The temperature of ebullition is not affected by the presence of substances in suspension in the liquid considered ; but when those substances are in solution they produce a very distinct effect, and in the proportion that the new ingredient is more or less volatile than the liquid, will the boiling point be lowered or raised. Salts dissolved in water retard its ebullition, and they do so in an increasing ratio until tho point of saturation has been reached, beyond which the boiling point (remains constant ; but it is to be remarked that the precise temperature of ebullition of a saline solution is not to be judged by the degree of heat that solution may temporarily attain ; for it is possible to raise the temperature to a certain degree, but directly the salt begins to be precipitated the temperature may fall. Thus, a solution of the carbonate of potash was observed by Legrand to attain 284' without parting with that substsnee ; but suddenly a great effervescence took place, and the temperature fell to 275°, accompanied by a copious deposition ; after which the temperature remained constant. Df. Legrand gave a table of the boiling points of the different saturated saline solutions, from which the following statement is copied ; the temperatures are of course given in this table, and throughout this article, unless otherwise mentioned, on the Fahrenheit scale.
The boiling point of water being 212', that of concentrated nitric acid will be 186'•8; that of alcohol will be 174°'2; that of oil of turpentine will be 3144; and that of linseed oil 600'48. Sea water, containing A of its own weight of the chloride• of sodium, boils at 213'2 in the open air ; if the proportion of salt be increased, the temperature of ebullition will increase also ; thus, for A of salt it becomes 214'44 ; for it becomes ; for A, 2161 ; for A, 217'9 ; for A, 219' ; for A, 221'44 ; for I?, 223'7 ; for d, or the point of saturation, 226'. It may be interesting to the practical reader here to state, that marine boilers are usually worked with proportions of salt equal to A of the weight of the water ; but that occasionally those proportions are raised as high as A witbout much inconvenience.
For mechanical purposes, it is necessary that steam should exercise a pressure in excess of that of the atmosphere ; and, upon Dalton's law above quoted, this additional pressure, or elastic force, can only be obtained by causing the water to boil under such conditions as shall bring to bear upon its surface an effort equivalent to the one the steam is desired to exercise. It is a matter of the greatest importance in the arts, then, to ascertain the temperature of the water which corresponds with steam of certain pressures, and numerous experiments have been made for this purpose. Messrs. Dulong and Arago, M. Regnault, and
Macquorn Rankine, may be cited as the greatest authorities on the subject ; and the reader who may desire to study this branch of physical science in detail is referred to their works, as also to those of Wollaston, Lavoisier, Laplace, Gay-Lussac, Petit, De Pambour, Lubbock, Mosely, &c. The tables drawn up by Messrs. Arago and Dulong of the elastic force of steam at the different temperatures observed during their experiments, or ascertained by calculation by means of a formula deduced from those experiments when the elastic force of the steam exceeded 39248 lbs. on the superficial inch, are sufficiently accurate for practical purposes, though M. Regnault's later researches are more elaborately correct. Arago and Dulong's table is to be found in Daguin's Trait6 de Physique,' and elsewhere ; the pressure being indicated in atmospheres of 1447 lbs. on the superficial inch, and the temperatures are given on the centigrade scale. Macquorn Rankine, however, gives in his' Treatise on the Steam Engine,' p. 564, a table, from which the following statement of the tension and the boiling points is extracted ; the tension being given in pounds, the tempera ture of the water, producing the steam of that elastic force, in degrees of Fahrenheit's scale.
Table of temperature of water able to produce steam possessing the elastic force indicated, in lbs. avoirdupois per superficial inch :— In practice, the pressure of steam is reckoned without any reference to that of the atmosphere; or, in other words, it is only the pressure in excess of the atmospheric pressure which is noticed. Ordinary condensing engines, said to be working at from 16 to 40 lbs. pressure, would, according to the above table, work with steam of an elastic force of from 3047 to 54.716s. ; high pressure engines, working at from 60 to 120 lbs. pressure, really work with steam of 74.7 and 1341 lbs. of the table, and the temperature of the water would in both cases corres pond with the higher degrees of elastic force. The laws connected with the relations between the temperature and pressure above stated are, moreover, sometimes applied practically in the arts as a pre caution against danger from an accidental increase of pressure ; for, on the continent, it is usual to place In a steam-boiler, intended to work under high pressure, plugs of mixed metals able to fuse at tempera tures which would generate steam of a dangerous nature. When these plugs fuse, the water escapes, and consequently puts out the fire ; they do not, however, dispense with safety-valves.