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Mr. J. A. Craw. On the Filtration of [Dec. 1, 



through the gelatine. From his observations with " wet " gelatine filters, 

 i.e., filters containing gelatine from which the normal content of water had 

 not been removed by drying in air or otherwise, Martin* concluded that 

 gelatine was impermeable to colloidal substances such as globulin, albumin, 

 glycogen, and soluble starch, but partially permeable to albumoses and 

 dextrin, and completely permeable to solutions of crystalloids, e.g., urea and 

 dextrose. It seemed, then, as if the gelatine filter was an instrument 

 destined to play an important part in the investigation of physiological 

 fluids. 



Martin and Cherry (1898) applied the filter to the investigation of the 

 course of the reaction occurring between diphtheria toxin and antitoxin, and 

 likewise to the reaction of snake venom with antivenene, the toxin and venom 

 being filter-passers, whereas the anti-bodiesf were retained. 



From these experiments it seemed as if the toxin was completely neutralised 

 by the antitoxin, but further investigation of the mechanism of gelatine 

 filtration shows that no such absolute conclusion can be drawn. E. Waymouth 

 Eeid (1901) showed conclusively that crystalloids do not pass "wet" 

 gelatine filters in unaltered concentration, and that although filters which 

 had been dried to constant weight in dry air allowed certain crystalloidal 

 solutions to pass unchanged, yet dextrose and sodium oleatet were partially 

 retained. Further, he found that the filtrate from serum§ had not the same 

 composition as the proteid-free serum, and that the residual fluid left in the 

 filter case had a much higher concentration of organic substances of non- 

 proteid character than either the original serum or the filtrate. The Martin 

 filter is not, therefore, a simple means of separating crystalloids from colloids. 

 One must not, however, under-estimate its value as an instrument for the 

 analysis of physiological fluids, for although the filter shows considerable 

 differences in permeability to various crystalloidal substances, I find (IV, 

 1905) that these and inferior colloids are, on the whole, retained to a 

 small extent compared with typical colloids. The partial retention of 

 filter-passers has an important bearing on the conclusions to be drawn from 

 gelatine nitrations, for, if the concentration of the filter- passer to be tested 

 for be small in the original fluid introduced into the filter, the gelatine may 

 retain practically the whole amount and the filtrate contain only a quantity 

 below the experimental error of observation. This was found to be the case 

 for neutral mixtures of megatherium lysin and antilysin, and mixtures 



* Loe. tit. 



t Cf. also Brodie (1897) (1900). 



I Krafft (1902) considers soap solutions, such as sodium oleate, to be colloidal. 

 § Cf. also Starling (1899). 



