NATURAL PHILOSOPHY. 113 



monstrous battery, it is put up in two large halls of the rear building. 

 It is kept heated day and night, the heat and steam of the acids thus 

 becoming intolerable. From the rear building wires are carried over 

 insulated posts, as in the telegraph, and run up the outside of the 

 tower, whence they come in contact with the coal-points, which are 

 square rods one quarter of an inch thick and five inches long, in- 

 closed in brass boxes, and regulated as to their distance from each 

 other by screws. The coal burns to the length of about half an inch ; 

 beyond this glowing point a. large lens is placed, which greatly in- 

 creases the light. The change of color is caused by the burning of 

 the coal when the two poles do not touch, the light appearing blue, 

 or yellow, or red, according as the distance between them is greater 

 or less. At the negative pole the coal burns rapidly, and must be 

 renewed almost every half-hour ; this interrupts the light, and is, in 

 fact, the worst feature in the whole experiment. A more complete 

 apparatus is now in course of erection, in which the coals burn in 

 vacua, and are regulated by clock-work instead of by the hands. An 

 enormous lantern is also placed on the tower. The same galvanic 

 light has been applied at Dresden, to imitate the rising of the sun in 

 the representation of Meyerbeer's Prophet. The disk of the sun is 

 formed by a parabolic concave mirror of about one foot in diameter, 

 the coal-points burning in the focus. On account of the dazzling bril- 

 liancy of the light thus produced, it appears to be more suitable for 

 such uses than for illuminating the streets. New York Tribune 

 April 10. 



VOLTAIC ARC. 



IN a paper presented to the French Academy on Feb. 23, M. Mat- 

 teucci communicates some results to which his experiments upon the 

 voltaic arc have led him. They are the following. 1. The temper- 

 ature of the positive pole is always greater than that of the negative, 

 and the difference is greater as the conducting power of the metal 

 used is less. The voltaic arc can only be formed when the poles 

 have been brought in contact and then separated, because in this way, 

 in proportion as they are separated, the conducting power of the cir- 

 cuit becomes less at the point of contact, and consequently the heat of 

 the extremities increases, whence results the disaggregation of their 

 parts. 2. The voltaic arc, which is formed of matter detached from 

 the poles, often in a state of combustion, has a conducting power dif- 

 fering with the different metals ; this power is not proportional to that 

 of the metal of the poles, but rather varies with the quantity of the 

 metal which disappears in the experiment, and as this quantity is 

 greater with bad than with good conductors, it follows that the con- 

 ducting power of the voltaic arc is greater with the former than with 

 the latter. This power is greater than would be at first supposed. 

 Thus, in an entirely metallic circuit we obtain in 60 seconds, and in 

 the voltametre, 46 cubic centimetres of gaseous mixture, while we ob- 

 tain in the same time quantities of the same mixture, expressed by the 

 following numbers, varying with the metal used, there always being 



10* 



