E. L. Troxell — Plaster of Paris. 207 



more complete some of the briquets, aside from those put into 

 the gum solution, were soaked in clear water, while still others 

 were not treated at all. This gave a basis from which to judge 

 more fairly the effect of the gum. 



The one forty-eighth solution decreased the strength of the 

 plaster markedly but two parts of gum in 48 of water almost 

 doubled this decrease. From this point less difference was 

 caused by the denser solutions and the average of the curves 

 (fig. 5) runs almost horizontal, the very dense one-fourth 

 solution showing a slight rise. Since only a small increase is 

 noted in the use of the densest mixture, it seems probable that 

 there is a selective absorption and that from the surrounding 

 liquid only a weak solution penetrates. 



From the experiments it is very evident that gum arabic 

 weakens plaster of pans, generally as much as one-third and in 

 many cases a half. The effect is greater than apparent, for the 

 surface hardening counterbalances some of the evil and in part 

 makes up the loss. 



Since the surface hardening is such an important result of 

 the use of the gum, it has a decided advantage in small or 

 irregular bodies where the surface area is great in proportion 

 to the volume. 



The Use of Glue. 



In the earlier days of vertebrate paleontology glue was 

 quite extensively used for mending broken bones. In spite of 

 the fact that it is reported to have failed in its purpose it is 

 still used by a number of museums. Its very great strength 

 and convenience gives it an important place in the preparator's 

 workshop. At the present time its use is, or should be, 

 limited to the mending of clean breaks or to the cementing of 

 smooth surfaces where only a thin seam is necessary. In this 

 service it is sometimes combined with plaster which furnishes 

 a body to the glue, rendering it more stable. 



The chief objection to the use of glue is that it is hygroscopic, 

 taking up moisture and giving it off again. Where this 

 happens it causes an expansion and contraction which makes 

 it impossible for an adhesive to retain its grip. A sheet of 

 glue, if laid flat on the hand, or other moist surface, will curl 

 up rapidly as the under side absorbs the moisture. A stick 

 of carpenter's glue put into cold water did not dissolve, but 

 took up enough water to expand to three times its former thick- 

 ness. It became very pliable and jelly-like but remained tough. 

 It is reasonable to suppose then that glue, exposed to the air 

 with its changing humidity, might take up moisture and prove 

 inefficient. 



