150 Chronicles of Science. [Jan., 



at the same time protected by a sort of screen, annulling in an 

 almost hemispheric region all the above-mentioned disturbing causes. 



The substance chosen for such a pui^iose should be at the same 

 time a bad conductor of heat, and j^ossessed of great powers of radia- 

 tion, conditions fulfilled to a gi'eat extent by lime, magnesia, and earthy 

 oxides ia general. The experiment was first efi'ected with cyhnders 

 of magnesia compressed according to the process of M. Caron, and 

 manufactm-ed for j)urposes of oxyhydric illumination. By placing 

 the base of one of these cylinders, whose diameter is about 8 milli- 

 metres, at a short distance from the charcoal points of an electric 

 lamp, in such a way that the magnesia may be, as it were, hcked up 

 by the voltaic arc, it will assume an incandescence equal to that of 

 the most luminous portion of the charcoal. At the same time the 

 light acquires remarkable constancy from the fixity of the arc, 

 which may be drawn to greater length than in ordinary cases, 

 because as the magnesia forms a screen and maintains the elevation 

 of the temperature, the chances of the arc being broken are greatly 

 diminished. 



The magnesia may thus be kept m contact with the voltaic arc 

 for more than an hour without sufficient consumption to cause any 

 apparent change in the conditions of the experiment ; its surface 

 becomes hollow during the first few moments, but if the bar of this 

 substance is kept fixed, the power of the arc abating at a very shght 

 distance, it will no longer be consumed. Another kind of alteration 

 will, however, ensue. The magnesia will imbibe the sihcious vapours 

 emitted by the voltaic arc, and combine with them in a sort of glass, 

 which, when cold, is of a pale greenish hue, and extremely hard. 

 This fact is disadvantageous, inasmuch as it greatly diminishes the 

 irradiating power of the magnesia, and renders the production of a 

 commercial pure carbon in an approj)riate condition for the purj)Ose 

 of electric illumination still more desirable. The arrangement of a 

 brilliant voltaic arc between two pencils of charcoal, and in the 

 presence of magnesia or any other earthy oxide, would constitute 

 one of the most beautiful sources of hght possible to reahze. 



M. Kindt has made known the nature of the phosphorescence 

 developed by heat in the three minerals, chlorophane, Estremadura 

 phosphorite, and the green fluor spar. He has analyzed the hght 

 emitted : the first is a simple green ; the second is a yellow-tinted 

 light, composed of green, yellow, and red ; and the third gives two 

 black rays, the one in the green, and the other near the orange. 



]\T. Becquerel has invented an electric pyrometer for the mea- 

 surement of high temperatures. Metallurgists will probably find 

 this application very advantageous. To solve the prolilcm of the 

 commercial apphcation of a thermo-electric current as a pyrometer, 

 it was necessary to make a thermo-electric couple with two unalterable 



