j5. L. Troxell — Plaster of Paris. 



203 



square inch, the next 113, while the third briquet broke while 

 being placed in the testing machine. With a one forty-eighth 

 solution the best briquet gave a 350-pound test, which is little 

 more than half the strength of the best pure plaster briquet. 



In contrast to this the tests with pure plaster afterward 

 soaked in dextrine gave uniformly good values regardless of 

 the density of the solution used. (See curves, fig. 2.) The 



Fig. 2. 



mo 



400 



S 300 



ZOO 



wo 



10 



IZ 



Parts of Dextrine in 48 parts of water. 



Fig. 2. Curves showing the strength of plaster which had been soaked in 

 a varying solution of dextrine. 



strength of a briquet soaked in a one-half solution was 393 

 pounds ; the next highest test was from a one-eighth solution : 

 342 lbs. The third highest gave a strength of 318 lbs. and 

 had been treated with a one forty-eighth solution. 



A thick solution of dextrine leaves a heavy coating on the 

 briquet which on drying cracks and falls away. It therefore 

 does not make as good a surface as gum arabic. 



Dextrine, because of its yellow stain, its greater insolubility 

 and its failure to harden plaster effectively, is very little used 

 in the restoration of fossils. 



Gum Arabic. 



Gum arabic is one of the most useful adhesives in the paleon- 

 tologist's laboratory and is well-nigh indispensable. It may be 



