218 Scientific Intelligence. 



SCIENTIFIC INTELLIGENCE. 



I. Ghemistry and Physics. 



1. On an Electrochemical method for changing an Alternating 

 into a Direct Current. — It has long been known that when an 

 aluminum plate is made the anode in an electrolytic cell, the 

 oxygen set free upon this plate produces there a coating of oxide 

 which greatly increases the resistance of the cell. Hence a cell 

 having such an electrode acts like a trap, the current being per- 

 mitted to flow freely when the aluminum plate is made the 

 cathode, while when it is the anode the current is very feeble. 

 Geaetz has now proposed to utilize such cells for the purpose of 

 changing alternating currents into direct ones. For this purpose 

 a number of such cells are arranged in series, the number being 

 so chosen that the polarization at the anode is either equal to or 

 is greater than the current-pressure. Under these circumstances 

 the current in that direction for which the aluminum plate would 

 act as the anode is greatly diminished, while the opposite current 

 passes freely. The issuing current therefore is practically a cur- 

 rent in one direction. — Ann. Phys. Ghent. , II, lxii, 323-327, 

 September, 1897. G. f. b. 



2. On the Electrochemical Equivalent of Carbon. — It has been 

 noticed that in electrolyzing liquids evolving oxygen, using car- 

 bon as the anode, this carbon is acted upon chemically, the char- 

 acter and extent of this action depending upon the electrolyte 

 employed. Thus Coehn has determined that when a current of 

 0*12 ampere is sent through six electrolytic cells having pure 

 carbon anodes and platinum cathodes, the electrolyte being sul- 

 phuric acid diluted with 1, 10, 20, 50, 100 and 500 volumes of water, 

 for ten hours, the most concentrated acid remains colorless, the 

 most dilute becomes dark brown, while the intermediate solutions 

 have intermediate shades ; at the same time there being con- 

 siderable mechanical disintegration. At higher temperatures, this 

 disintegration becomes less, while the electrolyte is still more 

 deeply colored. Thus at 100°, in a solution made up of equal 

 volumes of sulphuric acid and water, the mechanical disintegra- 

 tion becomes inconsiderable. The author determined the loss of 

 weight of a carefully purified carbon anode under these circum- 

 stances, and obtained the value 3*5 as the electrochemical equiva- 

 lent of carbon ; but this number is probably too high, as some 

 mechanical loss had taken place. He then made a number of 

 experiments at the ordinary temperature, using sulphuric acid 

 diluted variously with from 10 to 500 volumes ot water, the frag- 

 ments of carbon mechanically separated being collected and 

 weighed, and their weight subtracted from the total loss of the 

 anode. The value of the electrochemical equivalent of carbon 

 thus obtained varied from 2*7 to 3*0. It thus appears that this 

 number is independent both of the concentration of the acid and 



