204 



The Structure of Protoplasm 



(b) Control of Pressure. In order to establish a pressure differ- 

 ence between the two compartments, the opening of one compart- 

 ment is connected with a passage leading to an aspirator tube and a 

 manometer, while the opening of the other compartment is kept open, 

 as shown in Figure 5. 



Aspirator As, used for controlling the air-pressure, is made of a 

 thick, rubber tube, the middle part of which is placed between two 

 solid metal plates (shaded in Fig. 5) . The upper plate is equipped 



so that it can be moved 

 gently up and down by 

 means of a screw S, with 

 the result that the inner 

 volume of the rubber as- 

 pirator tube can be con- 

 trolled most exactly. 



After opening the stop- 

 cock SC, the aspirator is 

 pushed down gradually by 

 tightening the screw S. 

 The air pushed out by this 

 depression is let off at the 

 stop-cock SC. When the 

 aspirator tube is pushed 

 down to about half of the 

 stroke, the stop-cock SC 

 is closed. The whole sys- 

 tem consisting of compart- 

 ment B, the aspirator As 

 and the manometer M is 

 now air-tight. If the screw 

 S is tightened farther, the 

 inner pressure of the entire system is increased, i.e., the pressure 

 of compartment B becomes higher than that of compartment A. If, 

 on the contrary, the screw S is loosened, being turned in the opposite 

 direction, then the aspirator tube will expand because of the elas- 

 ticity of its rubber wall. The inner pressure of the system, including 

 compartment B, becomes, therefore, lower than atmospheric pres- 

 sure. The pressure of compartment B can thus be controlled to any 

 desirable degree within the necessary range, both in regard to 

 positive and negative compression, by means of a single screw. The 

 range desired is usually between + and — 30 cm. of water. 



Fig. 4. A. The double chamber, after in- 

 verting the glass slide with the agar sheet 

 and the material. The thin strand of proto- 

 plasm runs through the agar wall. B. The com- 

 pleted preparation: finished observation cham- 

 ber fastened between metal frames with two 

 rubber gaskets. 



