308 Sir J. Conroy. Refractive Indices and Densities of 



" On the Eefractive Indices and Densities of Normal and Semi- 

 normal Aqueous Solutions of Hydrogen Chloride and the 

 Chlorides of the Alkalis." By Sir JOHN CONKOY, Bart.,, 

 M.A., F.B.S., Fellow and Bedford Lecturer of Balliol College, 

 and Millard Lecturer of Trinity College, Oxford. Beceived 

 December 15, 1898, Bead January 19, 1899. 



A very large number of observations have been made of the refrac- 

 tive indices and densities of aqueous solutions of inorganic salts and 

 acids : in England, more especially, by Dr. J. H. Gladstone, who in a 

 paper in the * Philosophical Transactions ' for 1870, gave the values he 

 had obtained for the refractive indices and densities of some 160 salts 

 and acids; and in a series of papers published subsequently in the 

 'Journal of the Chemical Society,' has given the results of further 

 observations. 



Most, however, of these determinations have been made with solu- 

 tions of different strengths, and at different temperatures, and, there- 

 fore, I venture to bring before the Boyal Society an account of some 

 observations I have made of the refractive indices and densities of 

 normal and semi-normal aqueous solutions of hydrogen chloride, and 

 the chlorides of the alkalis at a uniform temperature of 18. 



The method was the ordinary hollow prism one, but special care was 

 taken to keep the solution at a definite known temperature. An 

 alteration of 1 in the temperature of water in the neighbourhood of 

 20 makes a difference in the refractive index of nearly one unit in the 

 fourth decimal place.* 



The goniometer used was made by Messrs. Troughton and Simms - 

 it has an 8-inch circle divided into 10', and is read by means of two 

 micrometers, directly to 10", and by estimation to single seconds.. 

 The prism was made by Steinheil ; the value of its refracting angle, as 

 determined by ten independent measurements, was 60 1' 44". The 

 prism table was supported independently of the divided circle by 

 means of a steel axis. 



In order that the temperature of the prism might be kept constant,, 

 it was surrounded by a water-jacket, the prism being in actual contact 

 with the metal casing containing the water, as, in. some previous 

 experiments,! it had been found that when the prism was merely 

 surrounded by the water-jacket, without being in actual contact with 

 it, a considerable time elapsed before the prism and water-jacket were 



* Cow/. Landolt and Bernstein's ' Tables,' p. 419. 



f " On the Refractive Index of Water at Temperatures between and 10," 

 ' Koy. Soc. Proc.,' vol. 58, p. 228. 



