COLLOIDS 519 



a freshly prepared gelatin solution will not precipitate arsenic sul- 

 phid, but it will do so after it has stood for some hours. What is 

 now greatly needed is more data on a greater variety of colloids that 

 have heretofore been investigated and work directed toward the 

 preparation of colloidal solutions of definite character. Until some- 

 thing has been accomplished in these directions all biological anal- 

 ogies and the like cannot be anything more than qualitative, and the 

 same holds true for many of the physical and chemical conclusions 

 which have been discussed in this chapter. 



