ON THE SPECIFIC REFRACTION OF SOLIDS. 155 



Report of the Committee, consisting of Dr. J. H. Gladstone, 

 Dr. W, R. E. HoDGKiNSON, Mr. W. Carleton Williams, and 

 Dr. P. P. Bedson {Secretary), appointed for the purpose of 

 investigating the Method of Determining the Specific Refraction 

 of Solids from their Solutions. 



The specific refraction of a substance, i.e., the index of refraction, minus 

 unity divided by tlie density, is, according to Messrs. Gladstone and Dale, 

 a property uninfluenced by the passage from the solid to the liquid state. 

 Further, since the specific refraction of a mixture of liquids is the mean 

 of those of its constituents, it .follows that tlie specific refraction of a 

 solid may be deduced from that of a solution containing it ; the specific 

 refraction of the solvent being known. In support of the method of 

 determining the specific refraction of a solid, based upon these observations, 

 these authors have advanced some direct proof ('I^hil. Trans.,' 1869, 

 {)p. 13, 14), and also a large amount of collateral evidence.' The method 

 has, however, been called in question, more especially by Janovsky 

 (Sitzungsber. Wien. ' Akademie,' Ixxxii. 1880,148), Avho denies the possi- 

 bihty of determining the refractive index of a solid from solution ._ Hence it 

 was thought desirable to submit the method to further examination, (1) to 

 test the accuracy of the statement made by this investigator, and (2) to 

 ascertain how far this method is applicable to determine the specific and 

 molecular refraction of solid compounds. 



In order to test the method, the specific refraction of liquid phenol has 

 been determined and compared with that obtained from its solution in 

 alcohol and glacial acetic acid. Further, the specific refractions of rock 

 salt, borax, and boric acid in the solid state have been compared with 

 the values obtained from their aqueous solutions. The instrument used to 

 detei-mine the refractive indices was a spectrometer obtained from Becker 

 (Meyerstein's successor) ; its graduated circle is divided into tenths of a 

 degree, and by means of verniers these divisions are further divided, 

 so that readings can be made to one second. The refractive indices have 

 been determined for the hydrogen lines a, ft, and y. in some cases for 

 the sodium line also ; from these, by the aid of Cauchy's formula, the 

 index of refraction (A) for a ray of light of infinite wave-length has 

 been calculated. The refractive indices are deduced from at least four 

 readings ; the refractive indices, calculated in two sets of observations 

 from such readings, differing from one another in the fifth place of 

 decimals only. 



In the case of liquids, a hollow prism was used of about 15 ccm. 

 capacity, closed by two plates of glass, the surfaces of which were 

 parallel. The prism has an aperture for the insertion of a thermometer, 

 by means of which the temperature of the liquid under experiment may 

 be ascertained. The thermometer used was one of Geissler's, graduated 

 to two-tenths of a degree, and, consequently, readings to one-tenth of a 

 degree could be made with accuracy. The temperature of the liquid 

 in the prism was raised to and maintained at the point desired, by means 

 of the circulation of warm water through a series of glass tubes placed 

 at the back of the prism and under the table upon which it stood. 



The specific gravities of liquids and solutions were determined in an 



« Dr. Gladstone's article, Phil. Mag., 1881, pp. 64-60. 



