We recommended the precaution of slackening the fire a little at the beginning of the operation, only to prevent the Lead from being too hastily converted into litharge, lest it should penetrate and corrode the test before it had wholly dissolved the ore: but if we were perfectly certain of the vessel's being so good as to be in no danger of penetration by the Lead, this precaution would be needless. It is proper to add eight parts of Lead for one of ore; though so much is not always absolutely necessary, especially when the ore is very fusible. The success of this operation depends chiefly on the completeness of the Scorification; and therefore the addition of more Lead than enough is attended with no inconvenience: for, as it always promotes the Scorification, it can never do any harm. If the ore be mixed with such earthy and stony parts as cannot be separated from it by washing, it is the more difficult of fusion, even though the stones should be such as are most disposed to vitrify; because the most fusible earths and stones are always less so than most metallic substances. In that case it will be necessary, for effecting the Scorification, to mix thoroughly with the pulverized ore an equal quantity of Glass of Lead, to add twelve times as much granulated Lead, and then to proceed as directed for a fusible ore; exposing the mixture to a degree of fire strong enough, and long enough kept up, to give the scoria all the properties above required as signs of a perfect scorification. Silver ore is sometimes mixed with Pyrites, and the ore of Arsenic, or Cobalt, which also make it refractory. As the Pyrites contain a large quantity of Sulphur, which is very volatile as well as Arsenic; in this case it is proper to begin with freeing the ore from these two extraneous substances. This is easily done by roasting: only be sure, when you first expose the ore to the heat, to cover the vessel in which you roast it, for some minutes, with an inverted vessel of the same width; because such sorts of ore are very apt to fly when they first feel the heat. After this uncover it, and leave it exposed to the fire till no more sulphureous or arsenical matters rise. Then mix it with the same quantity of Glass of Lead as we ordered for ores rendered refractory by the admixture of earths or stones, and proceed in the same manner. It is the more necessary to roast Silver ore infected with Sulphur and Arsenic, because, as Sulphur obstructs the fusion of Lead, it cannot but do hurt, and protract the operation; and Arsenic does mischief, on the other hand, by scorifying a very great quantity of Lead too hastily.