130 Progress in Science. (January, 
removed by means of a towel, and the small slit at the margin of the plug 
carefully closed by means of hog’s lard, which should be spread in a thin layer 
around the entire margin of the plug, to prevent the spirit from in any way 
reaching the cavity above the plug. The screw (p) must not be touched until 
after the freezing is completed, in case this accident occur. The tissue to be 
frozen, together with an imbedding fluid, are placed in the hole. For ordinary 
purposes a solution of gum arabic may be employed, prepared in the following 
manner :—Add to 1o ounces of water, 2 drachms of camphorated spirit and 
5, ounces of picked gum arabic; when the gum has dissolved, strain the fluid 
through calico, and preserve for use in a corked bottle. The gum when frozen 
can be sliced as easily as a piece of cheese. The gum or other fluid should 
be first placed in the hole of the machine, and when a film of ice has formed 
at the periphery the tissue should be introduced and held against the advancing 
ice until it becomes partially frozen. In this way a portion of tissue may be 
secured, in any position, for the process of section. Lay a piece of gutta- 
percha upon the brass plate (B) so as to cover the cavity containing the tissues 
and prevent the entrance of heat, and the accidental entrance of salt from the 
freezing mixture. Place in the freezing-box (c) alternate quantities of finely 
powdered ice and of salt, and take care they are pushed round the tube of the 
machine, and also that the tube (H) is kept open, in order to permit of the con- 
stant egress of the water from the melting ice. The freezing can be most 
rapidly effected by the addition, at short intervals, of ice and salt, and by 
repeatedly stirring the mixture, in order that the escape of water may be 
facilitated. It is possible, especially in winter, to have the tissue frozen too 
hard to permit of its being readily cut. It splinters when it is too hard. 
This is prevented by discontinuing the further addition of the freezing mixture, 
or by dropping water or a three-quarter per cent salt solution, at the ordi- 
nary temperature, on the surface of the frozen tissue, or by heating the razor 
slightly. With regard to the cutting tool, it may be stated that a razor 
answers perfectly well for all ordinary purposes. ‘The blade should always be 
hollow on both surfaces (R, Fig. 3). It is a mistake to employ a flat knife, for 
it is scarcely possible to keep the surface of the brass table of the machine 
smooth enough to permit of the knife lying quite flat. The knife should be 
pushed obliquely through the tissue, which should be cut at one sweep. This 
is not possible with a frozen tissue, if the ice be too hard. For an unfrozen 
tissue embedded in the paraffin the knife should be wetted with methylated 
spirit. Inthe case of freezing it is not necessary to wet the knife, for the 
melting ice does so readily. The machine has been made by Mr. Baker, of 
High Holborn. 
Mr. John Barrow* recommends naphthalin as a material for embedding soft 
tissues for the purpose of cutting sections. The advantages claimed for 
naphthalin over wax and other substances are—A low fusing-point, absence of 
contraction, the minimum of injury to the edge of the knife, and very ready 
solubility after cutting in benzol or spirit, so that the substance is at once 
removed from the section without injury. 
Mr. F. H. Wenham, at the October meeting of the Royal Microscopical 
Society, exhibited, under the microscope, a roughed pattern on a piece of 
glass produced by the American ‘‘ sand-blast,”’ remarking that the microscopi- 
cal appearance of the “ greyed”’ surface is quite distin& from that of ordi- 
nary ground glass. Observations under a low power give some insight into 
the modus operandi, It was stated, at the late meeting of the British Associ- 
ation, in the discussion that followed the description of the process, that a 
large crystal of corundum was speedily perforated with ordinary sea-sand and 
a blast-pressure of 300 lbs. per square inch. Corundum is several degrees 
beyond emery in hardness, approaching near to that of the diamond. But it 
was further stated that, under the conditions named, diamond itself became 
speedily worn away. At first sight it seems extraordinary that the hardest 
known material should quickly be destroyed by one considerably softer. The 
* Manchester Literary and Philosophical Society, Microscopical and Natural History 
SeGtion, April 21, 1873. 
