Page:Experimental researches in chemistry and.djvu/264

1829.] formed by the adhesion of a particle of dirt or grit), it should be examined, and removed by the point of a knife, if necessary, and its place also marked. All these places and the whole surface of the plate should then be examined for holes by a still stronger test, namely, by holding the sheet of metal before and close to a bright light, as a candle or lamp, in a dark room, and every hole observed, marked. In making this examination, it must be done carefully and minutely, holding the plate in different directions to the light (for sometimes the holes are oblique), and being careful that no reflexion from illumined objects, as the hands, on 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 platinum soldered with gold; for gold, like platinum, 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 platinum foil into small square or rectangular plates: a sufficient heat can usually be obtained by the use of the spirit lamp and mouth blowpipe. ln 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 platinum 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 platinum 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