Being nothing more nor less than animal charcoal, ivory or bone black had best not be compounded with organic pigments, in water at least. It is well known that this charcoal possesses the singular property of completely absorbing the colour of almost any vegetal or animal solution, and of rendering quite limpid and colourless the water charged with it. If a solution of indigo in concentrated sulphuric acid be diluted with water, and animal charcoal added in sufficient quantity, the solution will soon be deprived of colour. The more perfect the ivory or bone black, the more powerful is its action likely to be: either over or under calcined, animal charcoal is less energetic; in the former case, because it is less porous; in the latter, because the animal matter, not being wholly consumed, makes a kind of varnish in the charcoal which interferes with its acting. To a greater or less extent, gums, oils, and varnishes serve similarly as preventives, thereby decreasing the danger of employing these blacks in admixture; but, in the compounding of colours, nothing is gained by needless risk. To mix with organic pigments, therefore, blue or lamp blacks should be substituted for those of ivory or bone; that is, vegetal charcoal should be used instead of animal. It is a question whether even with inorganic pigments the adoption of the former in admixture would not be advisable. It was once the general opinion that the action of animal charcoal was limited to bodies of organic origin, but it has since been found that inorganic matters are likewise influenced. "Through its agency," says Graham, "even the iodine is separated from iodide of potassium;" whence probably pigments containing iodine would suffer by contact. The investigation of Weppen appears to prove that the action of the charcoal extends to all metallic salts; with the following, no doubt remains of this being so, to wit:--the sulphates of copper, zinc, chromium, and protoxide of iron; the nitrates of lead, nickel, silver, cobalt, suboxide and oxide of mercury; the protochlorides of tin and mercury; the acetates of lead and sesquioxide of iron; and the tartrate of antimony. Whether animal charcoal exercises any deleterious influence on pigments consisting of these metals, and, if so, how far and under what circumstances, can only be answered when our knowledge of the properties of pigments is greater than it now is. At present, perhaps, it is safer to choose vegetal charcoal for mixed tints, inasmuch as, although it shares the property of bleaching in a certain degree, it does not possess the same energy. 294. LAMP BLACK,