160 ELECTROLYSIS. 



the use of coating materials having red or white lead as a basis. 

 M. Oudry junior's process also offers some dangers to the work- 

 men's health, due to the use of dry copper dust ; the latter 

 becomes fatally introduced into the respiratory organs when the 

 operation of scrubbing is carried on. It is said that in time of 

 cholera this inconvenience might turn out to be an advantage ; 

 but although it is nowadays the fashion to advocate the use of 

 copper, we cannot consider its use as healthy. In the shape 

 of dust, particularly, it is eminently toxic, and certain precau- 

 tions should be taken to avoid its dangerous effects. 



In conclusion, we must add that the cost of the coppering 

 of cast iron over an insulating layer is comparatively high, 

 since the thickness of the deposited coating is necessarily 

 great, and that, notwithstanding its thickness, it is liable to be 

 deformed under the slightest shock. Many pieces get torn at 

 the angles, and these defects have to be concealed by means of 

 certain contrivances, the most generally used being the appli- 

 cation, when hot, of a cement composed of rosin and copper 

 dust on to the damaged parts. Almost all the pieces have to 

 be mended in that fashion, and it is not one of the least incon- 

 veniences of the process. 



In principle, it can be admitted that the pieces coppered 

 by that process cost a price varying between 1 franc and 

 1 fr. 50 c. per kilogramme. 



WEIL'S PKOCESS. M. Weil obtains a direct coppering in 

 an alkaline liquor. This liquor consists of sulphate of copper 

 dissolved in an excess of potassic or sodic tartrate, alkalised 

 with caustic soda. The objects of the excess of alkali are : to 

 keep the copper in a state of dissolution (for cupric tartar alone 

 is insoluble in water) and to prevent the cast iron being attacked 

 by the acid, which secures the adherence of the copper to the 

 cast iron. The property which alkalino-orgamc solutions pos- 

 sess of dissolving ferric oxide without attacking the iron itself 

 is availed of for the scouring of the pieces. The bath scours 

 the iron at the same time that it coppers it. 



The objects are suspended in the bath by means of zinc wires. 



The current is produced by the dissolution of zinc in the 

 alkali ; the expenditure of electricity is therefore very small ; 



