424 RECENT PROGRESS IN PHYSICS. 



treatise already mentioaed, in § 64, adds, in reference to this fact, the 

 following observations : . 



When, in forming the arc, a positive metallic point is opposite to a 

 negative plate, the point becomes ignited throughout, while on invert- 

 ing the poles the negative point is heated at its extremity only. 



If two points of the same metal are opposed to each other the posi- 

 tive one becomes more intensely ignited, and over a greater length. 

 If they are of different metals^ of course that one becomes most in- 

 tensely ignited which is made of the worst conducting metal. 



To this category belongs also an observation of Walker, made with 

 a Daniell's battery of 160 cups. — (Trans, of the Lond. Electr. Soc, pp. 

 65 and 71; Pog. Ann., LV, 62.) He laid the pole wires crosswise, 

 but so that after the contact they were again moved to a little distance 

 from each other, and a short arc of light passed between them. Under 

 these circumstances the positive end of the wire, from the point of 

 crossing, became so intensely hot that it softened and bent, while the 

 negative end remained comparatively cold. 



Experiments on the heating effects of the voltaic arc have been 

 made on the greatest scale by Despretz. He collected, in Paris, 500 

 sine-carbon cups, and arranged a battery of 124 elements, each con- 

 sisting of four Bunsen's cups. When a piece of sugar carbon, in a 

 glass globe exhausted to 5 millimetres, was brought between the poles 

 it became intensely ignited and the globe was covered with a dry, 

 crystalline black powder. Carbon from gas retorts produced the same 

 effects. This shows a sublimation of the carbon. 



Desprelz thinks too that he observed traces effusion of the carbon. 

 At any rate his experiments show that carbon evaporates more readily 

 than it melts. He believes that it could be melted in metallic vessels 

 in an atmosphere of compressed nitrogen. Similar in behavior to 

 carbon are lime, magnesia, oxide of zinc, &c. Alumina, rutil, 

 anatase, nigriue, oxide of iron, &c., form at first small globules, but 

 afterwards evaporate. 



Previous to these experiments with 496 cups Despretz had used a bat- 

 tery of 165 elements, and combined the heat of its arc with that of the 

 oxy-hydrogen blow-pipe and of the sun concentrated through a sec- 

 tional lens 90 centimetres in diameter. The effect of the galvanic 

 battery was increased by the addition of the other sources of heat. — 

 (Comptes Rendus, July, 1849, No. 3 ; Dingler's Polytechnic Jour- 

 nal, CXIV, 342.) 



§ 67. Influence of magnetism upon the voltaic arc. — That magnetic 

 forces have an influence upon the position and form of the arc has 

 already been observed by Davy, and it is known that this arc is 

 affected by a niagnet in the same manner as a movable conductor when 

 a galvanic current is passing through it ; the terrestrial magnetism, 

 therefore, must also act upon it. By the motion of the heated air the 

 arc of light is always carried upwards, so as to form a curve, convex 

 above. If we conceive a perpendicular plane to be passed through 

 the carbon points lying horizontally, the action of terrestrial magnet- 

 ism will be such that the highest point of 'the arc will never be in this 

 plane, but on one side or the other. 



