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capable of containing the products of respiration during five or ten 

 minutes, or must absorb them as fast as they are emitted. There 

 must be no adverse pressure upon the respiration. In collecting the 

 expired air in a bag, there will be the fallacy of not being able to 

 empty the bag completely ; and unless special care be taken, there 

 will be an adverse pressure from the weight of the sides of the bag, 

 and from their cohesion. Moreover, it is impossible to measure the 

 expired air by such means ; and if it be passed through a spirometer, 

 there will be a fallacy from the pressure required to move the instru- 

 ment ; or if it be passed into a graduated tube, there will be a change 

 of bulk from temperature and pressure. If only a part of the col- 

 lected air be submitted to analysis, it will be very difficult to obtain 

 a fair sample, since the specific gravities of the component gases vary 

 much. In seeking the absorption of the gas, it is essential that the 

 expired air should not be forced through a layer of fluid, since the 

 adverse pressure upon the respiration would cause either defective 

 expiration or an increased effort to expire, and in both cases error, 

 but in opposite directions, would occur. No arrangement of solid 

 absorbents with moistened surface can be so made that it shall absorb 

 all the carbonic acid during the process of expiration. No combina- 

 tion of tubes within tubes, with a view to increase the absorbing sur- 

 face, can be arranged within a manageable space and weight suited to 

 this purpose. Hence it is requisite that the expired air be passed 

 over a fluid ; and the fluid must be capable of rapidly and certainly 

 absorbing the gas, and offer so large a surface that it may be found 

 by experiment capable of absorbing the whole of the carbonic acid 

 during the period of expiration. The air should be exposed in thin 

 layers to the surface of the absorbent, and only a small column of it 

 be offered at the same moment, so as to allow a long period to elapse 

 before each small portion of the expired air shall have traversed the 

 whole surface. A test apparatus should be attached at the end, and 

 the test be occasionally applied during the inquiry, so as to ascertain 

 if any unknown cause of error exists. Any portion of the absorbing 

 fluid which may have been carried along by the current of the pre- 

 viously dried air must be arrested before the air escapes into the at- 

 mosphere, and no element of the expired air, besides the carbonic acid 

 which the absorbent might retain, must be allowed to enter the 

 absorbing apparatus. By this method the amount of carbonic acid 



