INTRODUCTION 25 



collodion sacs such as have been employed since for the introduction of 

 bacterial cultures into the peritoneal cavity of laboratory animals. 

 Borrel next utiHzed them for the filtration of toxins. But Malfitano 

 should receive the credit for having applied them in 1904 to the sys- 

 tematic study of colloids. The method to be described is that devised 

 by him. It is preferred to the method of Bechhold (or those of other 

 investigators) devised two years later, which Ukewise makes use of 

 collodion membranes, but in which the apparatus is far more com- 

 plicated and an assurance of aseptic technic is lacking. 



Collodion is a solution of nitrated cotton in a mixture of abso- 

 lute alcohol and sulfuric ether.* The viscous Hquid which results, 

 spread out in a thin layer, becomes gradually impoverished in ether, 

 and then in alcohol, as they evaporate, leaving a layer of nitrocellu- 

 lose, homogenous and relatively resistant. 



At the beginning of the drying the ether evaporates much more 

 rapidly than does the alcohol, and, as the nitrocellulose is insoluble 

 in alcohol alone a gel of sufficient toughness is obtained at a certain 

 stage of the desiccation. At this moment it is necessary to interrupt 

 the drying by plunging the membrane into water, for if drying becomes 

 complete, the membrane is impermeable. Spongy membranes are thus 

 obtained, permeated by an infinite number of ramifying invisible, or 

 microscopic, pores, the latter being, according to the composition of 

 the collodion employed, from some hundred-thousandths of a milUmeter 

 to one or two millionths of a millimeter in diameter. 



The greater the amount of alcohol present in the collodion and the 

 lower the content in nitrate of cotton, the larger the pores, and con- 

 sequently the filters are the more porous and allow a more rapid filtra- 

 tion. It can be seen from this that to obtain all possible degrees of 

 porosity it is only necessary to vary the relative proportions of the 

 three elements which enter into the composition of the collodion. The 

 most open membranes still retain bacteria with certainty and readily 

 allow ultra viruses to pass through; the tightest retain both. 



Membranes prepared from a collodion with the following formula 

 permit the passage of the ultra viruses: 



Alcohol, 96 per cent 500 cc. 



Ether, 65 per cent 500 cc. 



Nitrate of cotton 20 grams 



* The officinal collodion containing castor oil can not be utilized for it gives 

 an absolutely impermeable film. As a general rule, to insure success, one must 

 even prepare the collodion. 



