1322 



Journal of Applied Microscopy 



connect tank and machine. When the microtome is screwed directly upon the 

 carbon-dioxide tank it is necessary that the latter lie in a horizontal position. On 

 the other hand, if an L-shaped piece of tubing be utilized to connect freezing 

 microtome and tank, the tank may be placed at any desired angle. 



The valve of the tank is used to control the escape of gas into the machine. 



The axis and main support of the instrument consists of a stout tube with a 



narrow lumen (K-D, Fig. 2). This axial tube is united by a nut (J, Figs. 1 and 2), 



either directly to the nozzle of the tank, or, in case a connecting tube is used, to 



the latter. 



On the top of the axial tube the freezing stage (A, Fig. 1, A-C, Fig. 2), is 

 screwed. This stage piece consists of two parts, a base and a cover. The base 

 is the part screwed into the upper end of the axial tube (C, Fig. 2). To this base 

 the cover piece is screwed (A, Fig. 2). Between the base of the stage and the 

 axial tube is placed a thin brass plate (D, Fig. 2), with a very narrow aperture at 

 its center. Through this narrow aperture the carbon-dioxide escapes into the 

 lumen of the stage piece (C, Fig. 2). The difference in pressure on the two sides 

 of the brass plate causes a very rapid expansion of gas between the cover and 

 base of the freezing stage. The passage open for the escape of gas from the 

 lumen of the base (C, Fig. 2) to the external world is in the form of a spiral pas- 

 sage which finally opens out through the side of the cover, as shown in Fig. 1, A. 

 Between the cover and base of the freezing stage an asbestos washer is placed. 



The expanding gas, therefore, can absorb 

 little heat from the base of the stage. Almost 

 all heat absorption must take place from the 

 cover. This heat absorption is greatly facili- 

 tated by the metallic spiral, which projects 

 down from the cover so as to give rise to the 

 spiral passage through which the gas escapes. 

 Through the mechanism here described 

 far the greater part of the heat absorbing 

 power of the expanding gas is utilized to 

 lower the temperature of the surface of 

 the cover of the freezing stage. The tem- 

 perature of the rest of the machine is but 

 little altered. Good control of the tempera- 

 ture of the freezing stage can be thus 

 maintained. This control is further rendered 

 possible by the valve of the tank. If this 

 valve be turned on full the temperature of 

 the cover of the freezing stage will be quickly 

 reduced to a very low point. Tissue placed 

 on it is quickly frozen. On the other hand, 

 if the gas is not permitted to escape from the 

 tank with full force, the difference in pressure 

 on the two sides of the brass plate is less and 

 heat absorption from the cover is less marked. 



A 



CiJ 



H 



Fig. 2. 



A. Cover of freezing stage ; B. Glass track for 

 carrying knife ; C. Aperature in base of freez- 

 ing stage ; D. Aperture in tliin brass plate ; 

 E. Spiral spring ; F. Tubal base of knife stage ; 

 G. Check for limiting movements of knife-stage ; 

 H. Groove for G ; I. Wheel ; J. Nut for at- 

 taching axial tube to tank ; K. Opening into 

 lumen of axial tube. 



