188 A New Freezing Microtome. 



replace it ; screw it down to any desirable extent, and there leave 

 it. The object of this is to prevent the screw from becoming fixed 

 by the freezing. The spirit which has come up above the ping (K) 

 must be thoroughly removed by means of a towel, and a small slit 

 at the margin of the plug carefully closed by means of hog's lard, 

 which should also be spread in a thin layer around the entire margin 

 of the plug to prevent the sphit from in any way reaching the 

 cavity above the plug. The screw (D) must not be touched until 

 the freezing is completed, in case this accident occur. The tissue 

 to be frozen, together with an imbedding fluid, are placed in the 

 hole. If we desire to be very careful of the protoplasts of the 

 tissue, this fluid must be a so-called " neutral " fluid, such as blood- 

 serum, albumen, or a three-quarter per cent, solution of common 

 salt. It is, however, difficult to cut these fluids when frozen, owing 

 to their becoming crystalhne. For ordinary purposes it is prefer- 

 able to employ a solution of gum arabic. This solution is made in 

 the following manner : — Add to ten ounces of water two drachms 

 of camphorated spirit and five ounces of picked gum arabic ; when 

 the gum has dissolved, strain the fluid through calico or tow, and 

 preserve for use in a corked bottle. The gum, when frozen, can be 

 cut Avith great facility ; indeed, it can be sliced as easily as a piece 

 of cheese. The gum or other liuid should be first placed in the hole 

 in 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. Secure 

 the gutta-percha by inverting, say, a small tumbler or beaker upon 

 it, and place a weight thereon. Surround the box from side to side 

 by two or three layers of flannel. These can be readily fixed by 

 pushing them between the machine and the table to which it is 

 fixed. Place in the freezing box (G) alternate quantities of finely- 

 powdered ice and of salt, and take care that they are pushed round 

 the tube of the machine, and also that the tube (H) is kept open in 

 order to permit of the constant egress of the water from the melting 

 ice. The freezing can be most rapidly eff'ected by the addition, at 

 short intervals, of small quantities of ice and salt, and by repeatedly 

 stirring the mixture, in order that the escape of water may be 

 facilitated. This is to be preferred to covering the whole machine 

 up in a bag of the freezing mixture. This process is, when prac- 

 tised, really very simple, and can be fully carried out in from 

 fifteen to twenty minutes. A number of tissues may be frozen and 

 cut at the same time. It is possible, especially in winter, to have 

 the tissue frozen too hard to permit of its being readily cut. It 

 sphnters when it is too hard. This is prevented by discontinuing 



