MECHANICS AND USEFUL ARTS. 49 



electric action required to decompose a milligramme of water, and to 

 ascertain the electric state of all the parts of a protected metal ; and 

 thus to find the laws on which a system of protection might bo based. 

 In order to conduct these experiments on a large scale, the Imperial 

 Minister of the Marine placed at M. Becquerers disposal all the means 

 desired, with the assistance of the most eminent naval engineers in the 

 port of Toulon. The result of these experiments show, with regard 

 to copper, that, as on every metallic surface, there circulates by the 

 intermediate liquid which wets it, currents producing electro-chemical 

 decomposition ; if we wish to preserve a surface so as to avoid these 

 deposits we must arm the surface with an electro-motive power equal to 

 that point at which these deposits become insensible. For this pur- 

 pose laminae of iron and zinc have been employed. Lamiuge of cop- 

 per, armed with iron, and lamime of iron protected by zinc, present 

 similar effects, with the difference that the sphere of electric activity 

 is less, where the extent depends on the difference between the electro- 

 motive forces of the protecting and protected metal. Whenever the iron 

 is covered with several coats of white lead it is preserved so long as the 

 paint remains ; but when it is once removed, either by friction or the 

 slow dissolving action of the sea, the metal is attacked in various parts ; 

 those which have lost the paint become negative and the local altera- 

 tions are gradually disseminated. M. Becquerel trusts that the means 

 proposed will be Ibund to be successful in preserving vessels from the 

 action of sea-water and from the attachment of marine animals. 



In a subsequent communication to the Academy, M. Becquerel made 

 some additional statements relative to his researches. He stated that 

 he had considered that he had demonstrated that when one of the ex- 

 tremities of a bar of iron or steel is placed in salt water, in contact with 

 a bar of zinc, the surface of which is a hundred times less, the intensity of 

 the currents derived from the surface of the protected metal, winch re- 

 sults from the oxidation of the zinc, diminishes in proportion as it be- 

 comes more and more distant from the points of contact of the two 

 metals ; so that protection takes place at considerable distances. In 

 fresh water the effects are very different. The derived currents, which 

 are the cause of the preservation of the iron as well as the electro-motive 

 force, lose their power. They, however, still retain sufficient to preserve 

 from oxidation large surfaces of iron formed of pieces superposed on 

 each other, or juxtaposed and armed with zinc or a suitable alloy. In 

 conclusion, M. Becquerel asserts that if iron or steel be placed in either 

 fresh or salt water, so that the curve of the intensities of the electro- 

 motive force derived from contact with zinc shall never meet the right 

 line of the iron, the iron is protected. The process is also adapted to 

 the preservation of iron water-pipes in wet earth, 



The Protection of Iron Bridges from Oxidation, ~ A new iron 

 bridge of the Thames at London is to be protected from oxidation by 

 a new process patented by Messrs. Morewood & Co., and is alike im- 

 portant from the great cost which will be incurred, and the testing of 

 a rather abstruse chemical formula for the preservation of iron from ox- 

 idation and decay. The process is as follows: "The iron is to be 

 thoroughly cleaned and heated to the requisite temperature in a fur- 

 nace planned by the inventors. When this temperature is attained, it 

 is to be plunged into a bath of prussite of potash and chloride of potas- 



