ENCAUSTIC PAINTING is a kind of painting in which, by heating or burning in, the colours were rendered permanent in all their original splendour. It was not, how ever enamelling, but a mode of painting with heated or burnt wax, which was practised by the ancients. Pliny describes three modes of encaustic painting. In the first mode, the wax was melted, mixed with as much earth colour finely powdered as it could imbibe, and then this mass spread on wood, or on a wall, with a hot spatula. When it became cold it was the ground, in which the designer cut the lines with a cold pointed tool (style, cestrurn). In the second mode, ivory tablets were covered with red or black wax, and the design cut into it with the style, the object being to use the clear and smooth surface of the ivory for the lines, that they might look the more beautiful. The third kind was the applying the colours with the pencil ; the wax was dissolved, the colours mixed with it, and laid on with the pencil, and the painting then finished by care ful approximation to the fire : for this purpose a hot iron (cauterium) was used. When painting had been greatly improved by the invention of the pencil, a new method of en caustic was attempted. Encaustic wax paint ing had hitherto been designing on a coloured ground ; it now became painting with wax colours burnt in. When the artist had laid on the wax ground, and traced the outlines with the style, he proceeded to the colouring. From the wax mixed with the colours he sepa rated with the hot style as much as he wanted to cover a certain space, and spread it over the ground, put a second, third, &c., colour next the first, so that he had local tint, half tint, and shade together, which he softened into each other with the hot style.
This art, having been long lost, was revived by French and German artists in the 18th century, and is now occasionally practised. ENDOSMOSE and EXOSMOSE. These names are given to a remarkable filtering pro cess which takes place through membranes.
Endosmose is the attraction through an animal or vegetable membrane of thin fluid by a den ser fluid. M. Dutrochet found that if he filled the swimming bladder of a carp with thin mu cilage and placed it in water, the bladder gained weight by.attracting water through its sides : to this phenomenon he gave the name of Endosmose. He also found that if he filled the same bladder with water and placed it in thin mucilage, it lest weight, its contents being partially attracted through its sides into the surrounding mucilage : this counter-phe nomenon he named E X0S77103C. The same circumstances occur in the transmission of fluids through the tissue of plants. The parts of vegetables may be gorged with fluid by merely placing them in water, and may be emptied again by rendering the fluid in which they are placed more dense than that which they contain. This phenomenon takes place with considerable force. Water thickened with sugar in the proportion of 1 sugar to 2 water, is productive of a power of endosmose capable of sustaining a column of mercury of 127 inches, or the weight of 41 atmospheres.
Dutrochet considers endosmose to be owing to what he calls intercapillary electricity, grounding his opinion partly upon the experi ment of Porret, who found that when two liquids of different levels are separated by a membrane, they may be brought to a level by establishing an electrical current between the two, thus rendering the membrane permeable ; and partly upon experiments of his own. But M. Poisson, on the contrary, has demon strated that endosmose may be the result of capillary attraction joined to differences in the affinity of heterogeneous substances.
A few applications of this principle have been made in the arts ; but it still remains chiefly in the domain of science.