70 



CEMENTS, LIMES, AND PLASTERS. 



recognizable needles; these have the form of the crystalline plaster 

 of Paris (CaSO^H^O. As floor-gypsum contains no water, the needles 

 are evidently pseudomorphs after plaster of Paris. It may be added 

 that stucco gypsum, though consisting essentially of. half hydrate is 

 far from showing the crystalline development of the floor-gypsum. 



"In view of this striking crystalline condition, we directed our in- 

 vestigation not only to the influence of the burning temperature but 

 also to the possible influence of this factor (crystalline form). 



"To get an idea of the hardening property, observation is naturally 

 directed first to the process as carried out in practice, and we did not 

 fail to follow it on a small scale. However, we soon felt the need for a 

 process which would enable this hardening to be traced with greater 

 accuracy quantitatively. For this purpose we used the change of 

 volume in hardening and the weighing of the quantity of water taken up. 



"1. Change of volume on hardening. When the practicability of 

 the volume process was first tested in the case of stucco gypsum, a 

 great irregularity manifested itself, which was not observed in the case 

 of floor-gypsum. Hence we might pass those preliminary experiments 

 without mention did they not furnish the explanation of the well-known 

 fact that the hardening of gypsum, though accompanied by contraction, 

 nevertheless may lead to the breaking of the vessels in which it takes 

 place. 



"The apparatus consisted of a small flask with narrow neck; the 

 sample of gypsum, having been weighed, is introduced, and then a closed 

 capillary is sealed to it, which at a short distance above the flask bears 

 a lateral branch with stop-cock. By means of this the apparatus is 

 connected with the air-pump, and, after evacuation, is placed in a ther- 

 mostat filled with gypsum-water at 25. The stop-cock is opened and 

 gypsum-water is allowed to flow in. Gpysum-water is used instead 

 of ordinary water, in order to avoid the change of volume accompanying 

 the solution of the gypsum. After closing the stop-cock and breaking 

 off the upper end of the capillary, the reading of the level in the latter 

 may be performed by means of a millimeter scale one minute after the 

 introduction of the gypsum-water. 



"The following table shows the result of such an observation: 



