70 Biology of Micro-organisms 



For the determination of the nature of the gases produced, 

 Theobald Smith has recommended the following method : 



"The bulb is completely filled with a 2 per cent, solution 

 of sodium hydroxid (NaOH) and tightly closed with the 

 thumb. The fluid is shaken thoroughly with the gas and 

 allowed to flow back and forth from the bulb to the 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 CO 2 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 hydroxid solution gives the proportion of 

 CO 2 absorbed. The explosive character of the residue is 

 determined as follows: The cotton plug is replaced 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. The relative propor- 

 tion of gases resulting from the fermentation is frequently 

 of importance for the differential diagnosis of related bac- 



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teria. Smith has designated this relation of ^ o - as the 'gas 

 formula.' The colon bacillus has a gas formula corre- 



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spending to ^ f . Other aerogenic bacilli sometimes 

 show a formula ^j = ^." 



Liquefaction of Gelatin. As certain organisms grow in 

 gelatin, the medium becomes partly or entirely liquefied. 

 This peculiarity is apparently independent of any other 

 property of the organisms, and is manifested alike by patho- 

 genic and non-pathogenic forms. The liquefaction is sup- 

 posed to be dependent upon a form of peptonization. Bit- 

 ter* and Sternbergf have shown that if from a culture in 

 which liquefaction has taken place the bacteria be removed 

 by filtration, the filtrate will retain the power of liquefying 

 gelatin, showing the property is not resident in the bacteria, 

 but in some substance in solution in their excreted products. 



* " Archiv Fur Hygiene," 1886, Heft 2. 

 t" Medical News," 1887, No. 14. 



