T I N

TIN

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treated as in the lad experiment, hut with theaddition of fome leaf-tin and the copper wasnow compleatiy tinned.

4- Tiie lilt experiment was repeated, butwithout an excels of acid, and no eifedt wasproduced on the copper.

5. Cream of tartar, tin filings, and a pieceof copper, were boiled together, and the copperwas compleatiy tinned.

Thefe experiments fiiew that in order to pre-cipitate tin upon copper by means of tartar, itis neceffary, both that fome metallic tin fhouldbe prefent in the folution, and that the i'olutionfhould comain an excefs of acid.

6. Some oxyd of copper ariiing from thedecompofition of blue vitriol by an alkali, wasfirft boiled in a folution of cream of tartar, af-ter which both tin-leaf and copper were intro-duced, and the whole boiled together for threehours. Both the copper and tin were coveredwith a cruft of blackifh metallic alloy.

7. Some copper was tinned by boiling withtin and a folution of tartar. The liquor afterthe copper was withdrawn was not changed toblue by volatile alkali, and hence was inferredto contain no copper, though when expofed tothe light fome faint blue ltreaks were obferv-able.

8. The fame experiment was repeated withtartar, alum, and fait, (the ufual mode of tin-ning) and no indications of copper were fur-niflied by the volatile alkali.

9. Copper was tinned with each of theabove three falts fingly, and in neither inftancedid the volatile alkali give any indications ofdiffolved copper in the liquor.

10. Tin-leaf was boiled for an hour with afolution of cream of tartar. The tin was thenwithdrawn and a bar of iron introduced, andthe boiling continued. The iron was foonblackened at its furface, and after three quartersof an hour, was covered with a mixture of re-duced metallic tin and oxyd of tin, and the ironhad loft weight in the procefs.

Hence as iron was found to precipitate tin ina metallic ftate from its folution, it was con-jectured that iron added to a folution of tin, inwhich copper was immerfed, might determinethe tinning of the copper. The experimenthowever fueeeeded but imperfectly when tartarwas ufed, owing to the languor of its adtion oniron, and even when a few drops of fulphuricacid were added, the tin was precipitated folelyon the iron, whilft the furface of the copperremained unaltered, but with alum it was dif-ferent.

11. Some tin-leaf was boiled with a folutionof alum for half an hour and then withdrawn.Into the folution were then introduced a pieceof polifhed copper, and fome iron filings. Thelatter were adlcd on with much rapidly, andin a few minutes the copper was covered witha pellicle of tin, which in haft an hour gave itthe appearance of fine filver unpoiiihed.

12. Tin-leaf and a lolution of alum wereboiled together. The tin was then taken out,and into the folution both iron and copperwere introduced, each poliftied. After a fewminutes boiling both thefe metals were highlytinned.

13. Some fulphat of copper was diffolved inwater, tin leal was added, and the mixturewas expofed to a moderate heat for fome daysin a well clofed phial. In a few days thecolour of the folution entirely difappeared, andthe copper precipitated to the bottom of theveflel. The clear fupernatant fluid was diftri-buted into three phials, into the firft of whichwas put copper and tin ; into the fecond, cop-per and iron; and into the third, copper alone.They were well corked, and after digeftion inmoderate heat for fome hours the copper in thefirft phial, and the copper and iron in the fe-cond were well tinned, but the copper in thethird remained unaltered.

From the above experiments we may clearlymake out that the ftate of the folution of tin,when it is difpofed to precipitate in a metallicform on the furface of copper immerfed in it,is that of the loweft degree of oxydation, whichis obtained either by keeping fome undiffolvedtin in the folution or (Exp. 10 and 11) by in-troducing iron. The great difficulty which oc-curred to the author of thefe experiments wasto account for the precipitation of the tin whenthere did not appear to be any copper diffolvedduring the procefs, for in all other metallicprecipitations (as that of copper on iron for ex-ample) the feparation of one metal from thefolution is attended with the folution of a cor-refponding quantity of the other metal.

The difficulty is increafed too by the fa£Sthat when tin is immerfed in a folution of cop-per the copper is precipitated, and a portion ofthe tin is diffolved, fo that the diredt affinity ofacids for tin is greater than for copper.

The hypothefis given by Prof. Gadolin toexplain this is highly ingenious. He obfervesthat as a very ftrong affinity exifts between tinand copper, this affinity operates on the me-tallic part of the folution of the tin immediatelyin contact with the copper, and caufes its fepa-