1905.] on the Development of Spectro-Chemistry . 123 



statement that the diamond mnst also be combustible, because it 

 refracts light so powerfully. This statement is indeed remarkable 

 when we remember that in those distiint times no one had any idea 

 of the chemical composition of the diamond, or any conception of 

 the nature of combustion. 



But centuries of patient work were needed in order to recognise 

 clearly the connexion between the chemical composition of different 

 bodies and their power of refracting and dispersing light in different 

 ways, i.e. of producing differently constituted spectra. To account 

 for this connexion has become the task of " Spectro-Chemistry." 



§ 2. Newton was also the first to fix a standard for measuring 

 the refractive power of bodies. Starting from the emanation theory 

 of hgbt which he had himself founded, he diminished the square of 

 the refractive index, n^ by unity and regarded 



n- - 1 



as the expression for the refractive power. This power, reduced to 

 constant density, i.e. divided by the specific gravity d of the body : 



??r - 1 



d 

 was called by Newton the " absolute refractive power." 



§ 3. It was fully a hundred years later that Laplace, in his 

 celebrated "Mecanique celeste," laid down the principle that the 

 expression for refraction, derived from Newton's emanation-theory, 

 must for one and the same sudstance be unaffected by changes in 

 density caused by temperature and pressure — unaffected, that is, by 

 the accidental density of a substance. 



§ 4. The hitherto purely hypothetical formula for refraction 

 acquired further scientific importance from the researches of Biot and 

 Ai'ago (1806), and Dulong (1826), on the refractive powers of gases 

 and vapours. These, the first quantitative measm-ements of re- 

 fractivity, seemed actually to confirm the theory that Newton's 

 formula denotes a constant quantity which always remains unaffected 

 by the accidents of temperature and pressure. 



But with the triumph of the modern wave-theory of light, 



Newton's expression for refraction, — , lost its theoretical im- 



a 



portance. New experiments soon showed that even its supposed 



independence of temperature and pressure did not in fact exist. 



II. 



§ 5. In 1858, John Hall Gladstone, who was for some time 

 Professor in this Institution, began his splendid series of optical 

 researches, which he pursued with great success for over forty years. 

 At first in collaboration with the Rev. T. Pelham Dale he investi- 

 gated the dependence of refractivity on temperature in the case of 



