MUSCLE 



539 



filled with mercury. The tube is inserted into a small glass bottle,"* 

 and fastened in its neck by a cork or a plug of sealing-wax which 

 does not quite fill the opening, so that the interior of the bottle is 

 still in communication with the external air. The upper end of the 

 tube is connected by a short piece of rubber-tubing with a glass 

 T-tube as in Fig. 154. The bottle is partially filled with 5 to 10 per 

 cent, sulphuric acid, under which the capillary dips. By means of a 

 small reservoir made from a piece of glass tubing filled with mercury, 



B, bottle containing sulphuric acid ; Hg, 

 mercury ; E, E', platinum wires. E dips 

 into the mercury in the vertical tube, 

 and E' is fused through the bottom 

 of B, so as to make contact with the 

 mercury in B, the other end of it passing 

 out through a small hole in the wooden 

 platform F, on which B rests. F is 

 fastened to the stage of the microscope, 

 S, by a pin, G, passing through one of 

 the clip-holes, and to the wooden upright, 

 D, by the pin, H. D fits tightly over the 

 microscope stage, but can be moved 

 laterally a little so as to bring the capil- 

 lary into the middle of the field. I, 

 stem of glass T-tube passing through 

 a hole in D. L, rubber-tube connecting 

 the capillary point with the vertical por- 

 tion of the T-tube. A is a reservoir con- 

 taining mercury connected by the rubber- 

 tube M to I. A can be raised or lowered 

 by sliding it in the clips K. In the figure 

 the capillary tube appears as if the mer- 

 cury extended to the very point of it. 

 This should not be the case ; the sul- 

 phuric acid should rise for some distance 

 in the capillary, so that the mercury shows 

 a sharply-bounded meniscus in the tube 

 as is represented in C, the magnified 

 image of the capillary as seen with the 

 microscope. 



FIG. 154. A SIMPLE CAPILLARY 

 ELECTROMETER. 



and connected with the stem of the T-tube, a little mercury is forced 

 through the capillary so as to expel the air in it. When the pressure 

 is lowered again, sulphuric acid is drawn up, and now lies in the 



* A parallel-sided bottle is best, as it gives the clearest image of the 

 meniscus. But it is easiest to make a cylindrical bottle from a piece of 

 wide glass-tubing, and to insert a platinum wire into it before closing it 

 at the bottom in the blow-pipe flame. The tube can then be firmly 

 fastened with sealing-wax in a depression in a piece of wood, the wire 

 being brought out through a hole in the wood. Once the instrument is 

 arranged, there is little chance of the capillary getting broken, and there 

 is very little evaporation of the acid'. 



