1884.] NATURAL SCIENCES OP PHILADELPHIA. 185 



peculiarly substantial appearance, with entire reflexed margins, 

 yet in the present preparation they could with difficulty be 

 detected as mere ghosts of their normal shapes. The two discs 

 rarely remained together, their characteristic entire margins were 

 gone, the rotules being represented merely by a line of very fine 

 rays. The speaker ventured no suggestion as to the influences or 

 the method by which these changes had been efiected, but referred 

 the fact to the consideration of students more competent to deal 

 with the mechanical and chemical constitution of these bodies. 



Lieut. Thos. L. Casey, Eng. Corps, U. S. A., was elected a 

 member. 



July 1. 



Mr. Thos. Meehan, Yice-President, in the chair. 



Thirteen persons present. 



A paper entitled " On a supposed new species of Cristatella," 

 by Edw. Potts, was presented for publication. 



Volcanic Dust from Krakatoa. — Prof. H. Carvill Lewis 

 remarked that in connection with the cause of the beautiful red 

 sunsets of last autumn and winter, which had been recently the 

 subject of much discussion in the scientific periodicals, he had 

 been interested in examining some volcanic dust which had been 

 ejected from the volcano of Krakatoa, and which he had received 

 through the kindness of Rev. Wayland Hoyt,D. D., of this city. 



This dust, which, on August 27, 1883, fell thickly upon the 

 decks, rigging and masts of the bark William H. Besse, bound 

 from Batavia to Boston, is of a light gi'ay color and harsh to the 

 touch. It is essentially a pulverized pumice, by far the greater 

 part of it consisting of fragments of volcanic glass. These 

 fragments are sometimes twisted, but generally in flat angular 

 transparent scales, which are filled with minute bubbles, and, of 

 course, are isotropic. Angular fragments and crystals of trans- 

 parent plagioclase, occasionally showing the hemitropic striations, 

 and giving bright colors in the polai'iscope, together with more 

 irregular and rounded fragments of dark green and brown 

 pyroxenic minerals, probably augite and h^persthene, are scat- 

 tered very occasionally among the glass particles. Grains of 

 magnetite, often well rounded, also occur, and may be picked 

 out and examined separately by a magnet covered with tissue- 

 paper. 



As it is this dust which is regarded as tlie cause of the uni- 

 versal red skies wliich followed so soon after the eruption, 

 attempts have been made, both in Europe and America, to dis- 

 cover traces of it in snow or elsewhere, 



13 



