318 THE HUMAN MOTOR 



divisions or volumes of air to analyse. This is represented by N. 

 A reading thus made backwards Will prevent the gas stopping in 

 the tube d, which would give rise to small errors. 



By a similar operation, a sample of the solution of potassium 

 can be taken, bringing it to the middle of the tube g and then 

 expelling it immediately. If, afterwards, various samples of 

 air are passed through it, all the carbonic gas will be absorbed. 

 Bring it to 600, the division on the left being, for instance, 165 ; 

 therefore X' .= 600 165 = 435 volumes. It will be seen that 

 450 435 =15 volumes in 450 represents the carbonic acid gas 

 content. 



Next take a sample of pyrogallate and absorb the oxygen : 

 Let N" = 600 240 = 360 volumes. 



The oxygen content is, therefore : 435 360 = 75 volumes 

 in 450. 



The proportions will be : 



C0 2 = x 100 or 3-33% ; 



=X 10 r 16 ' 66 " 



The calculation is therefore very easy. N, N' and N* being 

 obtained ; then : 



N 

 N 



co 2 % = x :-^ x 100 ; 



o/ _ 



/O - 



XT 



The rapidity of the operations (about 15 minutes) render it 

 possible to dispense with any correction for temperature varia- 

 tions. It should be noted, however, on the thermometer t. The 

 absorption of oxygen will be accelerated by shaking the column 

 of gas, which will reduce the total duration to 10 minutes. The 

 sensibility of the measurements can be increased by reading to 

 half divisions. 



Precautions. Make two or three readings before passing from 

 one reagent to the next. After having shaken the gas, bring 

 it quickly to the graduated part, and take at least 400 volumes 

 for each analysis. 



The movement of the reagents should be very slow. At the 

 end of the experiment, draw in a drop of hydrochloric acid, clean 

 the tube, and finish the cleaning with distilled water and dry air. 



In view of the small capacity of the tube, one cubic, centimetre 

 of air will be sufficient for several analyses, and in addition the 

 whole apparatus is quite portable. 



