74 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Feb. It, 



a solution of any astringfent matter, green tea for instance, but a solution of 

 galls is best, wliicb changes it to a dark purple, forming, inileed, ink. It is 

 on this account that spring water, which frequentlj contains iron, spoils tea- 

 Prussiate of potash, also) may be used, which gives a dark blue, which is 

 Prussian blue. 



These are the principal constituents of the soil. But there are other sutj- 

 stances also present, which, though only in minute quantity, and once over- 

 looked as unimportant, are now considered to be of vital consequence to 

 plants, and which will be co»sidered in the next lecture. 



ELECTRO-METALLURGY. 



The importance of the recent discoveries in electro-magnetism, have in no 

 instance been more completely shown, than in their applications to metalhc 

 productions. The application of electro-magnetic power for plating, emanates 

 from Mr. Spencer, of Liverpool, who was so shamefully used by the British 

 Association. The Messrs. Elkington had, however, long before taken out 

 patents for plating, although on other grounds. They subsequently made 

 experiments with regard to the use of electro-magnetism in gilding and silver- 

 ing, and the result has been several valuable processes of manufacture. From 

 a very interesting little work, published by Messrs. Elkington, we extract the 

 following descriptions:— , ,. . , v 



Galvmuc Battcry.-\n speaking of the discovery of this Art, we have been 

 led to describe what a Galvanic Battery is. Many different arrangements ot 

 battery have been suggested, all more or less valuable, according to the parti- 

 cular object for which the arrangement is required, but we do not think it 

 important to notice them. 



In Electro-Metallurgy there are two distinct states (if such an expression 

 may be allowe.lj of galvanism or electricity, viz., electricity in its condition 

 oiqumMly, and inteiisiUj, and as it « ill be necessary to quote these terms, we 

 shall endeavour to explain them. . 



The ?«an(;(i/ of electricity generated in a battery, is in proportion to the 

 surface of zinc exposed to the exciting fluid, whether such consist of one or 

 more pairs of plates ; but if more than one pair be used, tlion the zinc ot the 

 first pair must be connected with the zinc ot the second pair, and the copper 

 vith the copper; and thus the quantity may be increased .as the numbers ot 

 pairs are augmented. 



A, Battery Trough. B, Zinc Plates. C, Copper Plates. D, Connecting Wires. 



Intensity is necessary when, from the nature of the solutions, or from any 

 other cause, there is a resistance to be overcome; the electricity requiring 

 intensity or power to force its way through , but the amount of metal de- 

 posited, depends wholly upon the quantity of the battery, and not upon the 

 intensity. 



Intensity is produced by arranging two or more pairs of plates, by con- 

 necting the zinc of the one pair with the eopper of the next pair, and so on 

 to any number of pairs— any numtjer thus connected, forming a battery equal 

 in qnanlity to one pair of plates only. 



other substances, forming compounds, capable of being dissolved, and held in 

 solution by acids and certain alkalies; thus, tor example:— When pure silver 

 is put into pure nitric acid and heated, the metal is rapidly dissolved, forming 

 the nitrate of the oxide of silver, or, as commonly designated, the nitrate of 

 silver, and this may be held in s ilution in water. In forming a metallic salt 

 for the purpose of electro-deposition, one general rule holds good in all 

 cases, and may be considered a law (and if this law is attended to, no difficulty 

 whatever can e.sist in electro-depositing) ; and this is, that the metal dissolved 

 in acid or other solvent must have a greater affinity for such solvent, ihan the 

 metallic article to be coated with the metal thus held in solution— for this 

 simple reason, that if the article to be coated hA% a greater affinity for the 

 solvent than the metal held in solution, a chemical substitutionjpreceding 

 the galvanic action) takes place ; the acid in preference combining with the 

 metal, for which it has the greatest affinity, forming an oxide upon its sur- 

 face, which oxide intervenes between the article and the metal depoiited 

 upon it. Such failures therefore as have taken place in electro-depositing 

 have not been the result of any defect in the principle, but from want of 

 knowledge in the operator, and principally from ignorance of the law just 

 stated. For this reason the nitrate of silver cannot be used ; the nitric acid 

 having a stronger affinity for most other metals than (or silver ; another and 

 better solvent therefore must be obtained, and this will be spoken of under the 

 metal, silver. 



Operation.— In proceeding to plate metals, it is necessary that the battery 

 should be so arranged, that the quantity of electricity generated, should cor- 

 respond with the surface of the articles to he coated, and the intensity should 

 bear reference to the state of the solutions ; that is, that the quantity should 

 be sufficient to give the required coating of metal in a given time, and the in. 

 tensit,, such as 10 drive the electricity through the solution to the articles. It 

 is also essential that the plates of metal forming the positive pole in the 

 solution, should be of corresponding surface to the articles to be coated, and 

 face them on both sides. 



Having procured the metallic salt best adapted for the purpose, the same is 

 placed in an appropi late vessel, the articles to be coated are then suspended in 

 the solution from wires attached to metallic rods, which cross the vessel at 

 intervals, and which are connected with the zinc or negative terminal of the 

 galvanic battery by means of long rods, placed longitudinally upon the vessel ; 

 plates of the same metal as that held in the solution are then placed therein 

 upon either side of the articles; and connected with the positive or copper 

 terminal of the same battery, by means of aslip of copper, fixedupon the ejge 

 of the vessel. 



A, Battery Trough. B, Copper Plates. C, Zinc Plates. D, Connecting Wires, 



With this knowledge a battery of several pairs of plates may be arranged 

 So as to produce bolh the quantity and intensity required. 

 Metallic Salts.— M. ordinary metals have the power of combining with 



A, Vat or vessel coitaining tlie solution. B, Battery with Zlne pole Z, connected with 

 roiis KRi and Copper pole C, eonnected with the metallic sheets PP, in the solution; hy 

 means of the Copper slip F; DD, are articles suspended in the solution by wues from the 

 rods RR ; S, the solution. 



The particular arrangement of battery shown in the above sketch we find 

 the most useful in practice. The coppers being continued above the zincs in- 

 stead of below, prevents their being affected by the mercury, which amalga- 

 mates the zinc. The plates also resting upon a shelf a tew inches above the 

 bottom of the trough, are always In contact with the least saturated portion 

 of the e.«iting fluid, the sulphate of zinc formed, being specifically heavier, 

 falls to the bottom. , 



So soon as the articles, which are connected with the negative pole of the 

 battery, and the metallic sheets with the positive, are both immersed in the 



