6 4 



Scientific Proceedings (51). 



For this we have sought analogy. Fresh dog serum precipitates 

 colloidal trisulphide of arsenic. Heated dog serum precipitates 

 the arsenic only when small quantities of the serum are added. 

 Larger quantities again disperse the precipitate and then protect 

 the colloidal metal from precipitation by subsequently added 

 fresh serum. 



The addition of further arsenic trisulphide to such protected 

 mixture will disturb the balance and lead to precipitation. 



It should be possible to make up two clear solutions, each con- 

 taining the three bodies, but in entirely different proportions, such 

 that upon mixing the two, the protective action of the heated serum 

 is so reduced that precipitation will occur. Slight, slowly appear- 

 ing precipitates have been produced in this way, but the results 

 thus far obtained have merely sufficed to emphasize the likelihood 

 of the explanation we offer. The protective action of the heated 

 serum is so enormous that it requires a very accurate adjustment 

 of relative amounts to ascertain the exact limit of protection. 



