APPLICATION OF PLATINUM UPON OTHER METALS. 
115 
itself, and forms an alloy with the interior without covering 
the surface. 
Finally it was attempted to cover some of the metals with 
platinum in the moist way. It is known that a metallic bar 
plunged into a solution of another metal, precipitates this lat- 
ter, if it is the most oxidisable metal, and if the new salt 
formed is soluble. But this metallic deposit takes place in 
an irregular manner, frequently in small plates, in powder, or 
in crystalline grains, and does not apply itself upon upon the 
metal in such a manner as to cover it completely. An en- 
deavor was made to ascertain whether the metal of the solu- 
tion could not be compelled to remain applied to the surface 
of the precipitating metal, and thus cover it completely. 
When a bar of iron or copper is plunged into an ordinary 
acid solution of chloride of platinum, the platinum is precipi- 
tated, but, as a gray powder, and in small plates; it does not 
remain applied to the bar, but soon falls to the bottom of the 
vessel. It is probable that the disengagement of hydrogen 
which takes place at the same time as the precipitation, oc- 
curring between the two metals, detaches the latter as soon 
as it is deposited; moreover, the metal becoming covered 
with a coating of oxide, this is of itself sufficient by its inter- 
position to prevent the adhesion of the metal. 
By modifying in different manners the circumstances of 
this experiment, a method of applying the metal in uniform 
and smooth layers has been obtained. But it is necessary to 
observe many precautions, without which the attempt will be 
imperfect, or fail completely: 1, the solution of platinum 
must be neutral or alkaline; 2, it must be much diluted; 3, it 
should be hot, (about 60 c.;) 4, the contact of the metal (well 
polished) with the solution should be of short duration; 
5, finally, the piece covered with platinum should be imme- 
diately washed in pure water. Without these precautions the 
platinum is precipitated as a black powder, but little adherent, 
which is easily rubbed off, and cannot acquire any polish. 
Of these three processes the last is certainly the most easy, 
the quickest, and the most economical; a solution of platinum, 
