HALL AND AYRES. — HEAT CONDUCTION IN IRON. 285 



and strongly adherent coating would be obtained upon the latter at the 

 first trial. The final procedure, which worked well, was as follows : — 



Two electrolytic baths were prepared, one consisting of a cyanide of 

 copper solution purchased ready made from a nickel plater, the other 

 being an ordinary sulphate of copper solution of specific gravity about 

 1.10, acidulated by about one drop of strong sulphuric acid to ten cubic 

 centimeters of the solution. Each solution contained two vertical plates 

 of copper, somewhat broader than the disk to be coated, placed several 

 centimeters apart. 



A hole about O.o cm. in diameter was bored a short distance into the 

 curved side of the disk, and in this was fixed one end of a steel rod, 

 which was to serve the double purpose of a handle and a conductor of 

 the current from the disk. After being rubbed tolerably bright the disk 

 was boiled in a strong solution of caustic potash for ten minutes, then 

 rinsed in flowing water, then scoured with powdered pumice and water 

 by means of a bristle brush, then dipped some seconds in a 20% solution 

 of hydrocliloric acid, then rinsed again in flowing water, then dipped 

 again in the acid solution, then rinsed again, then placed between the 

 two plates of copper in the cyanide of copper solution, which was at a 

 temperature near 70° C, and kept there half an hour with a current of 

 about 3.5 amperes flowing through it. At the end of this time the sur- 

 face of the disk, including its curved side, was well coated with copper. 

 Accordingly, the disk was taken from the solution, rinsed, covered as to 

 its curved surface with a rubber band to prevent further deposit of copper 

 there, then jilaced in the sulphate of copper solution between two copper 

 plates about 8 cm. apart; and a current of 3 amperes or more was set to 

 flow through it. 



After a number of hours, beads of copper were seen to have formed at 

 the edges of the two flat faces of the disk, and the disk was removed 

 from the solution long enough to allow these beads to be broken or filed 

 off". It was then rinsed, dipped in the hydrochloric acid solution, rinsed 

 again, then replaced in the sulphate bath and again subjected to the cur- 

 rent. This course of operations was continued for several days, about 

 135 hours of current use, until the layer of copper on each face of the 

 disk was about 0.2 cm. thick. At one stage of the procedure it was 

 found necessary to resort to the cyanide bath again for a short time, the 

 filing off of the copper beads at the edges having exposed the iron at 

 certain parts of the convex surface. 



The coatings when completed were somewhat tWcker near the edge 

 than in the middle. Accordingly, they were turned off in the lathe to a 



