RECENT PROGRESS IN PHYSICS. 409 



a previous, althoiigli very thin, gildinfij. The younger Becquerel 

 (Ann.de Chira.and de Phys., March, 1845: Dingler's Journal, vol. 9G) 

 directed attention to the fact that the order of the colors thus obtained 

 corresponds to that obtained with transmitted light, and also main- 

 tained that the thickness of the layers decreases from the centre of the 

 rings in the inverse ratio of the radii. Zur Nedden (Dingler's Jour- 

 nal, vol. 94) attempted to refute the opinion that the colors were pro- 

 duced by " thin plates" at all, and thinks that they arise from the 

 colors of the precipitated combinations and of the metals themselves. 

 But without disputing the influence of these two causes upon the shade 

 of individual colors, there cannot be the least doubt that their order, 

 as obtained, for instance, upon German silver is exactly the same as 

 that obtained from a wedge-shaped plate of gypsum when the mirrors 

 of the polarizing apparatus reflect the light in planes at right angles 

 to each other. 



The above mentioned assertion of Edmund Becquerel^ that the thick- 

 ness of the layers is in the inverse ratio of the radii of the rings, based 

 upon theoretical consideretions and confirmed by measurement on two 

 selected plates, was opposed by Du Bois, Reymond and Benz, (Fogg. 

 Ann., vol. 71.) The former deduced, from accurate theoretical inves- 

 tigations, the law that the thickness of the stratum precipitated by 

 galvanic currents at different distances from the negative point must 

 be inversely as the cubes of the radii, and Benz proved by experiments 

 that this actually is the case. The discrepancy between these contra- 

 dictory experiments is not yet explained. 



The splendid colors obtained in the manner indicated are already 

 used in the arts for covering table bells, cups, and other small ware of 

 brass plate. The solutions of oxide of lead used for this purpose is 

 prepared in the following manner : One part of caustic potassa is 

 dissolved in 5-6 water, and boiled with finely ground litharge in 

 excess for half an hour in an earthenware vessel, being well stirred 

 all the time. The solution is then filtered and preserved in a well 

 closed vessel. For use, the liquid is poured into a vessel of lead or 

 brass large enough to hold conveniently the object to be coated, which 

 is connected with the positive pole of a Daniell's battery of 3 pairs, 

 and immersed in the lead solution, when the negative pole is con- 

 nected with the outside of the vessel. The strength of the current 

 must be regulated experimentally by means of tlie charging fluids, 

 so that the formation of the rings, of which usually only the first two 

 systems are well developed, proceeds somewhat slowly, by which 

 means the current may w^ith more precision be interrupted just when 

 the desired shade is obtained. Brass requires some modifications in 

 the jjrocess, as Becquerel had previously noticed; objects of tliis metal 

 must be gradually immersed into the solution with the sharp edge 

 foremost, the circuit having been previously closed. All the objects 

 must be well cleaned with Tripoli or English rouge, and should not 

 afterwards be touched with the fingers ; varnish is not usually em- 

 ployed. 



§ 185. Obtaining metals from their ores by mea7is of galvanism. — Bec- 

 querel devoted especial attention to the solution of this problem ; but 

 notwithstanding the promise of his first reports, and the considerable 



