12 



extends to many hours, or even several days, and during such pro- 

 tracted exposure to the air and to common atmospheric temperature 

 weak solutions of the proteids readily undergo chemical change. Again, 

 it is to be remembered that the peptones, classed with the proteids, are 

 diffusible in much higher degree than natural proteid material prior to 

 its exposure to the action of the digestive fluids. This fact has well- 

 known physiological importance in its bearing on the preparation of 

 proteid food for absorption from the alimentary canal, but in analysis 

 it tends to confound the particular group of the peptones with the 

 simpler amids. In the recovering from weak solutions of small quan- 

 tities of dissolved substances by the evaporation of large amounts of 

 water, further chemical change of the substance recovered is likely to 

 vitiate the results. 



Xevertheless, some twenty or more experiments were carried out 

 with dialysers made from the tubes of parchment paper which can now 

 be bought in Germany. Cylinders of about 6 inches long were cut 

 from these tubes, and closed at one end by thin glass, carefully tested 

 as to freedom from holes, and immersed in water contained in ordinary 

 glass beakers. A fairly clean separation was obtained of leucin, aspar- 

 tic acid, and kreatiu from solutions to which had been added egg or 

 serum albumin, the diffusate being in each case evaporated at a mod- 

 erate temperature over the water bath, and the residue weighed. But 

 the process of diffusion was inconveniently slow, and less satisfactory 

 results were obtained when the dialyzed solution was made more com- 

 plex by the addition of other substances. 



Subsequently the writer's attention was attracted by the paper of 

 Charles J. Martin ! on the use for a like separation of a Pasteur filter 

 in the pores of which a film of silicic acid has been deposited, the fil- 

 tration being brought about under pressure. It has been practicable 

 to make only two or three experiments in this way. The method is 

 decidedly promising, but it seems more likely to be useful in the 

 purification of substances in quantity than as a process of analysis. 

 With small quantities of material it can hardly be made available for 

 regular laboratory work in connection with nutrition investigations. 

 It involves the same difficulty as any other form of dialysis in cases in 

 which the proteoses and peptones are present. 



INTERACTION WITH NITROUS ACID. 



It was hoped that by varying the conditions of experiment with this 

 reagent some characteristic differences of behavior might be observed 

 as regards the evolution of elementary nitrogen. 



The most advantageous mode of producing nitrous acid in definite 

 amount was found to consist in bringing together, along with the mate- 

 rial to be acted upon, pure silver nitrite and a hydrochloric-acid solu- 

 tion of known strength. The action took place in a glass flask of 



1 Jour. Pbysiol., 1896, 20, pp. 361-371. 



