Miscellanies. 379 



afamt purple light when heated, and the effect continued to increase 

 as far as one hundred and sixty discharges. 



While all the calcareous minerals may be rendered phosphores- 

 cent by the electric discharge, none of the specimens of quartz, sili- 

 ceous or aluminous minerals that were tried, either possessed natu- 

 rally or would receive phosphorescence. 



Voltaic electricity was tried by the author as a source of phospho- 

 rescent power, but without effect. 



One conclusion which appears to be deducible from the author's 

 -experiments is, that the natural color of fluors, as well as the light 

 which they emit when heated, is dependent on the variations of struc- 

 ture produced by electricity and heat. — Jou7\ of Roy. Inst. I. 267. 



2. Difference in brilliancy of the primary and secondary rainbow. — 

 (Alfred Ainger.) — The superior brilliancy of the primary bow is not, 

 I think, quite accurately accounted for, when it is ascribed to the cir- 

 cumstance of its rays having suffered but one reflexion ; for the 

 double reflexions are made at angles so favorable, as nearly to coun- 

 terbalance this difference. I apprehend that the faintness in the lat- 

 ter case is owing to the following causes : — 



1 . That the rays, which suffer the maximum deviation in the pri- 

 mary bow, arrive at the surface at a much smaller angle of incidence 

 than those which suffer the minimum deviation in the secondary bow ; 

 the latter, therefore, are more copiously reflected from the first sur- 

 face, and enter the drop in much smaller quantities. 



2. That the angle, at which the ray is afterwards refracted from 

 the inner surface to the air, is, in the secondary bow, similarly favor- 

 able to reflexion, and unfavorable to refraction, so that only a small 

 portion of the already reduced quantity of admitted light is refracted. 



3. That the extent of the dispersion is increased by the second 

 reflexion, as is shown by the greater width of the secondary bow. 



The last circumstance may, perhaps, be considered as included in 

 the expression that the faintness is owing to the second reflexion, 

 though it is not very obvious that such is the meaning. — -Idem. 



3. On the discharge cf a jet of ivater under water ; by R. W, 

 Fox, Esq. — Having observed that a communication of mine " on the 

 discharge of a jet of water under water" inserted in No. XLVII of the 

 Philosophical Magazine, has been noticed in the last number of the 

 Journal of the Royal Institution. I will fake this opportunity of men- 



