146 
NOTES AND MEMORANDA. 
In addition to this, the instrument has been made larger (it 
measures 18^ cm. in length), by which means the object to be cut may 
be considerably longer. 
The most important drawback hitherto, however, was not being 
able to alter easily the direction of the section when the object was 
once screwed down. The whole object had to be taken out every time, 
and the body in which it was imbedded differently shaped. Those who 
have had to make longitudinal sections through hairs, &c., know what 
tliat is. Besides this, with the old arrangement it was quite impossible 
to make oblique sections through their whole length of embryos 
which were at all curved. It was generally the case that the sections 
gradually took another direction with respect to the longitudinal 
axis, and not unfrequently the oblique section passed finally into a 
frontal section. In the modified instrument the clamp for the object 
is now fastened to a round socket ; by this arrangement it becomes 
possible to alter with great rapidity the inclination of the section in 
every direction, at least within certain limits, more extensive however 
than most objects require. The great advantage of this arrangement 
is self-evident. The round socket ought to be kept oiled with good 
machine oil, and the clamp screw which acts on a lever to fix the 
round socket must never be screwed too tight. The foot is not made 
of cast iron, as before, but is a heavy brass plate. It is best to have 
four knives, two straight and two angular. With these improvements 
this instrument places even a novice in a position to produce excellent 
sections in the course of a short time.* 
The Movement of Microscopic Particles suspended in Liquids . — 
Professor Stanley Jevons records in the ‘Quarterly Journal of 
Science ’ for April, under the above title, the result of the investiga- 
tions he has made on this subject. He objects to the names “ mole- 
cular movement,” “ Brownian movement,” or Dujardin’s “ titubation,” 
and suggests ^‘pedesisf from the Greek TrrjhrjcrLs, leaping or bounding. 
The best possible exhibition of the motion is to be got by grinding 
up a particle of pumice-stone in an agate mortar, and mixing it with 
distilled water. The minute angular particles will be seen under the 
microscope to leap about with an incessant quivering movement, so 
rapid, that it is impossible to follow the course of a particle. 
The substance most convenient for experiments, he considers, 
however, to be fine pure china clay or kaolin, a small quantity 
of which shaken up with pure water makes a milky liquid, a drop of 
which will show the motion in great perfection. He considers -that 
he has completely disproved the suggestion that the motion is excited 
by rays of light or heat falling upon the liquid, or that it is con- 
nected with the shape of the particles ; and from the observations he 
made on the length of time during which the motion will continue, 
he disagrees with the opinion recently expressed by Professor 
Tyndall that it is due to surface tension. He then proceeds to point 
out the intimate connection between pedesis and suspension of par- 
ticles in liquids. In the absence of pedesis, suspended particles 
attract each other and become aggregated together into little groups, 
which then acquire sufficient weight to force their way down through 
* Dr. H. Keichenbach, Assistant to the Zoological Institute of Leipzig Uni- 
versity, in the ‘ Archiv fiir Mikroskopische Anatomic,’ xv. 1. 
