STEAM AND STEAM-ENGINE.- Steam is the gas or vapour given off by a liquid, when its temperature is raised to such a degree as to cause it to pass into a state of ebullition • but as the vapour of water so given off is the one most commonly in the mechanical arts, attention will be called principally to that particular kind of steam in the ensuing notice.
Steam or vapour in its natural state, like all gases, is transparent and colourless ; its visibility in air being caused by its partial condensa tion, which gives rise to the formation of small vesicles of water, enclosing transparent steam, of such a nature as to refract and absorb light. It seems that the phenomenon of the generation of steam is affected by the following general laws, namely: 1st, that the tempera ture of the ebullition of a liquid is the same at all times, under the same pressure and in a vessel of the same substance : 2nd, that the temperature of the liquid remains constant during the whole period of the ebullition, provided the pressure remain the same, and if a greater quantity of heat be communicated to the vessel containing the liquid at one stage of the process than at another, the only effect will be that a greater quantity of steam will bo evolved : the temperature of the liquid will not be raised : 3rd, that the volume of the steam will be much greater than that of the liquid furnishing it, and that the volume of the vapour of water, for instance, will be about 1700 times greater than that of the water itself. The temperatures at which particular substances pass into steam are very different, it may bo added; thus whilst water boils at 212°, ether boils at 93'6°; con centrated sulphuric acid, at 617'; mercury, at 630'; sulphur, at 752°, &c. Even the most refractory metals when properly treated can be made to pass into the state of vapour, and there are others which pass into the gaseous state without passing through the liquid form, as for instance camphor, arsenic, iodine, &c., under which circumstances they are said to sublime ; whilst liquids occasionally pass into the state of vapour at temperatures below those required for ebullition, and they are then said to evaporate. Water evaporates at all temperatures; ice, and snow, sublime in dry winds.
The law with respect to ebullition ascertained by Dalton, namely, "that the elastic force of the vapour given ont during the process is equal to the exterior pressure," allows us to calculate the former from the temperature at which ebullition takes place, if we possess tables showing the elastic force of steam at the different temperatures; and it is upon this principle that hypsometrie observations with respect to the altitudes of mountains are made, by ascertaining the boiling point of water in open vessels. Very great precautions are requisite in
observations of this description, but with the aid of the hypsometric thermometer of M. Regnault, very useful checks upon the other modes of ascertaining altitudes may be obtained. Amongst other essential points to be taken into occount in these observations the nature of the vase must not be neglected, for Gay-Lussac observed that water required a greater degree of heat to pass into a state of ebullition when it was boiled in a glass vase than when it was boiled In a metal one ; and it is well known that when the vase is covered with a material with which water may have less cohesion than it has with the metal, the boiling point may be lowered. Thus, if the boiling point of water in a metal vase be at any place 212°, it would usually be 213'496 in a glass vase; but if the glass be covered by a coating of shell lac, the boiling point would become about 211'464. It also appears that the quantity of air diffused through the water has a considerable effect on the formation of steam ; for unless there be air in dissolution, the cohesion of the particles of the water will oppose the disengage ment of steam, excepting upon the surface. Galy Cazalat found indeed that, by excluding air from water exposed to heat, it was possible to raise the temperature to about 253°'4 without ebullition, but that at that temperature part of the water flashed into steam with a violent explosion. There are some curious facts connected with the spheroidal condition of water in contact with incandescent materials which are worthy of the attention of the physical philosopher ; but as they have not hitherto led to any practical results, the reader is referred, for a view of the present state of our knowledge on this subject, to the works of Boutigny, Pouillet, Poggendorff, Perkins and Faraday.