﻿184 C. Barus and V. Strouhal — Strain-effect of sudden 



thickness possibly associated with diminution of birefractive 

 power. The solution of the drop in hydro-fluoric acid does 

 not materially interfere with its transparency. 



Our polariscopic experiments were made with the P. E. 

 drops, Nos. 1, 2, 4, 6 and Nos. 3, 5, 7, 8, 9. The first four of 

 these were simply annealed ; the remainder examined after 

 consecutive removals of shells. It is expedient to describe the 

 experiments with annealed globules first. These experiments 

 are so striking and so easily repeated on the one hand, and so 

 difficult to reproduce accurately in a drawing on the other, 

 that the following very careful freehand sketches of the main 

 features of the figures are here inserted principally for pur- 

 poses of record. The figures represent P. E. drop No. 6, in a 

 fixed position between parallel nicols. 



On annealing as far as T=200°, variations of figure are not 

 certainly perceptible. After very long annealing at this tempe- 

 rature (200°), an influence appears, but is restricted to changes 

 of color. It is particularly to be observed, that the position 

 and contours of the original polarization figure shows an obvi- 

 ous relation to the position of the included bubbles (black 

 spots in figure). This was the invariable result for all the P. E. 

 drops examined. It is in accord with our inference of "centri- 

 fugal " contraction* discussed elsewhere. At 200° therefore, 

 so far as these special manifestations are concerned, change 

 of strain in glass is incipient. The P. E. drop, if broken, is 

 still explosive ; though perceptibly less so, than the original 

 drop. After one hour of annealing in boiling mercury (360°), 

 the changes of the polarization figure are obvious. The whole 

 appearance is more diffuse, the colors bolder and broader and 

 the demarcation less distinct and delicate. These striking 

 observations can only partially be reproduced in a figure. We 

 have encountered very marked diminution of birefractive 

 power, after seven hours of annealing in boiling mercury, the 

 evidences of diminished birefractive power having visibly 

 accrued. The figure as a whole is simpler, the coloration more 



gross. 



L. C, p. 45.0. 



