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THE AMERICAN NATURALIST [Vol. LIV 



in equilibrium with ordinary air and therefore to contain at 

 16° C. 0.0006 mg. of C0 2 per c.c., has received additions in the 

 various tubes of known amounts of this gas. This is accom- 

 plished as follows : A current of pure C0 2 from the generator is 

 allowed to pass through a few c.c. of distilled water in the 

 medium sized test tube until the water is saturated at atmos- 

 pheric pressure. It is convenient to use in the test tube a stop- 

 per with two openings, one for the inlet tube which carries the 

 gas below the surface of the water and the other for an open 

 glass tube projecting from the upper part of the tube through 

 the cork and several inches into the air. This permits the excess 

 gas to escape, and since C0 2 is heavier than air, the test tube 

 soon becomes filled with a pure atmosphere of it, the outside air 

 not readily entering through the long and narrow escape tube. 

 It is necessary that the current of C0 2 shall be slow enough to 

 give no more than atmospheric pressure in the test tube. If only 

 a few c.c. of water at a time are charged, the added pressure of 

 2 or 3 cm. of water, due to the dipping of the inlet tube below 

 its surface, is not significant. The minimum time required to 

 saturate the water has not been determined, but it is the custom 

 of the writer when fresh water is taken to allow the current to 

 flow for at least thirty minutes ; afterwards, by keeping the tube 

 corked between experiments, the water remains almost saturated 

 and exact equilibrium may easily be established in five or ten 

 minutes. It is, of course, very important that neither the test 

 tube nor the tube admitting the C0 2 shall give off appreciable 

 amounts of alkali, hence the recommendation that Pyrex or 

 Nonsol glass be used or the same result be secured with inferior 

 glass by means of a thin coating of paraffin. 



Having a solution whose C0 2 content can accurately be calcu- 

 lated if the temperature and the barometric pressure are known, 

 the next step is to add measured amounts of it to successive por- 

 tions of the indicator solution. This is done as follows. The 

 first flask and the test tube in its stopper are filled with distilled 

 water, which by shaking has been brought into equilibrium with 

 out-of-door air, and emptied with the precautions already noted. 

 Five c.c. of the prepared indicator solution are then added, the 

 cork quickly inserted and the whole vigorously shaken. The 

 color of the solution should not change if the proper precau- 

 tions have been taken. The remaining flasks are then treated in 

 the same way. By inverting them, the color of the indicator 

 solution in their respective test tubes can be compared against a 



