STEAM is the name generally given to the visible vapour which is driven off from fluids or moist bodies by heat. It is most frequently applied, however, to denote aqueous vapour, or the vapour raised from wa ter by ebullition.
When water, exposed to the pressure of the atmos phere, is heated to the temperature of 212°, globules of steam, composed of heat and water in a state of combination, are formed at the bottom of the vessel, and rising through the fluid, may be collected at its surface. In its perfect state it is transparent, and consequently invisible, but when it has been deprived of a part of its heat by coming in contact with cold air, it becomes vesicular and of a cloudy appearance, as when it issues from a tea-kettle.
By increasing the heat, the temperature of the wa ter never rises above 212°, nor that of the steam which is generated; the only effect being a more copi ous production of vapour. But if the water is confined in a strong copper vessel, both it and the steam which is produced may be brought to any temperature.
Like all gaseous fluids, steam is highly elastic; but if it is separated from the fluid from which it is gen erated, it does not possess a greater elastic force than the same quantity of air. If, for instance, a copper vessel is filled with steam only, at 212°, it. may be brought even to the temperature of red heat, without any danger of bursting; but if water is also in the vessel, each additional quantity of caloric causes a fresh quantity or steam to rise, which adds its elas tic force to that of the steam already generated till the constantly accumulating force bursts the vessel in pieces.
The latent heat of steam,* according to the experi ments of different philosophers, is given in the follow ing table: The mean of these results is 950° agreeing with the measure obtained by Mr. Watt.
Since steam therefore of the temperature of 212° contains 900° of heat, which is not detected by the thermometer, while it retains the gaseous state, its real quantity of heat will be conse quently, if we mix a quantity o' steam with 51 times its weight of water, at 32°, the temperature of the water will rise nearly to the temperature of ebulli tion, because 51x32°--I-32°,-_-208°. Hence the great utility of steam not only in manufactures where great quantities of hot water are required, but also for heat ing large buildings, and for drying whatever is liable to combustion.
The elasticity of steam, arising, no doubt, from the great quantity of heat which it contains, is very great, and from its extensive application as an impel ling power, it has been investigated with considerable attention.
Mr. Watt was the first philosopher who made any accurate experiments on the elasticity of steam. The
following account of them which Mr. Watt drew up at the request of the editor of this work, for his edi tion of Dr. Robison's System of Mechanical Philo sophy, is as follows:— " In the winter made experiments at Glas gow on the subject, in the course of my endeavours to improve the steam-engine, and as I did not then think of any simple method of trying the elasticities of steam at temperatures less than that. of boiling wa ter, and had at hand a digester by which the elastici ties at greater heats could be tried, I considered that, by establishing the ratios in which they proceeded, the elasticities at lower heats might be found nearly enough for my purpose. I therefore fitted a ther mometer to the digester with its bulb in the inside, placed a small cistern with mercury also within the digester, fixed a small barometer tube with its end in the mercury, and left the upper end open. I then made the digester boil for some time, the steam issu ing at the safety-valve, until the air contained in the digester was supposed to be expelled. The safety valve being shut, the steam acted upon the surface of the mercury in the cistern, and made it rise in the tube. When it reached to 15 inches above the sur face of the mercury in the cistern, the heat was and at 30 inches above that surface, the heat was Here I was obliged to stop, as l had no tube longer than 34 inches, and there was no white glass made nearer than Newcastle-upon-Tyne. I therefore seal ed the upper end of the tube y was empty, and when it was cool immersed the low er end in the mercury, which now could only rise in the tube by compressing the air it contained. The tube was somewhat conical; but by ascertaining how much it was so, and making allowances accordingly, the following points were fou•td, which, though not exact, were tolerably near for an appercu. At 291 inches (with the sealed tube) the heat was at 751 inches the heat was 26,4°, and at 1101 inches 292°. (That is, after making allowances for the pillar of mercury supported, and the pillar which would be necessary to compress the air into the space which it occupied, these were the results.) From these ele ments I laid down a curve, in hich the abscissa re presented the temperatures, and the ordinates the pressures, and thereby found the law by which they were governed sufficiently near for my then purpose. It was not till the years 1773-4, that I found leisure to make further experiments on this subject, of which, though 1 do not consider the results as accu rate, I shall give an account here, as they were in point of date prior to any other that I was then acquainted with.