lighted is in common use on some railways, where passengers' foot-warmers are filled with a strong solution of sodium acetate. When sodium acetate is dissolved in water it manifestly exists in the liquid state, and it is presumably present in its anhydrous condition (i.e., not combined with water of crystallisation). The common crystals are solid, and contain 3 molecules of water of crystallisation--also clearly in the solid state. Now, the reaction NaC_2H_3O_2 + 3H_2O = NaC_2H_3O_2.3H_2O (anhydrous acetate) (crystals) evolves 4.37 calories (Berthelot), or 1.46 calorie for each molecule of water; and whereas 1 kilo. of water only evolves 1 large calorie of heat as its temperature falls 1° C., 18 grammes of water (1 gramme-molecule) evolve l.46 large calorie when they enter into combination with anhydrous sodium acetate to assist in forming crystals--and this 1.46 calorie may either be permitted to warm the mass of crystals, or made to do useful work by raising the temperature of some adjacent substance. Sodium acetate crystals dissolve in 3.9 parts by weight of water at 6° C. (43° F.) or in 2.4 parts at 37° C. (99° F.). If, then, a jacket round an acetylene apparatus is filled with a warm solution of sodium acetate crystals in (say) 3 parts by weight of water, the liquid will crystallise when it reaches some temperature between 99° and 43° F.; but when the generator comes into action, the heat liberated will change the mass of crystals into a liquid without raising its sensible temperature to anything like the extent that would happen were the jacket full of simple water. Not being particularly warm to the touch, the liquefied product in the jacket will not lose much heat by radiation, &c., into the surrounding air; but when the water in the generator falls again (after evolution of acetylene ceases) the contents of the jacket will also cool, and finally will begin to crystallise once more, passing a large amount of low-temperature heat into the water of the generator, and safely maintaining it for long periods of time at a temperature suitable for the further evolution of gas. Like the liquid in the seal of an isolated gasholder, the liquid in such a jacket will last indefinitely; and therefore the cost of the sodium acetate in negligible.