122 THE SO-CALLED INORGANIC ENZYMES 



The colloidal suspension begins at once, and in a few minutes a 

 fluid of a deep dark colour is obtained, which in the case of platinum 

 resembles the colour of reduced osmic acid solutions. Accompanying 

 the colloidal particles there is always a certain amount of metal in 

 coarser suspension, which may be removed by allowing to stand, 

 filtering, or centrifugalising. The dark-coloured solution remaining 

 is perfectly clear, it may deposit a little more platinum in the 

 first day or two, but the rest remains in solution for months, and 

 apparently indefinitely. 



In the case of gold the colloidal solution has a deep dark-red 

 colour, while the silver-sol varies in colour from dark reddish brown 

 to olive green, according to the dilution and fineness of the sub- 

 division. 



All the metal-sols are extremely sensitive to the presence of 

 electrolytes ; they cannot be prepared in normal saline, and even 

 addition of normal saline to them when prepared precipitates them 

 entirely from solution. 



This somewhat militates against their employment for intra- 

 venous injection as germicides in septicaemia, as has been recom- 

 mended in the case of silver-sol, for example, in septic endocarditis ; 

 for it is probable that the sol will be precipitated by the saline of 

 the plasma, and so its effectiveness diminished or destroyed. 



The writer has found that platin-sol in distilled water can be 

 injected in animals without any untoward results and apparently 

 without affecting the animal in any way. It was found that the 

 addition of 0'7 per cent, saline to the platin-sol in distilled water 

 had the effect even at this dilution of completely precipitating it. 

 An attempt to prepare the platin-sol in isotonic glycerine solution 

 was unsuccessful ; more success attended the addition of glycerine 

 to make an isotonic solution after first preparing in distilled water, 

 for some of the platinum remained in colloidal solution, but even 

 here there was considerable initial precipitation. It must be remem- 

 bered, however, that one colloid has often a great effect in preserving 

 another against precipitation by electrolytes, and hence it is possible 

 that on intravenous injection of the platin-sol in distilled water, the 

 proteids of the plasma may assist in retaining the platinum in 

 colloidal solution against the precipitating action of the plasma saline. 

 Thus Lobry de Bruyn has shown that when two salts which 

 form a colloidal precipitate, such as potassium chromate and silver 

 nitrate, &C., 1 are mixed in a warm 10 per cent, gelatine solution, the 



1 A similar action occurs in preparing a silver gelatine mass, as for silver 

 staining of mammalian lung epithelium, and is connected with the success of 

 such an operation. 



