en. xxiv.] 



VENTILATION 



383 



(p. 379) show this; and the following experiment is also quite 

 conclusive. A frog can be kept alive for some time after salt solution 

 is substituted for its blood. The metabolism goes on actively if the 

 animal is kept in pure oxygen. The taking up of oxygen and giving 

 out of carbonic acid must therefore occur in the tissues, as the 

 animal has no blood. 



Ventilation. It is necessary to allude in conclusion to this very 

 important practical outcome of our con- 

 sideration of respiration. 



Some Continental observers have 

 stated that certain noxious substances 

 are ordinarily contained in expired air 

 which are much more poisonous than 

 carbonic acid, but researches in this 

 country have failed to substantiate this. 

 If precautions be taken by absolute clean- 

 liness to prevent admixture of the air 

 with exhalations from skin, teeth, and 

 clothes, the expired air only contains one 

 noxious substance, and that is carbonic 

 acid. 



Absolute cleanliness is, however, not 

 the rule; and the air of rooms becomes 

 stuffy when the amount of expired air in 

 them is just so much as to raise the per- 

 centage of carbonic acid to 01 per cent. 

 An adult gives off about 0*6 cubic feet 

 of carbonic acid per hour, and if he is 

 supplied with 1000 cubic feet of fresh 

 air per hour he will add 0'6 to the 0'4 

 cubic feet of carbonic acid it already con- 

 tains; in other words, the percentage of 

 that gas will be raised to O'l. An hourly 

 supply of 2000 cubic feet of fresh air 

 will lower the percentage of carbonic 



acid tO 0-07, and Of 3000 CUbic feet tO Flo . 337.-Ludwig' S Mercurial Pump. 



0*06, and this is the supply which is 



usually recommended. In order that the air may be renewed with- 

 out giving rise to draughts, each adult should be allotted sufficient 

 space in a room, at least 1000 cubic feet. 



The Mercurial Air-Pump. 



The extraction of the gases from the blood is accomplished by means of a 

 mercurial air-pump, of which there are many varieties, those of Ludwig, Alvergniat, 

 Geissler and Sprengel being the chief. The principle of action in all is the same. 

 Ludwig's pump, which may be taken as a type, is represented in fig. 337. It con- 



