There is another objection to the use of white lead, and really a very valid one it is. Persons go on year after year laying out sums of money for having their houses painted with white lead, when other pigments which will keep their colour might well be employed. A house painted with white lead after some time darkens in tint considerably; the colour is changed by some influence that is acting upon it through the air, and that influence is sulphuretted hydrogen gas. If you paint with white lead doors placed near a drain from which this gas escapes, those doors will become browned and blackened. White lead is very often, particularly that procured at ordinary shops, adulterated with a substance called sulphate of baryta, or, commonly, barytes. This is much more transparent when ground with oil than white lead itself, and it will materially impair that property for which white lead is valued, viz. that of covering down well and solidly. White lead adulterated with barytes has, generally speaking, a bluish sort of look; it is semi-transparent. It has not that opacity that pure white lead has. If you take a small piece of white lead and put it into a test-tube, and add to it a little nitric acid, or aquafortis, and some water, if the lead is pure the whole of it will dissolve in the liquid, and you will have a pure solution. If it does not dissolve there is a white precipitate, which will fall down to the bottom of the tube, and that precipitate is sulphate of baryta. Sulphate of baryta is insoluble in aquafortis, but carbonate of lead, and most lead salts, are soluble in it. There is another excellent test for the purity of white lead, which is this. If you take a small portion, and grind it up with a little carbonate of soda into a small pellet about the size of a pea, and then put it upon a piece of charcoal and hold it in the middle flame of a blow-pipe for some short time, the sulphate of baryta becomes decomposed, and you get sulphide of sodium formed. If this sulphide of sodium be acted upon by an acid liquid, sulphuretted hydrogen is given off, which could not be formed from carbonate of lead, for in it there is no sulphur at all; and inasmuch as sulphate of baryta is the impurity for which we have to look, the presence of sulphide after this treatment indicates that it was with the white lead which was examined. LIME WHITE.--A name sometimes given to the white pigment prepared from sulphate of barium. See baryta white, p. 170. LITHOPHONE.--This is a fancy title for one of the several varieties of white pigment having the metal zinc as a basis, and described under zinc whites on p. 247.