92 



THE AMERICAN NATURALIST 



[Vol. LIV 



two well-known facts: (1) that the carbon dioxide content of 

 out-of-door air varies only slightly from day to day, and scarcely 

 at all during the course of an ordinary series of experiments and 

 (2) that the distribution of a quantity of carbon dioxide between 

 given amounts of water and air may readily be calculated from 

 the known absorption coefficients for this gas at various tem- 

 peratures, which may be obtained from the Landolt-Bdrnstein 

 Tdbellen or elsewhere. The first fact obviates. the necessity of 

 removing all of the carbon dioxide from the apparatus at the 

 beginning of the experiment and the second makes it possible, 

 with a minimum of trouble, to prepare standards for comparison 

 which contain any desired amount of carbon dioxide, thus enab- 

 ling measurements of quantity as well as of rate of production 

 to be made. 



The apparatus in its simplest form consists of a Nonsol test 

 tube about 75 mm. by 10 mm., which has been drawn out in the 

 Bunsen flame somewhat above its middle into a constriction ap- 

 proximately 30 mm. long and 4 mm. in diameter. It is closed 

 with a well-rolled cork which has been thoroughly soaked in, 

 and coated with, acid free paraffin. The indicator solution is 

 placed in the lower portion of the tube ; the constriction prevents 

 it, when the tube is agitated, from splashing on the organism con- 

 tained in the upper portion. To enable quantitative measure- 

 ments to be made, a series of standard tubes is required in which 

 known amounts of carbon dioxide have been added to the same 

 indicator solution as that used in the apparatus just described. 

 For the preparation of these standards the following device is 

 employed. A Pyrex or Nonsol flask with a capacity of about 150 

 c.c. is fitted with a well rolled cork through which a hole is bored 

 and one of the unaltered Nonsol test tubes forced in such a way 

 that when the cork is in the flask the bottom end of the tube pro- 

 jects freely upwards and its lip fits against the small end of the 

 cork. After being thus prepared, the sides and the lower sur- 

 face of the cork are thoroughly coated with paraffin of the best 

 quality, partly to prevent leaks but chiefly to protect the indica- 

 tor solution from actual contact with the cork, which would be 

 very likely to cause changes in its color. It is desirable, though 

 not absolutely necessary, to have as many of the flasks with the 

 prepared stoppers as the number of standards to be employed — 

 usually three to five. The only additional pieces of apparatus 

 required are a carbon dioxide generator, a box for comparing the 

 colors of the indicator tubes such as is commonly used in colori- 



