L. TECHNOLOGY. 455 



by the winds. Similar agencies exist in Australia on a large 

 scale, and specimens of bard rock cut into fantastic shapes 

 are not unfrequently found in mineralogical collections. 

 Boston Transcript. 



EFFECT OF HOT GLASS OX A CUTTING DIAMOND. 



It is said that when diamonds are used in cutting hot glass 

 in a glass factory, one will last for only one day, assuming a 

 milky aj)pearance ; but that, if the glass be cold, one will last 

 three months. Hot glass is cut, however, more readily than 

 cold. 5 A, October, 1870, 440. 



MIXTUEE OF ALKALINE SALTS WITH PLASTER OF PARIS. 



Persons occupied in making plaster casts have been for a 

 long time aware that unburned gypsum can be made to hard- 

 en by the use of an alkaline solution, and that if this be em- 

 ployed in connection with the burned gypsum, or the regular 

 calcined plaster, a much firmer mass is produced. Some de- 

 tailed experiments have lately been made by Mr. Schott, in 

 Brunswick, which may furnish some important hints in re- 

 gard to the use of sulphate of lime with potash. Thus, if 

 equal parts of powdered crystallized sulphate of lime and of 

 a neutral sulphate of potassa be mixed together, and then 

 reduced to a paste with water, the mass hardens perfectly, 

 and more quickly than gyjmirn in the ordinary treatment. 

 If equal parts of common calcined plaster of Paris and of sul- 

 phate of potassa be mixed together, they will harden in a 

 moment with less than an equivalent weight of water so 

 much so, indeed, that the mixture can not be poured out of 

 the vessel. If, however, one part of each of the salts and two 

 of water be used, they form a mass which can be poured out, 

 and the surface of which will be found coated with a crust 

 of sulphate of potash. The rapidity of hardening, therefore, 

 can be made to vary with the percentage of water, the mass 

 solidifying even if six parts of water be used. 6 C,Jiine 23, 

 246. 



IMPROVED METHOD OF TAKING PLASTER CASTS. 



As some of our readers may be interested to know a meth- 

 od by which plaster casts of objects in natural history can 

 be taken most conveniently, we present some instructions 



