206 THE BLOOD. 



raised from its present level to that of the highest of the 

 shelves, the stopcock being at the same time turned so that 

 the vertical tube communicates with the external air. but not 

 with the bulb, the mercury will rise till the whole of the verti- 

 cal tube is occupied. The stopcock is now turned so as to 

 make communication only between a c and the bulb, and the 

 mercury-holder is replaced in its original position. As the re- 

 sult of this manipulation, the air previously coirtained in the 

 bulb and the tube leading from it occupies the whole cavity, 

 and (according to Marriotte's law) is expanded, ?'. ?., dimin- 

 ished in density in the same ratio that the volume occupied by 

 it is increased. In other words, the density of the air in the 

 bulb, before the depression of -u, is to its density after as the 

 capacity of the barometer plus the bulb is to that of the bulb 

 alone. To repeat the operation, the stopcock must first be 

 placed in such a position that all channels are closed. > is 

 then raised and the stopcock again turned as at first viz., 

 the horizontal way closed, the vertical way open. The air 

 contained in a c having been discharged, the stopcock is again 

 opened horizontally and closed vertically, and v depressed. 

 The air remaining in the bulb is again expanded in the same 

 proportion as before. If the capacity of the tube, together 

 with its dilatation, be equal to that of the bulb and its tube, 

 it is obvious that the effect of each stroke of the pump will be 

 to halve the density of the air in the bulb ; consequently, if 

 the operation is repeated ten times, the density of the air con- 

 tained in the bulb (supposing it to be dry, and to have an ori- 

 ginal density of 760 millimetres) becomes 760x(?)'=0.74 mil- 

 limetre. By filling the bulb and the tube leading to it, before 

 attaching it, with water deprived of its gases by boiling, the 

 process of exhaustion can be very much shortened. Xo sooner 

 does the mercury sink in the vertical tube (a c) than the water 

 follows it, and can be discharged by raising the mercury- 

 holder with the stopcock open vertically and closed horizon- 

 tally, as before. A vacuum which is almost perfect is thus 

 obtained at a single working of the pump. In the pumps 

 recently made by M. Alvergniat, he has substituted a movable 

 support which works up and down the vertical board by a 

 winch. 



27. Geissler's Pump. The instrument (see fig. 198) con- 

 sists, like that just described, of a fixed vertical tube (a), which 

 is dilated into a large bulb near the top and communicates near 

 its lower end by means of a flexible tube of thick walled caout- 

 chouc with another vessel (b) which can be moved up and down 

 by turning a winch. Above the bulb, the vertical tube, which 

 is nearly a metre in length, ends in a stopcock (gr), so con- 

 structed that the bulb can be completely shut off, or may be 

 brought into communication eithei with the external air or with 



