PRUSSIAN BROWN.--An artists’ colour known by this name is prepared from Prussian blue, but as it has no superiority over Vandyke brown or umber, and is higher priced, it is not in general use. It consists essentially of carbon and ferric oxide, resembles bistre in tone, and possesses durability and good covering powers. The operation of calcining the Prussian blue should be conducted slowly, and is best performed in a closed vessel, though it may also be done in the open. The pieces of blue should not be larger than a hazel nut. They soon split, scale off, and become red, when the heating should cease. On breaking the cooled particles they will show a patchy coloration varying from yellowish-brown to black. On grinding, the mass assumes the desired brown hue. RUBENS BROWN.--Another name for Cassel brown or Cologne earth (_q. v._). SEPIA.--This is one of the few pigments derived from the animal kingdom. It is produced by several sea-inhabiting creatures belonging to the class called _Cephalopoda_, and more particularly by two members of the genus _Sepia_, known respectively as _Sepia officinalis_ and _Sepia loligo_. A peculiarity of these cephalopods is that they are provided with what is commonly called an ink bag, in other words a gland or sac filled with a blackish-brown liquid, which possesses intense colouring power. The object of the secretion is the protection of the creature from pursuit by its enemies, a portion of fluid being discharged at will, and so obscuring the surrounding water that escape is facilitated. For the sake of this pigment the cuttle-fish are sought after by fishermen in the localities frequented by the animals, notably in the warm waters of the Mediterranean. When the creatures are captured, their glands are carefully extracted and sun-dried so as to solidify the contents. In this state ink bags are sent into commerce. The colourman subjects the sacs to boiling in a solution of soda or potash, whereby the colour is dissolved out of the receptacle, and being filtered clear of all fragments of the animal tissue, is next precipitated by the addition of acid, collected on a filter, washed, and dried. It then forms an exceedingly useful pigment, having, according to Prout, the following average composition:-- Per cent. Black pigment (melanin) about 78 Calcium carbonate 10½ Magnesium carbonate 7 Alkaline chlorides and sulphates 2 Organic matter 1