30 Dr. Gladstone and the Rev. T. P. Dale on some 



results obtained by rubbing one against the other, must neces- 

 sarily be inexact, inasmuch as in many cases the electricity thus 

 developed depends upon the energy of friction. 



Lastly, it follows at once from the foregoing facts, that in 

 electrical researches, the dry-pile electroscope possesses import- 

 ant advantages over all others wherein the unknown quality of 

 the collected electricity is ascertained by means of comparison 

 with that developed by the friction of resin or glass. Amongst 

 the countless determinations of the nature of a given quantity 

 of electricity which, for centuries now, have been made, who 

 shall say how many have been erroneous ? 



VI. 071 so7ne Optical Properties of Phosphorus. By Dr. J. 

 H. Gladstone, F.R.S., and the Rev. T. P. Dale, M.A., 

 F.R.A.S* 



ONE of the substances examined during our investigation of 

 the influence of temperature on the refraction of light f, 

 was melted phosphorus; and so remarkable were the phsenomena 

 presented by this elementary body, that we have since examined 

 it more fully, and with reference to other optical properties. 



Phosphorus has the reputation of being extremely refractive ; 

 and the indices 2"125 and 2'224 have been assigned to it — num- 

 bers only exceeded by that of diamond, 2*439, and of realgar 

 and chromate of lead, which are reckoned to transcend even that 

 amount. Unfortunately, however, in the above determinations 

 of phosphorus it is not indicated what part of the spectrum 

 was measured, nor is the temperature specified — two very essen- 

 tial particulars. 



Our measurements, made with the instrument of the Rev. 

 Baden Powell, give the following as the refractive indices of solid 

 phosphorus at 25° C. : — 



On account of a peculiar difficulty in distinguishing the fixed 

 lines through phosphorus, to be noted hereafter, it is impossible 

 to say whether the extreme limit of the visible spectrum corre- 

 sponded precisely with H, or not. 



These numbers indicate not only a very high refractive power, 

 but an unprecedented amount of dispersion. Assuming the end 



* Communicated by the Authors. 



t Phil. Trans. lcS58, p. 887 ; also Phil. Mag. No. 113, p. 222. 



