CHAP. IT.] KESPIKATIOK 445 



The vacuum produced by the ordinary mechanical air-pump is 

 insufficient to extract all the gas from blood. Hence it becomes 

 necessary to use a mercury pump capable of producing a large Tor- 

 ricellian vacuum. In the form of mercurial pump which bears 

 Ludwig's name (Fig. 87) two large globes of glass, one fixed and the 

 other moveable, are connected by a flexible tube ; the fixed globe is 

 made to communicate by means of air-tight stopcocks alternately 

 with a receiver containing the blood, and with a receiver to collect 

 the gas. When the moveable globe filled with mercury is raised 

 above the fixed one, the mercury from the former runs into and com- 

 pletely fills the latter, the air previously present being driven out. 

 After adjusting the cocks, the moveable globe is then depressed 

 thirty inches below the fixed one, in which the consequent fall of 

 the mercury produces an almost complete vacuum. By turning the 

 proper cock this vacuum is put into connection with the receiver 

 containing the blood, which thereupon becomes proportionately 

 exhausted. By again adjusting the cocks and once more elevating 

 the moveable globe, the gas thus extracted is driven out of the fixed 

 globe into a receiver. The vacuum is then once more established 

 and the operation repeated as long as gas continues to be given off 

 from the blood. 



A modified form of pump working on the same principles as 

 that of Ludwig, but involving the use of only one globe to be made 

 vacuous and one moveable reservoir for mercury, has been constructed 

 by Pfluger. It presents several advantages over the one just de- 

 scribed, the chief being that (i) non-defibrinated blood may be used 

 for the extraction of its gases, (ii) the vacuum into which the gases 

 are evolved is large, (iii) this vacuum is kept dry by being con- 

 nected laterally with a vacuous chamber containing sulphuric acid. 

 The details of its construction are however complicated, and the 

 greatest care is required in its use to avoid breakage. Of later years 

 a simplified form of pump has been introduced for laboratory work. 

 It was first used by Gre'hant and Paul Bert, and is now frequently 

 called an Alvergniat's pump, from the name of its present maker. 

 Fig. 88 gives a diagrammatic representation of its construction. 



A is a glass bulb some five inches in diameter, blown on to a 

 glass tube a below and on to a vertical tube b above. The lower 

 end of a is connected by a thick-walled india-rubber tube with a 

 reservoir for mercury B, which can be raised and lowered by means 

 of a string passing over a pulley c. The vertical tube b is thickened 

 at one place, and into this thickened portion a three-way tap d is 

 ground. The upper end of b is prolonged (above the three-way tap) 

 into a fine point. This point passes by a tight joint through the 

 bottom of a vessel e, which can be partly filled with mercury, and 

 over which a receiver /, filled with mercury for the collection of the 

 gases, can be inverted. A tube g fused on laterally to one opening 

 of the three-way tap d places the latter in connection with a thick- 

 walled WoulfPs bottle C containing a layer of strong sulphuric acid. 

 The second tubulure of this bottle is similarly connected by an 

 elastic tube with the vessel Z>, into which blood or other fluid may 

 be introduced by means of the tap h. All the moveable joints of 

 the apparatus are protected by india-rubber tubes into which water 



