CHAP. IL] RESPIRATION. 555 



The vacuum produced by the ordinary mechanical air-pump is in- 

 sufficient to extract all the gas from blood. Hence it becomes necessary 

 to use a mercury pump capable of producing a large Torricellian 

 vacuum. In the form of mercurial pump which bears Ludwig's name 

 (Fig. 73) two large globes of glass, one fixed and the other moveable, 

 are connected by a flexible tube; the fixed globe is made to com- 

 municate 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 completely 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 be- 

 comes 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 con- 

 tinues 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 Pfliiger. 

 It presents several advantages over the one just described, 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 connected 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 Grehant and Paul Bert, and is 

 now frequently called an Alvergniat's pump, from the name of its present 

 maker. Fig. 74 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 J3, 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 Woulff's bottle C con- 

 taining 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 can be poured, and a metal casing round the 

 tap d, which may also be filled with water, similarly prevents the 

 possibility of any leakage here. 



