AGRICULTURE AT THE AMERICAN ASSOCTATIOK. 533 



utilizing- the free nitrogen of the air. Nitrogen assiniihition in the 

 soil is not the result of the activities of a single organism, hutol" sym- 

 biosis of niicroaerophiles (("/cy.s//vV////y//^><';,s'A'^i'/v'c///'^/y/, several species 

 of Granulobacter of Beijerinek. and Kadiobacter of Beijerinck) with 

 macroaerophiles (Azotol)a('ter of Beijerinek). Either of these forms 

 alone is unable to assimilate nitrogen, but mixed cultures of Azotobac- 

 ter with the other forms showed gain of nitrogen of 4 to 7 mg, per 

 p-ram of assimilated sugar in the medium. A form of Azoto'oacter 

 isolated from Delaware soil was without the power of assimilating 

 atmospheric nitrogen, 



W. M. Esten exhibited an electric thermo-regulator, previously 

 described by H. E. Ward, for regulating the temperature in incuba- 

 tors heated l)}^ electric lamps within the oven. Two pieces of this 

 apparatus have been made, which have proved very satisfactory and 

 efEcient in maintaining the incubators at a constant temperature when 

 they were being run at a higher temperature than the surrounding 

 air. The apparatus was highly commended on account of its sim- 

 plicity, reliability, and safety from tire. 



The same author described the preparation of a number of culture 

 media for determining the kind of bacteria in milk on the plate culture. 

 A oelatin containing milk from which the casein has been removed, 

 designated the custard- whey gelatin, was said to bring out the differ- 

 ences more prominently than a peptone-litmus gelatin, and to inhibit 

 the growth of liquefying l)acteria. By means of this custard-whey 

 gelatin it is quite possible to tell the age of the milk by the appearance 

 of the growth in the plate culture. 



A paper entitled Milk Agar as a Medium for Demonstration of 

 Proteolytic Enzyms, by E. G. Hastings, was presented by H. L. 

 Russell. The culture medium was prepared by the addition of 10 to 

 12 per cent of skim milk to agar, and was found very satisfactory for 

 showing the presence of digesting bacteria in dairy products. The 

 medium was readily cleared up by the growth of such bacteria. 



W. A. Stocking, jr., presented a paper on The Germicidal Action 

 of Milk, in which he reviewed the claims that milk soon after it is 

 drawn has a temporary inhibiting action on bacteria, as judged from 

 the inunber of bacteria in the milk, and presented data in opposition 

 to this belief which tended to show that no true germicidal action 

 exists. It was found that in nearly all experiments the total number 

 of bacteria during the first few hours decreased, but a study of the 

 different forms present showed that while the non-acid species 

 decreased the acid species increased regularly from the first. One 

 case was mentioned in which alkaline species were present and inhib- 

 ited or retarded the growth of the lactic organisms. The acid species 

 were mostly the true lactic acid organisms. There seem to be only 

 two or three types of acid organisms which persist, 09 per cent in 



