E. L. Troxell — Plaster of Paris. 



201 



some parts they almost coincide, suggesting a peculiar effect of 

 coloring matter when mixed up in a certain definite propor- 

 tion. The relatively lower position of the bone-black curve 

 may be due to the greater weakening effect of that substance 

 on plaster or may in part result from the manner of mixing or 

 a greater quantity of water. The curves are roughly convex 

 upward, which indicates that the decrease in strength is 



Fig. 1. 



300 



S50 



g> 2O0 



CD 



m 



10 



zo 



30 



40 



SO 



Parts of coloring in 300 of Plaster of Paris. 



Fig. 1. Curves showing the influence of coloring materials on plaster of 

 paris, based on the record of 67 briquets. Upper curve raw sienna, lower 

 bone black. 



accelerated with a constant increase in color. What the course 

 of the curve would be with a continuation of the series is a 

 matter of speculation, but it is probable that it would descend 

 quite rapidly. 



The dependence of the strength of these briquets upon the 

 amount of water necessary in their making, is a factor to be 

 considered. But whether it is the coloring itself which 

 weakens the plaster or the greater amount of water which the 

 coloring requires does not matter, for the result is the same and 

 points back to the same ultimate cause. 



Am. Jour. Sci. — Fourth Series, Vol. XLT, No. 242. — February, 1916. 

 14 



