STEAM. 1. The vapour of water ; or the elastic aeriform fluid generated by heating water to the boiling point. When water in an open vessel is heated to the temperature of 212°, or to the boiling point, globules of steam are formed at the bottom, and rise to the surface ; and the continued application of heat, even though increased indefinitely, will only cause a more copious and rapid formation of steam, and will finally eva porate the whole of the water, without raising the temperature of either. In this case, all the heat which enters into the water is solely employed in convert ing it into steam of the temperature of boiling water. But if the water be con fined in a strong close vessel, both it and the steam which it produces may be brought to any temperature ; and as steam at 212° occupies nearly 1700 times the space of the water from it is generated, it follows that, when thus confined, it must exercise an enormous elastic or expansive force ; which may also be shown to be proportional to its temperature. When the temperature is considerably above 212°, the steam formed under such circumstances is termed high pressure steam ; at 212° it is termed low pressure steam, and its pres sure is equal to that of the atmosphere, or 15 lbs. on the square inch. Steam in its perfect state is transparent, and con stantly invisible ; but when it has been deprived of part of its heat by coming into contact with cold air, it suddenly assumes a cloudy appearance, and is con densed into water. Hence appears anoth er important property of steam, its con densibility ; so that whenever cold is applied to it, it suddenly returns to the liquid state, and thus can be employed to produce a vacuum. From the pro-. perties above briefly adverted to, steam constitutes an invaluable agent for the production of mechanical force, as ex emplified in the vast and multiplied uses of the steam-engine. Steam is also em ployed as an agent in distributing the heat used for warming buildings, in heat ing baths, evaporating solutions, distill ing, brewing, drying, dyeing and even for domestic cookery. It is also the means of extracting wholesome and nutri tious food from most unpromising and unpalatable substances. 2. In popular usage, the visible moist vapour which rises from water, and from all moist and liquid bodies, when subjected to the ac tion of heat; as the steam of boiling water, of malt, of a tan-bed, &e. This is properly water in a minute state of subdivision arising from the condensa tion of steam.
History of Steam. The nature and properties of steam were altogether un known to the ancients. Some accounts have come down to us bearing a very early date of engines ; such, for exam plc, as that proposed by Hero of AlexanN dria, in which the mechanical agency of steam was more or less used without any correct notion of its mode of action. Even at a much more recent period the effects produced by steam were ascribed, not to the vapor of water, but to the force of air which was supposed to be expelled from water by heat. In -the beginning of the 17th century, De Cans proposed the construction of a machine by which a column of water was raised by the elastic force of steam. About the middle of the same century, Lord Wor cester published the description of a high pressure steam-engine, which has since formed so remarkable a feature in all histories of that machine. Towards
the latter end of that century, however, the actual properties of vapor began to be gradually unfolded. In 1683, Sir Samuel Moreland published a description of the force of steam, in which he as signed very nearly the exact numerical proportion in which water increases its volume when evaporated under the pres sure of a single atmosphere. A few years after, Papin discovered the method of producing a vacuum by the condensa tion of steam, and the circumstances attending its condensation, became grad ually better understood, having been ap plied to mechanical purposes by Savery, Newcomen, and others. About the mid dlo of the 18th century, the celebrated Watt applied himself to the improve ment of the steam-engine, and by various experiments determined the relative vol umes of steam as commonly used in steam-engines, and the quantity of heat absorbed in evaporation and evolved in condensation. About the same period Dr. Black was engaged in his well-known investigations respecting the phenomena of heat, and had discovered the pheno mena and found the theory of latent heat, which served to explain the effects which Watt had also observed. The re lation between the temperatures and pressures of the vapor of water was determined by Dalton, and confirmed by Gay-Lussac, Prof. Robison, Ure, South ern, and others. The discovery of the law in virtue of which the pressure of all gases and vapors increases in propor tion to their density at a given tempera ture was due to Mariotte, and is known as Mariotte's law. The discovery of the remarkable fact that all gases and vapors receive the same increase of pressure or volume for each degree of temperature, was first discovered by Dalton ; but was immediately afterwards discovered also by Gay-Lussac, who was not informed of Dalton's proceedings. The most im portant course of experiments which has since been made were undertaken by a committee of the French Institute, con sisting of MM. Prony, Arago, Gerard, and Dulong, in consequence of an appli cation from the French government to the academy to point out the best means of preventing accidents from the burst ing of the boilers of steam-engines. The experiments were conducted chiefly by Arago and Dulong, and were certainly not only the most delicate as to their management, but the most hazardous which science and art owe to the courage and zeal of philosophers. Steam was produced of a sufficient pressure to force a column of mercury up a glass tube to the height of nearly 43 feet ; an atmos phere being measured by a column of mercury measuring 29•922 inches. Steam has been made use of lately in many no vel modes as an effective agent in the arts, a remarkable employment of steam is in distilling substances at a low tem perature, which require a high tempera ture without its use. Turpentine, vine gar, and charcoal may he mentioned. The following table exhibits the temperatures and corresponding pressures of steam as determined by these experiments, up to fifty atmospheres.