I 



On Light. 411 



4. Of the absorption and disengagement of light or phospho- 

 rescence. 



1 . Newton first discovered that certain bodies exercise on hght 

 a pecuHar attractive force. When a ray passes obliquely from 

 air into any transparent liquid or solid surface, it undergoes at 

 entrance an angular flexure, which is called refraction. The 

 variation of this departure from the rectilineal path for any par- 

 ticular substance, depends on the obliquity of the ray to the re- 

 fracting surface ; so that the sine of the angle of refraction, is to 

 that of the angle of incidence, in a constant ratio. Now New- 

 ton found, that unctuous or inflammable bodies occasioned a 

 greater deviation in the luminous rays than their attractive mass 

 or density gave reason to expect. Hence he conjectured, that 

 both diamond and water contained combustible matter, — a sa- 

 gacious anticipation of future chemical discovery. 



Dr. Wollaston invented a very ingenious ajjparatus, in which, 

 by means of a rectangular prism of flint glass, the index of re- 

 fraction of each substance is read oflF at once by a vernier, the 

 three sides of a moveable triangle performing the operations of 

 reduction, in a very compendious manner. — Fkil. Trans. 1802, 

 or Nicholson's Journal, 8vo. vol. iv. p. 89. 



But transparent media occasion not merely a certain flexure 

 of the white sunbeam, called the mean refraction, they likewise 

 decompose it into its constituent colours. This effect is called 

 dispersion. Now the mean refractive and dispersive powers of 

 bodies are not proportional to each other. In some refracting 

 media, the mean angle of refraction is larger, whilst the angle of 

 dispersion is smaller ; and in other refracting media, the mean 

 angle of refraction is smaller, whilst the angle of dispersion is 

 larger. In short, the knowledge of the mean refractive power 

 of a given substance, will not enable us to determine its disper- 

 sive power, and vice versa. 



From the refractive power of bodies we may in many cases in- 

 fer their chemical constitution. For discovering the purity of 

 essential oils, an examination with Dr. Wollaston's instrument 

 may be of considerable utility, on account of the smallness of the 

 quantity requisite for trial. " In oil of cloves, for instance, I have 

 met with a wide difference. The refractive power of genuine oil 

 of cloves is as high as 1 -535 ; but I have also purchased oil by 

 this name which did not exceed 1-498, and which had probably 

 - been adulterated by some less refractive oil." This fine idea, 

 suggested by Dr. Wollaston, has been happily prosecuted by 

 M. Blot, with regard to gaseous compounds. I shall first give 

 general tables of the refractive and dispersive powers of different 

 bodies, and then make some remarks on their chemical ap])lica- 

 tions: 



J F 2 Index 



