228 Progress of Invention. 



When oxide of lead is employed, three parts litharge, four parts 

 caustic potash, and forty parts water are boiled together, and as soon 

 as the solution is complete, and has been allowed to cool, forty parts 

 water are added to it ; after which it is transferred to a porous 

 vessel, which is placed in water acidulated with one-twentieth part 

 nitric acid. The article to be coated is placed in the porous 

 vessel, and a metallic plate in the dilute nitric acid, the article being 

 connected with the positive, and the plate with the negative pole of 

 a constant battery. In a few minutes the article is covered with a 

 tine dark brown coating of peroxide of lead, which is not in the least 

 disturbed by the burnisher. During the process the litharge is per- 

 oxidized by oxygen derived from decomposed water. A coating of 

 oxide of iron is formed by heating protosulphate of iron and 

 ammonia, and using the liquid thus obtained in the same way as 

 the lead solution. It will not answer with articles of copper, on 

 account of the effect produced on that metal by ammonia. The 

 thinner the film of peroxide of iron thus formed, the more adhesive 

 it is ; and therefore the process should be continued only until a 

 sufficiently good colour is obtained, which takes place in a few 

 minutes. The iron is peroxidized during the process in the same 

 way as the lead. The article to be covered should in all cases be 

 well cleaned, and be roughened with pumice stone. 



Curve produced by a Vibrattxg String. — A very simple means 

 has been devised for rendering the curve produced by a vibrating 

 string visible. For this purpose the string is illuminated by a 

 bundle of solar rays, reflected by a heliostat. In the path of these 

 rays, a little in front of the string, is placed an opaque disc, present- 

 ing transparent diameters at right angles, and capable of revolving 

 on an axis which is at right angles to its plane, so as to make a few 

 revolutions per second. On the other side of the string is fixed a 

 lens of short focus, which forms an enlarged image on a white 

 screen that is placed at the distance of two or three yards. The 

 curve thus obtained will be distinctly visible to the audience of 

 a lecture room. 



Self-Eegistering Electric Thermometer. — Avery sensitive and 

 efficient instrument of this kind has been invented by General 

 Morin. It consists of a thermo-electric pile, and a modified multi- 

 plier. The thermo-electric pile, which is on the principle of M. 

 Becquerel, consists of thirty rods of iron and maillechort, ranged in 

 parallel grooves, that are formed around a cylinder of wood about 

 two inches in diameter, and have their alternate extremities, which 

 project about three-quarters of an inch beyond the ends of the 

 cylinder, soldered together in pairs. The number of rods that 

 should be employed will depend on the intensity of the^ current 

 which it is intended to produce when the pile is in operation — that 

 is, when one end is kept at a constant temperature by means of 

 melting ice, and the other end is in the medium, the temperatures of 

 which are to be registered. The multiplier, which the current from 

 the pile is made to traverse, consists of two bobbins, in the centre of 

 which a magnetized needle is suspended by a silk fibre. When in 

 use, the needle should, if no current is passing, lie in the plane of the 



