886 General Notes. [September, 
LEUCITE. — Professor H. Rosenbusch! of Heidelberg, has 
recently made the most interesting observation that the morpho- 
logical no less than the optical characters of the mineral leucite 
can be brought into full accord with the regular system bya 
sufficient increase of temperature. It is well known that this 
mineral was regarded as the very type of an isometric icosatetra- 
hedron until vom Rath showed that considerable variation from 
the calculated angles as well as frequent twinning lamellz paral- 
lel the face O necessitated the assumption of a tetragonal sym- 
metry. The double refraction of this substance was also a point 
in favor of this view. The recent studies of Klein, Merian and 
Penfield have, however, shown that above a certain temperature 
leucite, like boracite, becomes altogether isotropic and now Rosen- 
busch finds that by the same means the twinning lamelle, ordi- 
narily visible as a system of fine striations, may likewise be made 
to disappear. A crystal upon which these were unusually dis- 
tinct was brought in focus under the microscope by reflected 
light in such a manner that the main face appeared bright while 
the lamella were in the shadow. Heat was now gradually 
applied and the most remarkable effect observed. A kind of un- 
dulatory motion was noticed and whole groups of lamella would 
disappear at one point and reappear at another, until finally, at 
the requisite temperature, all were gone and the face was seen to 
be quite uniform and even. Upon cooling the lamellæ returned 
but in a different position from that which the original ones occu- 
pied. So great was the molecular disturbance here produced, that 
after some repetitions of the experiment on the same crystal it fell 
to pieces. The supposition is made that leucite crystallizes in the 
regular system at high temperatures, and in some other unknown 
system at ordinary temperatures. The effort of the molecules to 
suit their arrangement to the altered. conditions produces a ten- 
sion which finds relief in the formation of secondary twinning 
lamellz parallel to the slipping plane (“ gleitflache ”), which in 
this case is ġo O: en the temperature is raised this tension 1s 
of course removed. The attempt will be made by the same inves- 
tigator to measure a crystal of leucite on a reflection goniometer 
at the temperature necessary for the obliteration of the twinning — 
lamella, when it is expected that the interfacial angles will agree 
„perfectly with the regular symmetry. 
BOTANY.” 
_ THe Axsunpance or Asu Rust.—In Eastern Nebraska, this 
‘year has been remarkable for the great abundance of the ash rust 
~ '(Aicidium fraxin’) upon the leaves, petioles and twigs of the 
green ash. In many instances each leaflet contained from ten to 
a : Neues Jahrbuch für Min., etc., 1885.11, p. I. 
* Edited by ProrEssor CHARLES E, Bessey, Lincoln, Nebraska, 
