204 



E. L. Troxell — Plaster of Paris. 



used to harden the fragile bones or the matrix about them ; it 

 may strengthen plaster, give it a harder surface and with plaster 

 it is sometimes used as a cement. Since the gum is soluble 

 in water it is much more convenient in the field than shellac, 

 but its very solubility renders it less fit for hardening bones 

 where flour or plaster bandages are to be used in addition. In 

 this case shellac is better because it does not weaken when the 

 wet bandages are applied. Gum arabic is clear and therefore 



Fig. 3. 



500 



- 400 



u 300 



200 





100 



2 4 6 8/0 



Parts of Guni Arabic in 48 of water. 



12 



Fig. 3. Curves showing the effect of gum arabic solutions mixed with 

 plaster of paris, based on test of 42 briquets. 



does not discolor a surface. Like dextrine, however, it is a 

 sugar and will sour or mildew unless properly sterilized. It 

 is not hygroscopic. 



Plaster and gum arabic, combined to form a cement for 

 mending bones, has an advantage over the pure plaster in that 

 the gum adheres more readily to the broken surfaces. This 

 with its ability to give a hard coating constitutes its chief 

 value. Yery little strength is gained from the gum which goes 

 beneath the surface ; in fact it weakens the plaster. This may 

 be done by prying apart the minute crystals, and drying, renders 

 permanent the same weakening which wet plaster suffers. 



Experiments with Gum Arabic. — The use of gum arabic in 

 the original plaster mixture makes necessary an excess of 



