MOIST CHAMBER. 39 



separated from the stroma ; but as the colored ma- 

 terial is exuding it becomes coagulated by the astrin- 

 gent reagent, and in place of being dissolved in the 

 surrounding liquid, as after the action of acetic acid, 

 it remains attached to the stroma as a small, bright, 

 reddish projection. 



In the first part of the reaction, viz., the render- 

 ing the corpuscles globular, it acts similarly to other 

 weak acids. 



The most convenient strength of solution to use 

 is 1 per cent.; it. should be freshly prepared. It is 

 well, moreover, first to mix the drop of blood upon 

 which it is desired to test the action of the reagent 

 with an equal amount of salt solution ; otherwise 

 the tannic acid produces such a dense precipitate 

 with the albumen of the serum that the view of the 

 corpuscles is greatly obscured, and, indeed, the re- 

 agent with difficulty reaches them. 



Preparation 6. Action of Weak Alkalies. 

 A mixture of 1 part .of caustic potash to 500 of salt 

 solution may be used, a drop being added to the 

 preparation in the usual way (p. 37). The reaction 

 takes place very suddenly ; the corpuscles, both 

 white and red, swell up as soon as the reagent 

 reaches them, appear to burst, au.d then entirely dis- 

 appear. The white are affected by a weaker solu- 

 tion than the red. This is apparent from the fact 

 that, as the liquid slowly diffuses and mixes with 

 the blood, the white may be seen to become destroyed 

 in parts where the red are still unaffected. 



Preparation 7. Chloroform. To observe the 

 action of chloroform vapor upon blood a moist 

 chamber is used. This is an apparatus for keeping 

 a tissue or fluid under examination in its naturally 

 moist condition, whilst at the same time allowing 

 its surface to be exposed to air or to any desired gas 

 or vapor. The simplest form of moist chamber is 

 made out of a small piece of soft putty or modelling- 

 wax, which has been rolled out between the fingers 

 into a round cord about 2 inches long and J inch 



