190 SECI10N PHYSICS. 



that in washing out the silver contained in the vessel some parts of it 

 are easily broken off, because it covers the black platinum vessel with 

 a rough surface, and so some particles are lost. Then if it is not well 

 washed, foreign matters remain in it, and thirdly, if it is not well 

 dried, a great deal of water may adhere to it ; for instance, upon a 

 square centimetre of a plate covered with silver by this process, eight 

 milligrammes of water will adhere under ordinary circumstances. 



This instrument, devised by Professor Lenz, measures the decompo- 

 sition of mercury not by weighing it but by volumetric measurement. 

 There are two vessels connected by an inclined tube, and in the bottom 

 of the upper one a little mercury is put ; the lower vessel ends in a 

 glass tube which dips into a cylindrical iron vessel, which forms the 

 chief part of the measuring apparatus. Some mercury is poured into 

 the upper vessel and drips into the lower one, and goes through the 

 glass tube and into the measuring apparatus. Platinum wires, enclosed 

 in glass tubes, are immersed into both vessels, and they form the 

 electrodes. The platinum wire is quite immersed into the mercury, so 

 that the mercury really forms the electrode. Over the surface of the 

 mercury in both vessels a solution of mercury salt is poured. There 

 is a micrometer screw which works an inner cylinder which fits into 

 the iron vessel quite tightly, and which is carefully calibrated so that 

 the value of one division of the micrometer screw is easily ascertained ; 

 and by this screw one can lift the mercury in the tube or let it fall. 

 Before the commencement of the experiment, when the solution is 

 poured in, the mercury contained in the tube is lowered to a standard 

 point, which can be read off by means of a lens, and it is then lifted 

 again so that it forms a bead at the bottom of the vessel, which is 

 sufficient to cover the end of the platinum wire. The experiment then 

 begins ; the current flows through one wire to the other ; the decom- 

 position begins, and the mercury is evolved at the lower electrode, and 

 just as much as is evolved there would enter into combination from 

 the acid solution contained in the other vessel. By the decomposition 

 of the mercury in the lower vessel the fluid becomes lighter, but that 

 is prevented by the slanting tube, which allows it to flow from the 

 upper vessel, and it can also be stirred. After a certain lapse of time 

 the current is broken, the screw is lowered down, which brings the 

 mercury again to the standard point ; and then, by means of the 



