G, H. PARKER 693 



the weight in milligrams of 1 cc. of carbon dioxide at 24°C. and 762 

 mm. of barometric pressure, the conditions of the test. The result 

 of this operation was then multiplied by 100,000 to permit the final 

 number to be expressed in hundred- thousandths of a milligram (F). 

 In this way the weight of carbon dioxide delivered per second and 

 expressed in hundred-thousandths of a milligram was arrived at. 



In the use of this apparatus it is assumed that equal concentrations 

 of carbon dioxide produced the same color tints in the indicator. 

 Since in all four tests the readings were always begun at the same 

 tint and ended at another tint always the same, it follows that the 

 amount of carbon dioxide delivered per second in each test multiplied 

 by the number of seconds over which the test extended ought to 

 yield a constant. And such seems to be the case, as is shown in the 

 last line of Table II in which the product of the average times 

 (Table I) by the weights of carbon dioxide delivered per second are 

 given. These constants vary from 1,238 — to 1,318-f- and average 

 1,283.5 ; they are a measure of the amount of carbon dioxide necessary 

 in this particular piece of apparatus to change the indicator from 

 the tint characteristic of pH 7.78 to that for pH 7.36. 



Another way of expressing these relations is shown in the graph 

 (Fig. 1) in which the weights in hundred-thousandths of a milligram 

 of carbon dioxide delivered are plotted as abscissse, and the rates 

 at which the indicator changed as ordinates. As the plotting shows, 

 the relations are clearly linear. 



In using these results to determine the absolute amount of carbon 

 dioxide produced by an organism, it is convenient to express them 

 in the form of an equation thus: 



K= TXW 



where K is the constant already determined for the apparatus, 

 T the time in seconds for the change in the indicator from one pH 

 value to the other, and \V the weight of carbon dioxide in hundred- 

 thousandths of a milligram delivered per second. As the constant 

 and the time in the operation described are the known factors and 

 the weight the desired one, the most convenient statement of the 

 equation is 



