There is a principle underlying many of the phenomena of dry silver assaying which the student should endeavour to understand; and which serves to emphasise and explain some facts which without an explanation may present difficulties. If a button of melted lead be covered with a layer of slag rich in oxide of lead, and a second metal be added, this other metal distributes itself between the metal and slag in proportions which depend mainly upon the ease with which it is oxidised, and to a large extent upon the relative quantities of material present. Easily oxidisable metals such as zinc, iron, antimony and tin, will go mainly into the slag, and, if the proportion of the slag is large, very little will go into the metal. On the other hand, with metals oxidisable with difficulty, such as silver, gold, and platinum, the reverse holds true; nearly the whole of the metals will go into the lead, and very little into the slag. If, however, the slag be very rich, say in antimony, the lead will contain antimony; and, on the other hand, if the lead be very rich in silver, the slag will contain silver in appreciable quantity. Copper, which is near lead in the facility with which it is oxidised, will serve for the purpose of a detailed example. The results of actual analyses of metal and slag formed in contact with each other are shown in the following table:-- ---------------------------+-------------------------- Percentage Composition | Percentage Composition of the Metal. | of the Slag. ----------+----------------+-----------+-------------- Lead. | Copper. | Lead. | Copper. 6.8 | 93.2 | 71.4 | 21.4 20.0 | 80.0 | 78.0 | 17.0 28.0 | 72.0 | 80.0 | 12.5 32.0 | 68.0 | 86.0 | 6.7 85.0 | 15.0 | 90.0 | 3.6 ----------+----------------+-----------+-------------- It will be seen from this table that the slag is always much richer in lead and poorer in copper than the metal with which it is in contact. The ratio of lead to copper in these five samples is:-- In the Metal. In the Slag. 1 : 14 1 : 0.3 1 : 4 1 : 0.2 1 : 2.5 1 : 0.16 1 : 2 1 : 0.08 1 : 0.16 1 : 0.04