S. p. KRAMER 



559 



When we consider the nature of plaster of Paris a very interesting phenomenon is 

 found. Plaster of Paris is supposed to be calcium sulphate, but when filters were made 

 of calcined chemically pure calcium sulphate, it was found that such filters had no ac- 

 tion on either class of colloid dyes. B oth the basic Victoria blue B (or methyl green) and 

 the acidic Congo red readily passed through such filters. Calcium sulphate is neutral 

 and without charge. It was found that the plaster of Paris of commerce contained up 



Fig. 3. — This is arranged to show the method of 

 making the gypsum filter, yl is a heavy glass tube with 

 a I -inch lumen. 5 is a rubber stopper carrying a glass 

 tube D. C is a vaseHned glass rod acting as a core. 



The plaster of Paris, containing, in some instances, 

 25 per cent of MgO, is mixed with an equal weight of 

 water, rapidly stirred to get rid of large air bubbles and 

 then poured into A by means of a pipette to avoid air 

 bubbles. As soon as the plaster sets, the core C is 

 withdrawn and the "filter" is allowed to dry out. After 

 testing with a solution of Congo red it is ready for use. 



A 



B 



D 



Fig. 3 



to 5 per cent of calcium carbonate, and when calcium carbonate was added to chem- 

 ically pure calcium sulphate and filters made from this mixture, such filters acted as 

 did the filters made of commercial plaster of Paris. The calcium carbonate is alkaline 

 and has a positive electrical charge. It is probable that the calcium sulphate in the 

 filters acts as a binder for the calcium carbonate and that it is the calcium carbonate 

 which is the active component. 



It was thought that if a filter were made of plaster of Paris and silica or diatoma- 

 ceous earth intimately mixed before setting, we might produce a filter which would re- 

 move both positively and negatively charged dyes and colloids. When this was done, 

 however, it was found that such filters were neutral and that neither suspensoid was 



