RECENT PROGRESS IN PHYSICS. 423 



By this, or ratlier by a similar equivalent formula, Fizeau and 

 Foiicault compufced the results of their observations, and thus obtained 

 the following relative values for the intensity of the sources of light. 



Sun-light, in August and September, at noon, with a clear sky 1000 



Garhon-ligldj produced by 46 Bunsen's zinc-carbon cups 235 



Lime-light , 6.8 



The lime-light appears to be surprisingly little ; but Fizeau 

 and Foucault found with the common photometrical method a similar 

 relation between the lights from lime and carbon. No other com- 

 parative measurements are known to which these can be referred ; a 

 careful experimental re-examination of the matter is, therefore, de- 

 sirable. 



In reference to the change of the intensity of the carbon-light with 

 the number and magnitude of the galvanic elements we hud the fol- 

 1 lowing data in this memoir : While a battery of 46 Bunsen's elements 

 gave an intensity of light of 235, this was increased to 238 only when 

 the number of cups was augmented to 80 ; but a battery of 46 triple 

 cups gave an intensity of 385, after having been already in action 

 for one hour. 



In consequence of the rapid alteration of the fluid — the diluted sul- 

 j)huric acid becoming gradually a solution of sulphate of zinc — the 

 force of the battery, and with it the intensity of the arc of light pro- 

 duced by it, decreases rapidly. While 80 cups afforded at first an 

 intensity of 238, this after three hours was diminished to 159. 



It is to be regretted that these physicists have not measured the 

 force of current corresponding to the intensity of light, whereby 

 the value of the above given numerical relations would have been 

 very much enhanced. 



§ 66. Production of heat hy the voltaic arc. — The heat developed at 

 the poles, between which the arc is taken, is entirely too great to be 

 attributed to the mere passage of the electric current through these 

 conductors. According to the experiments mentioned in § 57, a cur- 

 rent, to make a platinum wire of 0.75 mm. in diameter incandescent 

 by its passage, must have at least a force of 1^>0. Therefore, to make 

 a platinum wire .of 3 mm. in diameter only white-hot requires, at the 

 very least, the enormous force of current of 640 ; and yet with the cur- 

 rent of a Bunsen's battery of 44 cups and a force of 80 to 100, we can 

 produce an arc in which the point of a platinum wire of more than 3 

 mm. in diameter may easily be melted into a globule, if used as one 

 pole of the battery while the other is formed by a carbon point. _ The 

 combustion of carbon is so trifling that it cannot essentially contribute 

 to the great heat produced ; besides, the fusion of the platinum wire 

 by the galvanic arc takes place in a vacuum as readily as in the opeu 

 air. 



The electric current, therefore, besides producing heat by its mere 

 passage through the conductors, in forming the arc rnust act at the 

 pole itself to produce heat in some other way, of which as yet we 

 know nothing- 



The development of heat is not equal at the two poles of the arc ; 

 it is greater at the positive than in the negative. De la Rive, in his 



