588 



Experiments 



causes, it is best for us first to get an accurate idea of the exactness 

 one may hope to secure by using the apparatus which we have 

 described. Let us consider that the vacuum is made, and that we 

 are bringing to the cock r the syringe containing, for example, 

 50 cc. of blood. Let us say first that it is impossible, considering 

 the caliber of the syringe, to determine this quantity very exactly; 

 we shall be below the truth in taking as possible errors either 

 49.8 cc. or 50.2 cc. Furthermore, there will remain in the rubber 

 tube and the cock r at least 0.5 cc. of blood which will escape 

 analysis: the truth is then that when we say we 

 have tested 50 cc, we have really introduced into 

 the apparatus 49.3 cc. or 49.7 cc. Let us now make 

 the extraction, and let us suppose it perfectly 

 complete: at least we have no means of measur- 

 ing the very small residue which may remain in 

 the apparatus. We shall obtain on the average 

 30 cc. of gases which will have to be collected 

 in two different tubes, if we wish to use narrow 

 tubes so that the readings may not bring too great 

 a cause of error. At the same time as the gases, 

 water vapor has penetrated into the pump and 

 has condensed; each of our tubes always contains 

 1 or 2 cubic centimeters of water. How much 

 carbonic acid in solution has this water absorbed? 

 We do not know. That is not all; since the gas 

 is at a high temperature, before measuring it, we 

 must immerse the tubes completely in little glass 

 mercury basins, narrow and deep, constructed for 

 ^JBL^ this purpose (Fig. 25); during this time, and 



l yij W& under pressure, a new quantity of carbonic acid 

 .^*™^^^ must enter into solution. Perhaps we can, for 

 each tube, estimate at 0.2 cc. or 0.3 cc. the total 

 quantity of this gas of which no account can be 

 taken. 



Now we have two tubes, one of which contains, I suppose, 20 cc, 

 the other 10 cc; if we refer to what has been said on the possible 

 errors of analysis by potash and pyrogallic acid, we shall see that 

 we can vouch for the exactness of the composition only between 

 limits analogous to the following: 



Fig. 25 — Small 

 mercury reser- 

 voir. 



