CHANGES IN- THE REACTION OF MEDIA 219 



is completely filled with a 2 per cent, solution of sodium 

 hydroxide (NaOH) and closed tightly with the thumb. 

 The fluid is shaken thoroughly with the gas and allowed to 

 flow back and forth from bulb to closed branch and the 

 reverse several times, to insure intimate contact of the CO 2 

 with the alkali. Lastly, before removing the thumb all the 

 gas is allowed to collect in the closed branch, so that none may 

 escape when the thumb is removed. If C02 be present, 

 a partial vacuum in the closed branch causes the fluid to 

 rise suddenly when the thumb is removed. After allowing 

 the layer of foam to subside somewhat the space occupied 

 by gas is again measured, and the difference between this 

 amount and that measured before shaking with the sodium 

 hydroxide solution gives the proportion of CO 2 absorbed. 

 The explosive character of the residue is determined as 

 follows: the thumb is placed again over the mouth of 

 the bulb and the gas from the closed branch is allowed 

 to flow into the bulb and mix with the air there present. 

 The plug is then removed and a lighted match inserted 

 into the mouth of the bulb. The intensity of the explosion 

 varies with the amount of air present in the bulb." 



Durham's Fermentation Tube. Durham employs a con- 

 venient modification of the ordinary fermentation tube, 

 which is constructed in the following manner: test-tubes 

 of about 10 or 12 c.c. capacity are placed in an inverted 

 position within a larger test-tube, and the latter plugged 

 with cotton in the usual way and sterilized. (See Fig. 

 37.) The small tube should fit loosely within the larger 

 one. The medium to be used is run into the larger tube 

 until there is present about 50 per cent, more than the 

 volume of the smaller tube. The whole is then steri- 

 lized in streaming steam by the fractional method. After 



