280 PHYSICS. 



eludes that this peculiar eifect is due to a film of nitrogeu dioxide which 

 collects upon the surface of the irou and protects it from further chemi- 

 cal action. This film, he asserts, is apparent when the surface of the 

 iron is examined under the microscope. If a current of carbon dioxide 

 or of hydrogen gas is passed through the liquid the passivity ceases and 

 the iron is dissolved. INIoreover, on placing the iron plate in vacuo 

 nitrogen dioxide is evolved from it. — {Nature, xxi, 117, December, 1879.) 



Pellat has examined the standard cell proposed by Latimer Clark, 

 and finds that it is not entirely free from variations in its electromotive 

 force. According to his obsertations two of these cells may diii'er from 

 each other by a quantity equal to the one-hundredth of the electromotive 

 force of a Daniell cell. The method employed was to couple up the two 

 cells in opposition to one another, and then by means of an electrometer 

 to measure the residual difference of potential. — [Nature, xxi, 117, De- 

 cember, 1879.) 



Debrun has constructed an ingenious capillary electromotor, based 

 on the theorem established by Lippmann, that if a mercury surface be 

 deformed by mechanical means there is produced an evolution of elec- 

 tricity which tends to arrest the motion of the mercury. Through a 

 capillary tube mercury is caused to run drop by drop, acidulated water 

 being introduced, by means of a reservoir at the top, between each glob- 

 ule. Wires are connected with the masses of mercury at the upper and 

 lower ends of the tube, and a current is observed to flow on closing 

 these in the direction in which the globules move. The tube actually 

 used was 2.5"^™ in diameter at top and 1™'" at bottom, 30''™ long, con- 

 taining not less than 20 nor more than 35 globules. The electromotive 

 force under these circumstances is 1.4 volts, so that with Wollaston's 

 points it can decompose water. The energy, however, is not very great, 

 since it is produced by the fall of only two kilograms of mercury per 

 hour through only 25*^™. In allowing the mercury to run for twenty- 

 four hours sufficient current was developed to silver strongly a five- 

 centime piece. The internal resistance of a column of acidulated water 

 -[mm jj2 diameter and 30"^ long being so considerable, a high-resistance 

 galvanometer should be used to test the currents produced by this in- 

 strument. — ('/. Phys., ix, 28, January, 1880.) 



Barrett has communicated to the Physical Society of London the fact 

 that the motion of a chalk cylinder under a metallic surface generates 

 an electric current, having an electromotive force of rather over one- 

 third of a volt. The strength of the current depends on the rapidity of 

 the rotation and the pressure on the chalk surface ; the latter only 

 dimiuishing the very high internal resistance. The discovery, he says, 

 resulted from a suggestion made to him four or five months before, to 

 try whether Edison's motograph telephone receiver could be used as a 

 transmitter. Then his experiments were not successful, but now he 

 finds that the voice is faintly but accurately transmitted on speaking 

 into the receiver while the chalk is made to rotate. But Edison had 



