Probably the fact that white lead possesses the body it has is the reason why it has been so largely used, and why so many paintings which have been painted with it have come to a most untimely end. Prof. Barff says he knows of no pigment so liable to change of colour as white lead. In saying this he expects that there are many who will not agree with him. They know that white lead works well and easily, and they like it because it covers down well; but then he points out some of the great defects under which it labours. If you take some oil, and if to that you add lime-water, the oil will mix with the lime-water, and form a kind of emulsion. Again, if you boil oil or fat with soda, a kind of soap is formed, and the process of manufacturing soap is termed the process of saponification. Now if, instead of boiling fat with soda or alkali, we boil it with plumbic oxide or oxide of lead, we shall form a soap, and that soap goes by the name, amongst medical men, of _emplastrum plumbi_, or lead plaster. This is a substance made by the saponification of oil with the oxide of lead. Because this oxide and carbonate of lead have the power of saponifying oils, you get in white lead that peculiarly smooth easy working which you do not get with any other white pigment; and it is on this account, for one reason, that it is liked by artists and painters. Taking a piece of paper coated with some of this lead plaster, if you throw a light upon it, you will see that the substance is semi-transparent. This is a peculiarity of lead that it will saponify and form this sort of transparent substance. The famous landscape painter, Mr. Wilson, made an addition to a room in his house. The old part of the room had been painted a dark colour; the new part, of course, when it left the workman’s hands, was perfectly white, and therefore the painters painted down the dark colour with white lead, until the whole room displayed one uniform tint. After a while, however, it was found that the part which was originally painted dark became dark again; the dark paint, in fact, showed through the white lead. Sometimes, possibly, when an artist wishes to put in figures upon a dark background that he has painted, he uses white lead, and the figures will stand out well and brilliantly at first, but after a time the dark colour upon which they are painted strikes through the lead, and the figures of course recede. Now, this striking through is owing to a slight process of saponification, no doubt owing to an interchange between the carbonic acid of the lead carbonate and the stearic and oleic acids of the oil with which the lead is mixed; so that, in time, the white lead, which has a body which makes it so great a favourite with artists, loses that body, and becomes a transparent or semi-transparent substance, something like lead plaster. Here is a reason why white lead should not be used unless the ground has previously been brought to a light colour.