TEANSACTIONS OF SECTION A. 525- 



crystalline and less adherent. The current density cannot he increased in nearly 

 the same proportion as the increase in the amount of silver nitrate in the solution, 

 and hence when sheet silver cathodes are used the most economical and most satis-- 

 factory method is to use weak solutions and large plates. Rolled silver gives 

 imsatisfactory results until the outside skin has been removed by use, polishing, or 

 scraping. 



The later experiments described in the paper refer for the most part to copper, 

 and include experiments on the uncertainty likely to arise from oxidation of the 

 plate in drying, and while in the solution ; the effect of the loss of weight due to 

 direct chemical corrosion of the plate ; the effect of density of solution ; of size of 

 plate, or current density ; and of the addition of acid to the solution. It was found 

 that there is no difficulty in washing and drying a plate without oxidising it 

 sensibly, and methods are described for doing this. The loss of weight by corrosion 

 in the liquid introduces an error which is nearly proportional to the surface of the 

 plate exposed, and judging from Dr. Gore's experiments it is also no doubt influ- 

 enced by the temperature, if that be not constant ; but the effect of temperature was 

 not investigated. The amount of corrosion is also influenced to some extent by the 

 density of the solution, and it appears to be least when the density is between 1-15' 

 and 1-10. It is concluded that in the use of copper for standardising purposes the 

 current density and the density of the solution must be known, and the proper 

 correction made on the electro-chemical equivalent to suit the particular circum- 

 stances. If this be attended to, copper is capable of giving residts within a tenth 

 per cent, of accuracy, and is much more easily managed than silver. Silver is- 

 preferable for the highest accuracy, but the accuracy which copper is capable of 

 giving is easily obtained, and it seems sufficient for most practical purposes. 



Experiments on the ratio of the electro-chemical equivalent of copper to that 

 of silver were also made, and led to the conclusion that when the proper cor- 

 rection for corrosion in the liquid is made the ratio is very nearly 0"2942, which, 

 on the assumption of •00II18 as the mass in grammes of the silver deposited by 

 one coulomb of electricity, gives for the corresponding number for copper 

 •0003290. 



"When a nearly saturated solution of copper sulphate is used, and a current 

 density of 5^5 of an ampere per square centimetre of the cathode, the amount of 

 copper deposited by a coulomb of electricity is •0003287 gramme. 



It is pointed out that if the solution does not contain free acid there is a danger 

 of obtaining too great an increase of weight, due apparently to the oxidation of the 

 copper as it is being deposited. The same remark as that made above with regard 

 to the silver anodes applies to the copper anodes if they are to be weighed. If the 

 copper anodes are so small as to give a current density exceeding about the fortieth 

 of an ampere per square centimetre the current is liable to stop almost completely,- 

 even when an electromotive force of as much as 25 volts is used to produce it : 

 this is apparently due to excessive resistance at the surface of the anode. After a 

 few minutes the current will again resume nearly its old strength, and gases will 

 be given off freely at the anodes. These gases are readily dissolved by the liquid, 

 and generally as the final result the gain and loss of copper do not differ very 

 greatly— although they always do differ — from those obtained when no gases are 

 given off. 



Lastly, the arrangements of one of the standardising tables in Sir William 

 Thomson's laboratory are illustrated, and the different pieces of apparatus and the 

 mode of using them described. 



11. Description of a new Calorimeter for lecture purposes. By T. J. Bakee. 



The instrument consists of two exactly similar metallic air-thermometers 

 mounted aide by side with their U-shaped thermometer-tubes adjacent, so that 

 their indications can be easily compared with each other. 



The air-vessel of each thermometer contains a cylindrical well, in which the 

 substance to be experimented with is immersed. Each well is provided with a 

 discharging tube furnished with a stopcock. The scale common to both ther- 



