54 REPORTS ON THE STATE OF SCIENCE. 



to be dried in an electric oven at a temperature of about 160° C. ; it is 

 placed in a desiccator to cool, and is afterwards weighed. 



The mass of the deposit, expressed in grammes, divided by the number 

 of seconds during which the current has been passed and by 0"001118, 

 gives the mean current in amperes. 



Preparation of the Weston Cadmium Standard Cell. 



The cell has mercury for its positive electrode, and an amalgam con- 

 sisting of from 12 to 12-5 parts by weight of cadmium in 100 parts of the 

 amalgam for its negative electrode. The electrolyte consists of a saturated 

 solution of cadmium sulphate, and solid cadmium sulphate is contained 

 within the cell. A paste, consisting of solid mercurous sulphate, mercury, 

 and solid cadmium sulphate, rests on the positive electrode. 



For the positive electrode, pure distilled }7iercury should be used. 



The amalgam may be made either by electrodeposition or by mechani- 

 cal mixing. It should be fused and freed from oxide by washing with 

 dilute sulphuric acid. 



For the preparation of the cadmium sulphate crystals and solution, 

 commercially pure recrystallised cadmium sulphate should be dissolved in 

 pure distilled water so as to form a clear saturated solution. Evaporation 

 at about 35° C. is then allowed to proceed, when crystals separate from 

 the solution. The crystals are washed with successive small quantities of 

 distilled water, and part of them is dissolved in distilled water to form 

 a saturated solution. The solution should be neutral to congo red. 



The mercurous sulphate should be quite pure, and its crystals should 

 not be so small as to have an abnormal solubility or so large as to be in- 

 efficient as a depolariser. The following is an example of a method for 

 preparing the salt satisfactorily : — - 



Add 15 cubic centimetres of pure strong nitric acid to 100 grammes of 

 pure mercury, and place on one side until the action is over or nearly over. 

 Transfer the mercurous nitrate thus formed, together with the excess of 

 mercui'y, to a beaker containing about 200 cubic centimetres of dilute nitric 

 acid (1 volume of acid to about 40 volumes of water) ; a clear solution 

 should result. Prepare about 1 litre of dilute sulphuric acid (1 volume of 

 acid to 3 of watei'), and while the mixture is hot add the acid mercurous 

 nitrate solution to it. The solution should be added as a very fine stream 

 from the narrow orifice of a pipette, and the mixture violently agitated 

 during the mixing. Mercurous sulphate is precipitated. Decant the hot 

 clear liquid and wash the precipitate twice by decantation with dilute sul- 

 phuric acid (1 volume of acid to 6 of water). The precipitate should then 

 he filtered and washed three times with dilute sulphuric acid (1 to 6), and 

 afterwards 6 or 7 times with saturated cadmium sulphate solution to 

 remove the acid. The mercurous sulphate should then be flooded with 

 saturated cadmium sulphate solution and left for one hour, after which the 

 solution is tested with congo red paper. In general no acid will be 

 detected, and if so the r:jercurous sulphate is ready for use. 



To set up the cell the H form of vessel is the most convenient. The 

 platinum wires inside the vessel should be amalgamated by passing an 

 electric current to each in turn through an acid solution of mercurous 

 nitrate. The vessel must afterwards be washed out twice with dilute 

 nitric acid and several times with distilled water ; it must be free from 

 stains and scrupulously clean ; it is dried by the application of heat. The 



