Properties purely physical, and therefore much simpler than those studied by the chemist, offer us noteworthy instances of rules that work both ways. For years the walls and doors of safes and bank vaults have been filled with gypsum as a substance all but impervious to heat. To-day Norwegian cooking chests, on much the same principle, are attracting public attention by their economy. A pot is filled with, let us say, the materials for soup, it is brought to a boil, and then placed in a chest thickly clad with a non-conducting coat of felt or even of hay, as illustrated on page 189. In an hour or so a capital soup is found to have cooked itself simply by its own retained heat. A resource long familiar to the builder of safes and strong-boxes is thus taken into household service with much profit. It is plain that whatever obstructs the passing of heat may be employed either to keep it in or keep it out. For years inventors busied themselves in finding non-conductors wherewith to cover steam-pipes and steam-boilers. To-day, in cold storage plants, these non-conductors are just as useful in covering pipes filled with circulating liquids of freezing temperatures. Take a parallel case in the field of physical research. In 1873 Dulong and Petit in their measurement of heat avoided losses of heat with a new approach to perfection by using glass vessels one inside another, with exhausted spaces in between. In 1892 Professor Dewar applied this device to keeping liquefied gases, of extremely low temperatures, from being warmed by surrounding bodies, an aim just the converse of that of Dulong and Petit. Often, as in these cases, the applications of a quality may come in pairs; one invention may suggest its twin. [Illustration: Copyright, Pach Bros., New York. THOMAS ALVA EDISON, 1906. ORANGE, NEW JERSEY.] This convertibility of principle may be observed as clearly in the phenomena of nature as in the creations of ingenuity. Water expands as it freezes; when this expansion takes place freely, the freezing temperature is 0° C., but when expansion is resisted, as when the water is confined in a strong gun-barrel, the freezing temperature is lowered, for now the ice has to do work in the act of crystallization. So with the boiling points of liquids; they rise as atmospheric pressure increases, they fall as atmospheric pressure is reduced. A prospector on Pike’s Peak cannot boil an egg in his kettle. Next day he descends a mine in the valley, to find the boiling point higher than when he built his fire beside the mouth of the mine. Cube Root Easily Found.