420 RECENT PROGRESS IN PHYSICS. 



always consists of oxydized particles of tlie metal used as tlie positive 

 electrode. 



If the negative pole has the form of a plate, while the positive pole 

 is a point, the deposit of the transferred matter upon the plate 

 forms a very regular ring, the centre of which is the projection of the 

 point upon the plate. 



When the arc of light is taken between a metallic point and an 

 opposite surface of mercury, the latter, when positive rises in a cone, 

 but forms a cavity when negative. In this case it is very difficult to 

 observe accurately the minutias of the phenomenon^ on account of the 

 great quantity of mercurial vapor evolved. 



De la Rive made experiments with plates and points of platinum, 

 iron, silver, and copper, but I cannot enter upon the details of the 

 experiments, because there is much that is not clear to my mind ; in 

 many cases, for instance, I cannot see in the individual experiments 

 the proof and confirmation of the generalizations announced. A 

 repetition of these experiments and an accurate description, illustrated 

 when practicable with figures, seems therefore very desirable. 



§ 65. Intensity of light of the voltaic arc. — Casselmann has made 

 experiments upon the intensity of light of the voltaic arc, which have 

 been described in the memoir already mentioned. They were after- 

 wards also copied into Poggendorf's Annals. — (Pog. Ann., LXIII, 

 576.) The photometer used in his experiments was constructed upon 

 the same principles as that described in the third edition of my Lehr- 

 huch der Physik, vol. II, 674. The carbon pieces, between which the 

 arc was taken, were of the same composition as that used in the cylin- 

 ders of Bunsen's battery, but prepared also in other ways, as some of 

 them were saturated in solutions of nitrate of strontium, boracic acid, 

 (fee, and then intensely ignited. Thus prepared they gave a very 

 steady light, differently colored, according to the solution employed ; 

 and the carbon points could (with a Bunsen battery of 44 cups) be re- 

 moved to a distance of 7 to 8 millimetres before it disappeared, while 

 the unsteady light of unprepared carbon went out at a distance of 5 

 millimetres. 



A tangent compass was at the same time inserted into the circuit, 

 80 that for each measurement of the intensity of light the correspond- 

 ing force of current could be determined. 



The brightest parts of the whole light, it is well known, are at the 

 points of the two pieces of carbon, upon which the arc rests. In the 

 following table the intensity of the whole light is compared with that 

 of a stearine candle, and for each kind of carbon, with the points once 

 at a very small, and then at the greatest possible distance. The values 

 of the force of current are reduced to the chemical unit. 



