158 REPORT — 1891. 



Method of Mold-)} g a Measurement, 



The platinum bowl is washed with nitric acid and distilled water, 

 dried by heat, and then left to cool in a desiccator. When thoroughly 

 dry it is weighed carefully. 



It is nearly filled with the solution, and connected to the rest of the 

 circuit by being placed on a clean copper support, to which a binding 

 screw is attached. This copper support must be insulated. 



The anode is then immersed in the solution so as to be well covered 

 by it and supported in that position ; the connexions to the rest of the 

 circuit are made. 



Contact is made at the key noting the time of contact. The current 

 is allowed to pass for not less than half an hour, and the time at which 

 contact is broken is observed. Care must be taken that the clock used 

 is keeping correct time during this interval. 



The solution is now removed from the bowl and the deposit is washed 

 with distilled water and left to soak for at least six hours. It is then 

 rinsed successively with distilled water and alcohol and dried in a hot-air 

 bath at a temperature of about 160'' C. After cooling in a desiccator it 

 is weighed again. The gain in weight gives the silver deposited. 



To find the current in amperes this weight, expressed in grammes, 

 must be divided by the number of seconds during which the current has 

 been passed, and by •001118. 



The result will be the time average of the current, if during the 

 interval the current has varied. 



In determining by this method the constant of an instrament, the 

 current should be kept as nearly constant as possible, and the readings 

 of the instrument taken at frequent observed intervals of time. These 

 observations give a curve from which the reading corresponding to the 

 mean current (time average of the current) can be found. The current, 

 as calculated by the voltameter, corresponds to this reading. 



Pkovisioxal Memorandum on the Preparation op the Clark's 

 Standard Cell. 



Definition of the Cell. 



The cell consists of zinc and mercury in a saturated solution of zinc 

 sulphate and mercurous sulphate in water, jJi^epai'ed with mercurous 

 sulphate in excess, and is conveniently contained in a cylindrical glass 

 vessel. 



Preparation of the Materials. 



1. The Mercurij. — To secure purity it should be first treated with acid 

 in the usual manner, and subsequently distilled in vacuo. 



2. The Zinc. — Take a portion of a rod of pure zinc, solder to one end 

 a piece of copper wire, clean the whole with glass paper, carefully remov- 

 ing any loose pieces of the zinc. Just before making up the cell dip the 

 zinc into dilute sulphuric acid, wash with distilled water, and dry with a 

 clean cloth or filter paper. 



3. Tlie Zinc Sul})hate Solution. — Prepare a saturated solution of pure 

 (' pure re-crystallised ') zinc sulphate by mixing in a flask distilled water 

 with nearly twice its weight of crystals of pure zinc sulphate, and adding 



