M. H. SOULE 255 



they are burned to CO2 and H2O. By this method the gaseous components may be deter- 

 mined with an accuracy of 0.02 per cent. 



The analysis of the gas over the culture does not give the total gas change since a con- 

 siderable and varying amount of CO. is taken up by the medium. The CO2 may be present 

 in mere physical solution, or, reacting with NajCOj, NaiHPO^, Na of proteins, it may form 

 NaHCO,; or combining with the NH3 and amines made by microbic action, it may yield cor- 

 responding carbonates. A culture of B. subtilis growing on 10 cc. of plain agar may produce 

 enough alkali to bind 15 cc. of CO2. 



When working with broth cultures, Van Slyke's' apparatus for the determination of CO2 

 in blood plasma can be used. Rogers and his co-workers, in their study of the colon bacillus, 

 removed the dissolved CO2, but not the fixed CO2, by evacuation. Obviously these methods 

 cannot be used when working with solid media. Satisfactory determinations of the total dis- 

 solved CO2 can be made in liquid or solid media by the aeration method which consists in 

 passing COi-free air through the acidified culture medium and then into known amounts of 

 a standard solution of Ba(0H)2. The medium can be kept at 90° C. to liquefy the material 

 if a solid substrate, such as agar, is used. The addition of the acid liberates the combined 

 CO2. After sufiicient aeration the tubes containing the standard solution are placed in ice 

 water, and the excess of alkali subsequently titrated with standard HCl. The number of 

 cubic centimeters of N/io hydroxide neutralized by the CO2 from the medium multiplied by 

 1. 1 1 29 gives the volume of CO2 in the medium in cubic centimeters at 0° C. and 760 mm. 



The determination of the CO2 content of a blood-agar medium requires a slight modifi- 

 cation of the foregoing procedure. It is impossible to liquefy the medium, since the blood co- 

 agulates at a relatively low temperature, thus making it difficult to secure proper aeration of 

 the material. The medium is, therefore finely comminuted with a flattened glass rod, and, 

 after adding the H2SO4, is aerated at a temperature of about 40° C. 



RESPIRATORY QUOTIENTS 



By respiration in its widest sense must be understood all those processes in the 

 cell whereby the potential energy stored up in chemical compounds of high complexity 

 is set free to furnish the energy required by an organism for its vital activities. The 

 object is effected by processes of oxidation; the result is the production of energy with 

 the formation of simple chemical substances such as H2O and CO2. The fact that man 

 and animals consume O2 and return CO2 was shown by Lavoisier in 1777. Dulong,^ 

 continuing the work, found a difference in the volume of CO2 returned per volume of 

 O2 consumed in dogs, rabbits, and fowls. He suggested that this might be due to a 

 difference in the character of food. Dulong's suggestion was substantiated by Re- 

 gnault and Rieset.^ The value obtained, by dividing the volume of CO2 produced by 

 the volume of O2 consumed, was later designated as the "respiratory quotient." The 

 ratio of this exchange is the same whether the volumes are expressed in cubic centi- 

 meters, or percentage, or as millimeters of pressure. 



On the assumption that the oxidation is completed to CO2 and H2O, the theoreti- 

 cal respiratory quotient of a carbohydrate is i.o, of a fat 0.71, and of a protein 0.8. 

 Thus, in the case of glucose, we have the equation 



C6Hi206+602 = 6C02+6H20 . 



' Van Slyke, D. D.: J. Biol. Chcm., 30, 347-68. 1917. 

 = Dulong, M.: Ann. d. chim. et d. phys. (3), i, 440. 1841. 

 sRegnault, V., and Reiset, J.: ibid., 26, 299. 1849. 



