PLATING. 



between the temperatures of 32 and 

 212 is expanded about 12 parts in 10,000, 

 while the expansion of platina is only 

 about 10. 



Platina combines with many of the me- 

 tals, and forms with them alloys, some 

 of which are of considerable importance 

 in the working- of this metal. Platina forms 

 an alloy with arsenic, which is brittle and 

 very fusible. It is in this state of alloy 

 that platina is susceptible of being 1 formed 

 into different utensils and instruments for 

 which it is peculiarly fitted. It is first 

 fused with this metal, ,and then cast into 

 moulds, at first in the form of square 

 plates. It is then exposedto a red heat, and 

 hammered into bars. By the heating and 

 hammering, the arsenic is driven off, and 

 the metal is purified and becomes infusi- 

 ble, but retains its ductility, so that it may 

 be wrought like iron. It has been found 

 extremely difficult to combine platina and 

 mercury. Guyton had observed that the 

 adhesive force of platina and mercury is 

 greater than that of metals which do not 

 combine with it ; and that it is not inferior 

 even to those which readily form alloys ; 

 from which he conjectured that the alloy 

 of platina and mercury might be effected 

 by the following process. He placed a 

 very thin plate of pure platina at the bot- 

 tom of a matrass containing a quantity of 

 mercury. The matrass was put upon a 

 sand bath, and heat applied, till the mer- 

 cury boiled and the matrass became red- 

 hot. When the platina was taken out, 

 it vyas found to have acquired additional 

 weight, and to have become verv brittle. 

 But this combination is different from the 

 other combinations of mercury with the 

 metals, for the platina did not lose its 

 solid form. M. Chenevix, in the course 

 of experiments and researches respecting 

 a supposed new metal called palladium, 

 succeeded in forming an amalgam with 

 p atina and mercury. He heated purified 

 platina in the form of fine powder, with 

 ten times its weight of mercurv, and rub- 

 bed them together for a long time. The 

 result was an amalgam of platina, which 

 >emg exposed to a violent heat, lost all 

 the mercury it contained, and the origi- 

 nal weight of the platina remained. Pla- 

 tina combines with copper by means of 

 fusion, and gives it hardness. When the 

 proportion of copper is three or four 

 times greater than that of platina, the alloy 

 is ductile, susceptible of a fine polish, and 

 is not altered by exposure to the air 

 his alloy has been employed in the fa- 

 'icatum of mirrors for telescopes. Gold 

 combines readily with platina, but it re- 



quires a very powerful heat for the fusion 

 of these two metals. Platina diminishes the 

 colour of gold, unless it be in very small 

 quantity. When the proportion of pluti- 

 na is above _I T , the colour of the gold be- 

 gins to be altered. There is no percep- 

 tible change in the specific gravity or the 

 ductility of gold from this alloy. 



Platina, on account of its peculiar pro- 

 perties, its infusibility, density and indes- 

 tructibility, could it be obtained insuffi- 

 cient quantity, and at a moderate price, 

 would undoubtedly prove one of the most 

 useful and most important of the metals 

 yet known. The importance and utility 

 of platina, on account of its scarcity, have 

 been hitherto limited to chemical pur- 

 poses ; and for different chemical instru- 

 ments and utensils, it has been found 

 peculiarly appropriate, as there are few 

 chemical agents whose effects it cannot 

 resist. There is indeed little doubt but 

 it might be employed with equal advan- 

 tage in the construction of instruments 

 and utenils, in various arts and manufac- 

 tures. 



PLATING, is the art of covering baser 

 metals with a thin plate of silver, either 

 for use or for ornament. It is said to 

 have been invented by a spur-maker, 

 not for show, but for real utility. Till then 

 the more elegant spurs in common use 

 were made of solid silver ; and from the 

 flexibility of that metal, they were liable 

 to be bent into inconvenient forms by the 

 slightest accident. To remedy this defect, 

 a workman at Birmingham contrived to 

 make the branches of a pair of spurs hol- 

 low, and to fill that hollow with a slender 

 rod of steel or iron. Finding this a great 

 improvement, and being desirous to add 

 cheapness to utility, he continued to make 

 the hollow larger, and of course the iron 

 thicker and thicker, till at last he discov- 

 ered the means of coating an iron spur 

 with silver in such a manner, as to make 

 it equally elegant with those which were 

 made wholly of that metal. The inven- 

 tion was quickly applied to other purpo- 

 ses ; and to numberless utensils, which 

 were formerly made of brass or iron, are 

 now given the strength of these metals 

 and the elegance of silver, for a small ad! 

 ditional expense. The silver plate was 

 formerly made to adhere to the baser 

 metal by means of solder ; which is of two 

 kinds, the soft and the hard, or the tin 

 and silver solders. The former of these 

 consists of tin alone, the latter generally 

 of three parts of silver and one of brass. 

 When a buckle, for instance, is to be pla- 

 ted by means of the soft solder, the ring 



