and Laboratory Methods. 1323 



In this way tissues placed on the cover may be slowly frozen without subjecting 

 them to severe cold. Thus, too, a constant low temperature may be maintained 

 by opening the tank valve to the required point. 



The mechanism for controlling the thickness of the sections is equally 

 simple. On the lower end of the axial tube a movable wheel (I, Fig. 1 and 

 Fig. 2) is placed. This wheel moves up and down the axial tube on a screw 

 thread, cut twenty-five threads to the inch. A complete revolution of the wheel, 

 therefore, raises or lowers it a millimeter. The margin of the wheel is divided 

 into fifty spaces, each of which therefore represents twenty microns. A pointer 

 (N, Fig. 1) serves to indicate the number of spaces passed in a partial revolu- 

 tion of the wheel, and thus to show the thickness of the sections cut. 



The knife-stage (F-B, Fig. 1 and Fig. 2), consists of a tubal base (F), which 

 surrounds an axial tube and rests on the movable wheel ; and of two flanges (B), 

 which extend above the freezing stage on each side for the support of the cutting 

 blade. The base of the knife stage is moved up the axial tube by screwing the 

 wheel upwards. It is forced down the axial tube by the spring (E, Fig. 1 and 

 Fig. 2) whenever the wheel is turned so as to be carried downwards. The flanges 

 of the knife-stage support parallel glass tracks upon which the cutting blade is 

 carried to and fro. 



For cutting sections a razor or a plane, or almost any good steel blade with a 

 straight edge, may be used. 



The advantages of the machine are as follows : 



1. But little carbon-dioxide is wasted. 



2. The temperature of the freezing stage can be controlled. 



3. The machine, including the tank, may be readily carried about. 

 This should render it of especial value to surgeons. 



4. Above all, it is simple in design, strong, and exceedingly unlikely 

 to get out of order.* 



Charles Russell Bardeen. 

 Anatomical Laboratory, Johns Hopkins University, Baltimore. 



MICRO-CHEMICAL ANALYSIS. 

 XIV. 



BARIUM— Continued, 

 ///. Barium unites with Potassiu7n Ferrocyanide to fonn a Ferrocyanide of 

 Barium and Potassium. 



BaClj -h K^FeCCN)^ = BaK2Fe(CN)g . 5H2O + 2KC1. 



Method. — To the test drop, which should contain no free mineral acid, add a 

 little acetic acid, then a little potassium ferrocyanide, and warm the preparation 

 very gently. In a few seconds rhombs of the double ferrocyanide will appear 



*The description of the microtome here given is essentially similar to one that will appear in 

 the May- June number of the Johns Hopkins Bulletin, 1901. 



