RECENT PROGRESS IN PHYSICS. 427 



same time that it was uplifted in the shape of a cone towards the 

 point. 



The first part of this is clear ; not so the last two paragraphs. The 

 passage between f and ft appears to indicate that the oblique direc- 

 tion of the arc of light only occurs when the plate is negative and 

 the point positive ; but somewhat further on we read that the direc- 

 tion in which the luminous arc is projected depends upon that of the 

 exciting current. It should, therefore, take place when the plate is 

 positive and the point negative. Besides, an obscure allusion to the 

 rotation of the arc is found in this passage, but so obscure that one 

 not previousl)'' acquainted with the phenomenon could form no idea 

 of it from this representation. That the Genevan physicist, in penning 

 this passage, actually had this rotation in view is evident from the 

 conclusion of the last paragraph. Similar faults frequently occur in 

 De la Hive's treatises ; his description rarely gives a clear and intelli- 

 gible representation of the phenomenon. It is much to be regretted 

 that in this way the results of many a beautiful and difficult experi- 

 mental research are only imperfectly presented to those engaged in 

 physical studies. 



§ 68. Use of the galvanic light for illumination. — It was to be expected 

 that the great intensity of the galvanic carbon light would soon lead 

 to the idea of employing it for illumination after its production was 

 so much facilitated by the invention of the constant batteries. 



Deleuil several times made public experiments with this kind of 

 illumination. At first he illuminated the pavilion of a mansion at 

 the Pontneuf, in Paris, with 98 zinc-carbon elements. Acherau made 

 similar experiments in the Place dela Concord. — (Dingler's Polytech. 

 Journal, vol. 91, p. 324.) 



Though the intensity of the galvanic carbon light is enormous, and 

 although a battery of 48 Buusen's elements produces as much light as 

 63 common gas barners, yet the use of the galvanic light for public 

 illumination appears unfit for practical application for the following 

 reasons : 



An immense quantity of light is here emanating from one single 

 point, and therefore very strong contrasts between light and shade 

 will be produced ; the darkness in the shade will be the more unideasant 

 just on account of its contrast to the dazzling light. At any rate, the 

 illumination obtained from 63 gas burners, perfectly distributed, will 

 be more uniform and agreeable than an equivalent light concentrated 

 in one |)oint. 



Another objection to the application of the galvanic carbon light, 

 is the difficulty of keeping its intensity uniform for a long time. 



In consequence of the formation of sulphate of zinc the conducting 

 power of the fluid decreases so rapidly that the force of the current, 

 even in half an hour, becomes considerably weaker than it was at the 

 beginning. But, apart from this, the maintenance of the battery is 

 extremely expensive, because much more zinc is consumed than the 

 current itself requires, and the nitric acid acts destructively upon the 

 metallic rings around the carbon cylinders. It is true the disadvan- 

 tages of this action of the nitric acid could be avoided by the use of 

 Daniell's elements, but then the battery must be considerably enlarged 

 to obtain the same effect. 



