FLOORING-PLASTERS AND HARD-FINISH PLASTERS. 



71 



"On the whole, therefore, a contraction takes place, as is required, 

 in fact, by the molecular volumes at 25: 



Hence for 



AV= -5.68. 



However, after a rather large contraction (from 540 to 370) a transient 

 expansion (from 370 to 438) manifests itself. 



"Although this peculiar behavior suggested the transient formation 

 of an intermediate product, yet all attempts to isolate such a product 

 were in vain. The explanation is found in another way: the solubility 

 of the stucco gypsum (CaSO^^O) is much greater than that of normal 

 gypsum (according to Marignac one part of CaS04 in the first form dissolves 

 in 110 parts of water, while of the second form one part dissolves only 

 in 479 parts of water at 24); thus the solution first formed is over- 

 saturated with gypsum and precipitates the latter. Now, as shown 

 by direct experiment, this process is accompanied by considerable 

 expansion; in the same apparatus, when 0.54 gram of gypsum was 

 thrown down from the oversaturated solution, there was a rise of 

 78 mm. in the scale, that is to say, quite comparable with the above value 

 (conversely the solution of gypsum was accompanied by a contraction). 

 Undoubtedly connected with this process is the sweating-out, which 

 is occasionally observed in casts of stucco gypsum. While, therefore, 

 the volume method of ascertaining the process of the hardening of 

 stucco gypsum is to be rejected, a satisfactory regularity appears in 

 the case of floor-gypsum, as shown by the following table : 



