TRANSACTIONS OF THE SECTIONS. 39 



for nearly forty years a favourite subject of study with the late Mr. Harcourt. 

 Having commenced in 1834 some experiments on vitrifaction, veith the object stated 

 in the title of this notice, he was encouraged by a recommendation, which is printed 

 in the 4th volume of the Transactions of the British Association, to pursue the 

 subject further. A report on a gas-furnace, the construction of which formed a 

 preliminary inquiry, in which was expended the pecimiary grant made by the 

 Association for this i-esearch in 1836, is printed in the Report of the Association 

 for 1844, but the results of the actual experiments on glass have never yet been 

 published. 



My own connexion with these researches commenced at the Meeting of the 

 British Association at Cambridge in 1862, when Mr. Harcourt placed in my hands 

 some prisms formed of the glasses which he had prepared, to enable me to determine 

 their character as to fluorescence, which was of interest from the circumstance 

 that the composition of the glasses was known. I was led indirectly to observe the 

 fixed lines of the spectra formed by means of them ; and as I used simlight, which 

 he had not found it convenient to employ, I was enabled to see fm'ther into 

 the red and violet than he had done, which was favom-able to a more accurate 

 measure of the dispersive powers. This inquiry, being in furtherance of the 

 original object of the experiments, seemed far more important than that as to 

 fluorescence, and caused Mr. Harcourt to resume his experiments with the liveliest 

 interest, an interest which he kept up to the last. Indeed it was only a few days 

 before his death that his last experiment was made. To show the extent of the 

 research, I may mention that as many as 166 masses of glass were formed and cut 

 ijito prisms, each mass doubtless in many cases involving several preliminary 

 experiments, besides disks and masses for other purposes. Perhaps I may be 

 permitted here to refer to what I said to this Section on a former occasion* as 

 to the advantage of working in concert. I may certainly say for myself, and I 

 think it will not be deemed at all derogatory to the memory of my esteemed 

 friend and fellow-labourer if I say of him, that I do not think that either of us 

 working singly coidd have obtained the results we arrived at by working together. 



It is well known how difiicult it is, especially on a small scale, to prepare 

 homogeneous glass. Of the first gi'oup of prisms, 28 in number, 10 only were 

 sufficiently good to show a few of the principal dark lines of the solar spectrum ; 

 the rest had to be examined by the bright lines in artificial sources of light. 

 These prisms appeared to have been cut at random by the optician from the mass 

 of glass supplied to him. Theory and observation alike showed that strife interfere 

 comparatively little with an accurate determination of refractive indices when 

 they lie in planes perpendicular to the edge of the prism. Accordingly the prisms 

 used in the rest of the research were formed from the glass mass that came out 

 of the crucible by cutting two planes, passing through the same horizontal line a 

 little below the surface, and inclined 221° right and left of the vertical, and by 

 polishing the enclosed wedge of 45°. In the central portion of the mass the striae 

 have a tendency to arrange themselves in nearly vertical lines, from the operation 

 of currents of convection ; and by cutting in the manner described, the most 

 favourable direction of the striae is secured for a good part of the prism. 



This attention to the direction of cutting, combined no doubt with increased 

 experience in the manufacture of glass, was attended with such good results that 

 now it was quite the exception for a prism not to show the more conspicuous dark 

 lines. 



On account of the inconvenience of working with silicates, arising from the 

 diflniculty of fusion and the pasty character of the fused glasses, Mr. Harcourt's 

 experiments were chiefiy can-ied on with phosphates, combined in many cases 

 with fluorides, and sometimes with borates, tmigstates, molybdates, or titanates. 

 The glasses formed involved the elements potassium, sodium, lithium, barium, 

 strontium, calcium, glucinum, magnesium, aluminium, manganese, zinc, cadmium, 

 tin, lead, thallium, bismuth, antimony, arsenic, tungsten, molybdenimi, titanium, 

 vanadium, nickel, chromium, uranium, phosphorus, fluorine, boron, sulphur. 



A verj^ interesting subject of inquiiy presented itself collaterally with the 

 original object, namely, to inquire whether glasses could be found which would 

 * Eeport of the British Association for 1862, Trans, of Sect. p. 1. 



