The purification of Silver by Nitre, as well as the process for refining it on the cupel, is founded on the property which this metal possesses of resisting the force of the strongest fire, and the power of the most active solvents, without losing its phlogiston. The difference between these two operations consists wholly in the substances made use of to procure the scorification of the imperfect metals, or semi-metals, that may be combined with the Silver. In the former this was obtained by Lead, and here it is effected by Nitre. This Salt, as we have shewn, hath the property of calcining and quickly destroying all metallic substances, by consuming their phlogiston, except the perfect metals, Gold and Silver, which alone are able to resist its force. This method may therefore be employed to purify Gold as well as Silver, or indeed both the two mixed together. In this operation the Nitre is gradually alkalizated, as its Acid is consumed with the phlogiston of the metallic substances. The Alkaline Salt and pounded glass are added, with a view to promote the fusion of the metalline calces, as fast as they are formed, and to fix and retain the Nitre, which, as we shall presently see, is apt to fly off in a certain degree of heat. The precaution of covering the crucible with another crucible inverted, which hath only a small hole in its bottom, is designed to prevent any of the Silver from being lost in the operation: for when the Nitre comes to be acted on by a certain degree of heat, and especially when it deflagrates with any inflammable matter, part of it flies off, and so rapidly too as to be capable of carrying off with it a good deal of the Silver. The little hole left in the covering crucible is necessary for giving vent to the vapours, which rise during the deflagration of the Nitre, as they would otherwise open themselves a passage by bursting the vessels. After the operation this vent-hole is found beset with many little particles of Silver, which would have been lost if the crucible had not been covered. If you should observe, during the detonation of the Nitre, that a great many vapours issue through the vent-hole with a considerable hissing noise, even without applying the coal, you must take it for a sign that the fire is too brisk, and accordingly check it; else a great deal of the Nitre will be dissipated, and with it much Silver. You must observe to take the Silver out of the fire as soon as it is in fusion: for if you neglect this, the Nitre being entirely dissipated or alkalizated, the calces of the metals destroyed by it may possibly recover a little phlogiston, communicated either by the vapours of the charcoal, or by little bits of coal accidentally falling into the crucible; by which means some portion of those metals being reduced will mix again with the Silver, prevent its having the desired degree of purity and ductility, and oblige you to begin the operation afresh. PROCESS IV.