Scientific Notes. 
[April, 
284 
vanes which had been stationary then began to turn in the positive direction 
at a speed of 100 revolutions per minute, whilst the disks turned as before, 
positively, but with less speed. (8.) When the exhaustion was carried beyond 
0-098 millim. the speed of the two disks and of the vanes increased till it ex- 
ceeded 600 revolutions per minute, and it did not seem to diminish with the 
highest rarefadtion, which was at o-oooi millim. According to the most 
recent determinations the number of molecules contained in a cubic centi- 
metre of air at the ordinary pressure is probably something like — 
1,000,000,000,000,000,000,000 (one thousand trillions) ; 
consequently, at an exhaustion of o-oooi millim., 100,000,000,000,000 are 
still left. This number is sufficiently large to justify the hypothesis — That 
when the molecules are set in vibration by a white-hot platinum wire they are 
still capable of exercising an enormous mechanical effedt. 
Mr. J. W. Groves, of the South London Microscopical Club, after cleaning 
glass slides for mounting microscopical objedts, by one of the usual processes, 
fastens them together by their edges, after the manner of the well-known 
artists’ sketching-blocks. This is easily done with a pile of slips, by fixing 
round their edges a piece of ready-gummed tissue-paper, 10 inches long, and 
of a width suitable to the number of slides, so that, although they are firmly 
bound together, their surfaces are left uncovered. The block is left to dry, 
when each slip may be detached by running the thumb-nail round its edges. 
The surface next the adjoining slip should be used for the preparation to be 
mounted on, as it is, of course, quite clean, although the exposed one may 
have become dirty : the fragments of tissue-paper are removed after the 
mount is completed. 
Mr. H. C. Sorby, F.R.S., has described to the Royal Microscopical Society 
a new arrangement for distinguishing the direction of the axes of doubly- 
refrabting substances. The usual method is to employ plates of selenite of 
various thickness : this, however, involves great difficulty in selecting one of 
nearly the same tint as that of the crystal examined under the polarising mi- 
croscope; and unless this can be done it is impossible to obtain so decided a 
result that the direction of the positive and negative axes can be seen at once 
and no consideration be required. Mr. Sorby employs a wedge-shaped plate 
of quartz, cut parallel to the principal axis, ij inches long and \ an inch wide. 
At its thickest end it is i-20th of an inch thick, and thins off to the sharpest 
possible edge : this is fixed on a glass plate so as to leave a space of glass 
4-ioths of an inch long by i an inch broad beyond this thin end of the quartz. 
The combined plates are fixed in a brass frame, like that for a micrometer, 
which slides into the eye-piece. On using polarised light with a crossed 
analyser over the eye-piece, and arranging the plate so that the part with only 
glass is in front, we see the objedt in its normal state, and the rest of the field 
black, and on pushing forward the quartz wedge we see the field of the mi- 
croscope crossed with coloured bands, gradually rising from the bluish white 
of the first order, through all the brighter orders of colours, to the faint reds 
and greens, and upwards to what cannot be distinguished by the unaided eye 
from white light. If some crystal, giving any tint, be on the stage of the 
microscope, we can usually see at once whether the tints are raised or de- 
pressed, by the manner in which it alters the colour of the bands ; and by 
pushing the quartz wedge backwards and forwards, there may be no difficulty 
in finding the exa< 5 t place where the plate of quartz so exadtly neutralises the 
action of the crystal that it appears black. If this does not occur in any 
place, and, on the contrary, the tints appear to be raised, the eye-piece and 
the plate must be rotated through an angle of go degrees, and the requisite 
place can then be easily found. The plate of quartz being so cut that its 
longer axis is parallel to the principal axis of the crystal, we know that this 
longer axis is positive, and thus also at once know which is the positive and 
which the negative axis of the crystal under examination. We can also at 
once see what is the true order of colour which it gives, since we can readily 
count it up from the bands due to the quartz alone seen crossing the field of 
