 with heat and electricity, was regarded as a material substance. In 

 no field was the corpuscular hypothesis more popular than in that 

 of luminescence. What more simple explanation than to suppose 

 that a phosphor retained the light particles within its pores, to emit 

 them slowly in the dark. The ideas on luminescence of two eminent 

 scientists will illustrate this trend. 



LUIGI BRUGNATELLI 



That light is a material thing, a light substance, was particularly 

 appealing to many chemists at the beginning of the century. For 

 example, the celebrated Luigi Gasparo Brugnatelli (1761-1818), 



