If hot-air engines and inflammable gas engines fail as yet to furnish power comparable to .hat which steam affords, without a very disproportionate increase of bulk, and for high powers fail to furnish it at all, the same objection will not hold in regard to\ho new motors now beginning to make their appearance, in which the motive power is derived from ammoniacal gas. The gas, which is an incidental and abundant product in certain manufactures, especially that of coal gas, and which makes its appearance in the destructive distillation of all animal substances, is found in commerce chiefly in the form of the aqueous solution. It is the most soluble in water of all known gases, being absorbed, at the temperature of freezing, to the extent of more"than a thousand volumes of gas to one of water; and at the temperature of 50° Fah., of more than eight hundred to one. What is most remarkable in regard to this property is, that, at low temperatures, the solution is sensibly instantaneous. This may be strikingly illustrated by transferring a bell-glass filled with the gas to a vessel containing water, and managing the transfer so that the water may not eome into contact with the gas until after the mouth of the bell is fully submerged. The water will enter the bell with a v iolent rush, precisely as into a vacuum, and if the gas be quite free from mixture wit 1 any other gas insoluble in water, the bell will inevitably be broken. The presence of a bubble of air may break the force of the shock and save the bell. This gas cannot, of course, be collected over water. In the experiment just described, the bell is filled by means of a pneumatic trough containing mercury. It is transferred by passing beneath it a shallow vessel, which takes up not only the bell-glass but also a sufficient quantity of mercury to keep tha gas im fJ risaned until the Arrangements for the experiment ' are completed. The extreme solubility of ammoniacal gis is, therefore, a property of which advantage may ba taken for creating a vacuum, exactly as the same object is accomplished by the condensation of steam. As, on the other hand, the pressure which it is capable of exerting at given temperatures is much higher than that which steam affords at the same temperatures; and as, conversely, this gas requires a temperature considerably lower to produce a given pressure than is required by steam, it seems to possess a combination of properties favorable to the production of an economical motive power. Ammonia, like several other of the gases called permanent, may be liquefied by cold and pressure. At a temperature of— 38°'5 C., it becomes liquid at the pressure of the atmosphere. At the boiling point of water it requires more than sixty-one atmospheres of pressure to reduce it to liquefaction. The same effect is produced at the freezing point of water by a pressure of five atmospheres, at 21° C. (70° Fah.) by a pressure of nine, and at 88° C. (100° Fah.) by a pressure of fourteen. If a refrigerator could be created having a constant temperature of 0° C., or lower, liquid ammonia would furnish a motive power of great energy, without the use of any artificial heat. The heat necessary to its evaporation might be supplied by placing the vessel eontaining it in a water bath, fed, at least during summer, from any natural stream. Such a condenser could not be economically maintained. A con denser at 21° C., however, and an artificial temperature in the boiler of 88° C., would furnish a differential pressure of five atmospheres, with a maximum pressure of fourteen. By carrying the heat as high as.50° C. (122° Fah.), a differential pressure of eleven atmospheres. could be obtained, with an absolute pressure of twenty. These pressures are too high to be desirable or safe. Moreover, condensation is more easily effected by solution than by simple refrigeration, and hence, iI). the ammoniacal gas engines thus far constructed, the motive power has been derived, not from the liquefied gas, but from the aqueous solution. The gas is expelled from the solution by elevation of temperature. At 50° C. (122° Fah.) the pressure of the liberated gas is equal to that of the atmosphere. At 80° C. (176° Fah.) it amounts to five atmospheres, and at 100° C. (212° Fah.) to seven and a half. At lower temperatures the gas is re- dissolved, and the pressure correspondingly reduced. in the ammoniacal engine, therefore, the expulsion and resolution of the gas take the place of vaporization and condensation of vapor in the steam engine. The manner of operation of the two descriptions of machine is indeed so entirely similar, that but for the necessity of providing against the loss of the ammonia, they might be used interchangeably. The ammonia engine can always be worked as a steam engine, and the steam engine can be driven by ammonia, provided the ammonia be permitted to esoape after use. The advantage of the one over the other results from the lower temper- aturetrequired in the case of ammonia to produce a given pressure, or fr8lIl the higher pressure obtainable at a given temperature. These circumstances are favorable to the economical action of the machine in two ways. In the first place, they considerably .diminish the great waste of heat which always takes place in the furnace of every engine driven by heat; the waste—that is, which occurs through the chimney without contributing in any manner to the operation of the machine. This waste will be necessarily greater in proportion as the fire is more strongly urged; and it will be necessary to urge the fire' in proportion as the temperature is higher at which the boiler, or vessel containing the elastic medium which furnishes the power, has to be maintained. In the second place, that' great loss of power to which the steam engine is subject, in Consequence of the high temperature at which the steam is discharged into the air, or into a condenser, is very materially diminished in the engine driven by ^moniacal gas. For instance, steam formed at the temperature of 150° C. (802° Fah.) has a, pressure of nearly five atmospheres (4'8). If worked expansively, its pressure will fall to one atmo sphere, and its temperature to 100° C. (212° Fah.) after an increase of volume as one to four. If, now, it is discharged into a condenser, there is an abrupt fall of temperature of 50°, 60° ', or 70°, without any corresponding advantage. If it is discharged into the air, this heat is just as much thrown away. In point of fact, when steam of five atmospheres is discharged into the air at the pressure of one, considerably more than half the power which it is theoretically capable of exerting is lost; and when, at the same pressure, it is discharged inte a condenser, more than one quarter of the power is in like manner thrown away. And as the expansion given to steam is usually less than is here supposed, the loss habit- n.ally suffered is materially greater. The ammoniacal solution affords a pressure of five atmospheres at 80° C. (176°.Fah.), and in dilating to four times its bulk, if it were a perfectly dry gas, its temperature would fall below 0° C. But as some vapor of water necessarily accompanies it, this is condensed as- the temperature fells .and its latent heat is liberated. The water formed by condensation dissolves also a portion of the gas, and this solution produces additional heat. In this manner an extreme depression of temperature is prevented, but it is practicable, at the same time, to maintain a lower temperature in the eondenser than exists in that of the steam engine. It must be observed, however, that owing to the very low boiling point of the solution it is not generally practicable to reduce the pressure in the condenser below half an atmosphere. The advantages here attributed to ammoniacal gas belong also, more or less, to the vapors of many liquids more volatile than water; as, for instance, ether and chloroform. Engines have therefore been eonstructed in which these vapors have been employed to produce motion by being used alone, or in combination with steam. The economy of using the heat of exhaust steam in vaporizing the more volatile liquid is obvious. But all these vapors are highly inflammable, and in mixture with atmospheric air they are explosive. The dangers attendant on their use are therefore very great. Ammonia is neither inflammable nor explosive, and if, by the rupture of a tube or other accident, the solution should be lost, the engine will still operate with water alone.' :rhe action of ammonia upon brass is injurious; but it preserves iron from corrosion indefinitely. It contributes, therefore, materially to the durability of boilers. A steam engine may be eonverted into an ammonia engine by replacing with iron or steel the parts eonstructed of brass, and by modifying to some extent the apparatus of condensation.