184 



Foreign Literature and Science. 



The paste of Sevres is composed of 

 .633 of kaolin of Limoges, washed ; 

 .105 of quartzose sand from the mound of Aumont; 

 .032 of chalk from Bongival ; 



.210 of fine sand extracted from the kaolin by washing ; 

 this sand is a mixture of quartz and feldspath. 



The porcelain of Sevres is known to be of an excellent 

 quality, and as refractory as any in Europe. 



The glazing is made of a rock composed of quartz and 

 feldspath, reduced to a very fine powder. It is composed of 

 Silica, - - - 0.730\ 

 Alumine, - - - 0.]62f r, qro 

 Potash, - - - 0.084 ( ^'^^^ 

 Water, - - - 0.006 1 

 It melts into a perfectly transparent and colorless glass. — 

 Annales des Mines. T. I. 469. 



31. On the measure of the intensity of Light. — The most 

 general method is to place an opake body before a white 

 card or pastebord, in front of the two lights to be examined, 

 to remove the latter from the screen until they produce shad- 

 ows of equal intensity, and then to take the squares of the 

 distances as the relative powers (inversely) of illumination. 



It has been found that the apparent intensity of the two 

 shadows varies with the position of the observer. If they 

 are equal when observed from a point situated perpendicu- 

 larly over the screen and equidistant from each shadow, 

 they will, if viewed from any other point appear to vary in 

 intensity, the nearer having the deeper shade, so that in 

 changing from right to left of the tabic, each will appear in 

 its turn of the deeper hue. These differences are greater in 

 proportion to the distances of the shadows from each other 

 and the smoothness of the surface of the screen. Hence 



