512 Experiments 



two or three times, a complete vacuum has been made in the whole 

 apparatus, including the rubber tube, as is proved by the sharp 

 click of the mercury (the "mercury hammer") against the closed 

 cock R, the force of which must be lessened by raising the reservoir 

 carefully. 



When this has been done, and when reservoir B has been 

 lowered as far as possible and cock R has been placed in position 

 3, I open the communicating cock between the bell-jar from which 

 I wish to take an air sample and the rubber tube. Evidently, a 

 certain quantity of air from the bell-jar rushes out and fills bulb A. " 

 As a precaution, I let out this air for fear that the vacuum has not 

 been perfect in the lateral tube and the barometric chamber, and 

 begin the same procedure again. But this time I invert over bowl 

 C a graduated tube full of mercury, and the gas which has been 

 compressed in chamber A as a result of raising reservoir B comes 

 into the tube bubble by bubble through a passage carefully con- 

 trolled in shifting from position 1 to position 2 of the cock. The 

 gas thus collected can easily be removed for analysis. 



The analysis is made over the mercury bowl by means of a 

 solution of potash to absorb the carbonic acid, then by another 

 solution of pyrogallic acid to absorb the oxygen. The differences 

 of level, measured in the graduated tube, by a very simple calcula- 

 tion give the percentage composition of the gas. This method of 

 analysis, extremely convenient and rapid when one is careful to 

 shake the tube vigorously, especially after the introduction of the 

 pyrogallic acid, seems to me superior to any other. 



A German physiologist, who visited my laboratory one day, re- 

 proved me severely for measuring the differences of level without 

 using a cathetometer; for not using the Bunsen method, by bubbles 

 of potash and phosphorus, which gives more exact results; for not 

 having deducted the value of the column of liquid, which lessens by 

 two or three centimeters of water the tension of the air contained 

 in the graduated tube; and for not having taken into account the 

 small quantity of oxide of carbon which may be formed during 

 the absorption of oxygen by the pyrogallate. I should not mention 

 these petty criticisms here if they did not furnish a very character- 

 istic example of a common mistake in method from the other side 

 of the Rhine, which affected pedants would like to import into 

 France. I have already had the opportunity to express my opinion 

 of this useless and dangerous- search for false exactness. I mention 

 • it in reference to the present analyses merely to state that the 

 causes of errors pointed out affect only the third decimal, which I 



