Mr. W. Sutherland on Molecular Refraction. 141 



strata of air more or less laden with aqueous vapour has been 

 advocated. The appearance of the spectrum of the electric 

 spark in steam certainly leads one at first to favour this hypo- 

 thesis. We have spoken of the marked brilliancy of the 

 hydrogen-line, and of a continuous red band near this line. 

 The continuous spectrum in the yellow is no less prominent. 

 The observations which have been made on the northern 

 lights do not enable one to make exact comparisons. The 

 lines given by different observers, however, do not appear to 

 coincide with the prominent lines and bands observed in the 

 air-spectrum heightened by steam, 



Other observers, among them Professors Liveing and 

 Dewar, have employed steam to obtain steam-lines : but we 

 have been unable to find any reference to the remarkable 

 economy in time and in waste of apparatus which results in 

 the use of a jet of steam in spectrum analysis when the spark 

 method of obtaining the spectra of metals is employed. 



Jefferson Physical Laboratory, 

 Cambridge, U.S., Dec. 22, 1888. 



XYII. Molecular Refraction. 

 By William Suthekland, M.A., B.Sc.^ 



ri^HE object of the present paper is to furnish a theoretical 

 -■- establishment of Gladstone's empirical law connectino- 

 the index of refraction of a substance with its density and to 

 consider certain consequences thereof, also to show that Glad- 

 stone's law expresses more accurately than any other which 

 has yet been proposed the connexion between index and 

 density. To this latter end it will be advisable to give a sketch 

 of the history of the subject, especially as that history is one 

 of the most interesting of those of the minor branches of 

 physics. 



The first indication of a connexion between index of refrac- 

 tion and density is due to Newton, who was led to discover it 

 by his investigation of refraction according to the emission 

 theory. In his ' Optics,' Book II. part iii. he enunciates 

 (chapter x.) thus :— " If light be swifter in bodies than in vacuo, 

 in the proportion of the sines which measure the refraction of 

 the bodies, the forces of the bodies to reflect and refract lioht 

 are very nearly proportional to the densities of the same bodies 

 excepting that Unctuous and Sulphureous bodies refract more 

 than others of this same density." Newton's proof of this 

 proposition may be stated thus in the terminology of the 



* Commmiicated by the Author, having been read in Section A of the 

 Australian Association for the Advancement of Science, August, 1888. 



