1879.] 



On the Capillary Electroscope. 



33 



of the mercury to different parts of the tube, but sometimes by a 

 minute trace of solid substance in the tube. With a very feeble 

 current this adhesion shows itself in several ways, usually by a more 

 or less sudden retardation or stoppage of the motion of the mercurial 

 column, by a delay in its commencement of motion, or by its non- 

 movement. The current I have employed has, in nearly all cases,, 

 been a very feeble one. obtained from two wires, one of copper, and 

 the other of platinum, each about 1 millim. diameter, immersed about 

 2 or 3 millims. deep in common drinking water. With a very sensitive 

 instrument, and a capillary tube as free from obstruction as could be 

 obtained, this power has been sufficient to move the meniscus 10 or 

 20 millims. 



A common cause of obstruction consists of impurities (especially 

 oxide) in the mercury ; the metal therefore should be as pure as 

 possible. I have obtained it sufficiently pure by distilling the ordinary 

 metal twice, then shaking it violently with dilute nitric acid, washing 

 it with distilled water, shaking it again strongly with a dilute solution 

 of potassic cyanide (which very effectually removes oxide), washing it 

 again, and filtering it through a minute hole in the point of a paper 

 funnel. The mercury should be kept in a closed bottle, and be fre- 

 quently filtered to remove dust from its surface whilst being used r 

 because the most minute speck of solid matter, even that which is in- 

 visible to the unassisted eye, is sufficient to stop the capillary tube. 

 The solution also should be kept perfectly clear by means of filtration. 

 Another cause of stoppage is the employment of glass tubing into 

 which dust has been allowed to enter; this may be obviated by 

 employing newly-made tubing, sealed at its ends by wax ; with such 

 tubing I have obtained no manifest advantage by washing it with 

 acids, alkalies, ether, &c, nor by allowing a stream of clean mercury 

 to run through it. After having taken all these precautions, there 

 remains a very common cause of stoppage of the motion of the 

 mercury, viz., portions of air or gas adhering to the tube ; these con- 

 sist either of minute films of air, or if the current is too strong, also 

 bubbles of gas liberated from the electrolyte, 



One of the most effectual modes of removing the air or gas is to 

 repeatedly cause by mechanical means (such as compression of the 

 pressure-chamber, or by change of position of the instrument from 

 horizontal to vertical and vice versa) the mercury and conducting 

 solution to move freely to and fro through the whole of the fine part 

 of the tube whilst the circuit is closed, and expelling occasionally a 

 few drops of mercury from the end of the tube ; employing also an 

 electric current in a suitable direction to assist the motion. If there 

 remain any adhering portions which are not removable by these 

 means, diminished pressure, combined with an electric current from 

 the point of the capillary towards its larger part, and the simultaneous 



VOL. xxx. D 



