18 
MR. FARADAY ON THE 
and being careful that no reflection from illumined objects, as the hands, on 
to that side towards the face shall give deceptive indications. In the marking, 
too, the indicating spot should always be made at a certain distance from the 
hole, as the fourth or the third of an inch, and on the same plate constantly in 
the same direction or towards the same edge ; the holes are then easily found 
again, and the mark remains during the soldering to guide the operator. 
32. The holes discovered by these examinations are to be closed by little 
patches of platina soldered with gold ; for gold, like platina, may be safely 
used in these experiments, when reducing matter is absent. The gold has been 
used in the finely divided state in which it is obtained by precipitation from its 
solutions by means of sulphate of iron, but it must be washed perfectly pure : 
the patches are formed by cutting a piece of clean new platina foil into small 
square or rectangular plates : a sufficient heat can usually be obtained by the 
use of the spirit lamp and mouth blowpipe. In the process of soldering, a 
little of the powdered gold is heaped upon the hole and slightly flattened by 
some clean instrument, the spirit lamp is applied underneath for a moment, 
which causes the gold to adhere slightly, a selected patch of platina is laid 
delicately upon the gold, and then the heat of the spirit lamp, urged by the 
blowpipe, is directed beneath against the place. Usually the gold will melt 
and run instantly, the platina patch will come into close contact with the plate, 
and the operation will be completed. If well done, the fused gold will appear 
all the way round in the minute angle formed by the edge of the patch, 
and also faintly at the hole on the opposite side of the plate. 
33. Sometimes when the patch is large, or in the middle of a plate, the heat 
obtained as above is hardly sufficient to melt the gold freely and cause perfect 
adhesion. In such cases, a single or double piece of platina foil loosely laid 
over the part, prevents loss of heat from the upper surface, and frequently 
causes such increased elevation of temperature as to render the soldering 
perfect and effectual. In the few cases where this expedient has not succeeded, 
I have resorted to the oxyalcohol blowpipe, using a small bladder of oxygen 
with a little attached jet for the purpose. This has never failed to produce an 
effectual heat, and 15 or 20 cubical inches of oxygen are sufficient for many 
operations. 
34. This application of patches and soldering is only secure for small holes. 
