Page:Popular Science Monthly Volume 9.djvu/578

552 whole is covered with a glass lid. A cylindrical reflector of silver directs the sun's rays upon the apparatus. With this marmite it takes less than four hours to prepare an excellent pot-au-feu, consisting of one kilogramme of beef and a quantity of vegetables, the whole being perfectly cooked, and very palatable, owing to the fact of the heat being applied with great regularity.

In this form of marmite, now superseded by a simple glass vessel fixed at the focus of a conical mirror of silver-plated brass, fruits, potatoes, all sorts of legumes, meats, and grains, are cooked by solar heat. So, too, an infusion of tea or coffee can be readily prepared, and for this purpose we may employ one of those bottles of colored glass in which Lyons beer is put up. To cook legumes or grains rapidly, a different course may be taken. A closed vessel containing water is set in the focus of the reflector, and, when the liquid begins to boil, the upper portion of the vessel is connected by a tube with the bottom of another containing the legumes or grains, which are quickly cooked by the steam.

To transform the marmite into an oven, a disk of wrought-iron is placed beneath the glass lid, and in less than three hours a kilogramme of bread is baked. The crust is hard and brown, and the pith light and well raised, as with bread baked in an ordinary oven.

The roasting of meat, not requiring the same amount of heat as does the vaporization of an equal weight of water, can be performed in the open air, by the action of the solar reflector alone, the piece of beef, veal, or mutton being fixed upon a spit. In less than an hour we have in this way a very fine roast. The use of butter must be avoided, lest the chemical rays, by transforming the butter into butyric acid, should spoil the flavor of the meat. By interposing a pane of green or red glass we can intercept the chemical rays which cause this fermentation, and then the result leaves nothing to be desired.

By substituting for the two lids of the solar marmite an alembic-head, the apparatus can be used for the purposes of distillation. To this end the alembic-head is connected, by an horizontal tube, with a worm, the latter descending in the form of a helix and dipping into a constant current of cold water, while the metallic vessel, containing two litres of wine, is inclosed in the glass cylinder and set in the focus of the reflector. The alcohol is collected after forty minutes of exposure to the sun. Inasmuch as the apparatus grows hot slowly and continuously, the alcohol is highly concentrated and possesses a very agreeable aroma.

In all the foregoing experiments, M. Mouchot at first used concave silver mirrors, cylindro-parabolic in form, i. e., cylindrical mirrors whose base-line is an open curve resembling a parabola. The reflecting power of cylindrical mirrors increases in proportion to their aperture, and hence the time required, for instance, to boil a litre of water is inversely as the aperture of the mirror, i. e., the greater the aperture