781 



STEAM AND STEAM-ENGINE. 



STEAM AND STEAM-ENGINE. 



782 



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 craparale. Water evaporates at all temperatures ; 

 ice, and snow, sublime in dry winds. 



The law with respect to ebullition ascertained by Daltou, namely, 

 " that the elastic force of the vapour given out 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 hypsometric 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. Hegnault, very useful checks upon the other modes 

 of ascertaining altitudes may be obtained. Amongst other essential 

 points to be taken into account 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 218*'96 

 in a glass vase ; but if the glass be covered by a coating of shell lac, 

 the boiling point would become about 21 1'64. 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. 



The temperatiire 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 the 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 substance ; but suddenly a 

 great effervescence took place, and the temperature fell to 275, 

 accompanied by a copious deposition ; after which the temperature 

 remained constant. M. 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 314'6; and that of linseed oil 600'8. Sea water, 

 containing ,'j of its own weight of the chloride- of sodium, boils at 

 21-V--2 in the open air; if the proportion of salt be increased, the 

 temperature of ebullition will increase also; thus, for ,f, of salt it 

 become* 214"-4 ; for A, it becomes 215-5 ; for , 216-7 ; for & 217'9 ; 

 for ft, 219' ; for A, 221'4 ; for jj(, 223*7 ; for if, 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 fc of the weight of the water ; but that occasionally those 

 proportions are raised as high as 3 * 3 without 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 392'8 Ibs. 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 'Traitd de Physique,' and elsewhere; the pressure being 

 indicated in atmospheres of 147 Ibs. 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 

 pouits 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 Ibs. 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 Ibs. pressure, 

 would, according to the above table, work with steam of an elastic 

 force of from 30'7 to 54'7 Ibs. ; high pressure engines, working at from 

 60 to 120 Ibs. pressure, really work with steam of 747 and 1347 Ibs. 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. 



A great number of formula! have been proposed for the purpose of 

 calculating the relation between the elastic force of vapour and the 

 temperature of the water producing it ; such are the formula; of Laplace, 

 Ivory, Poisson, Lubbock, De Prony. The simplest of them is un- 

 questionably the one given by Dr. Young, p = (l + 0-0029*)'; or, as 

 given by Arago and Dulong for the centigrade scale, F = (1 -t- 07153*) 5 ; 

 and Regnault found that the results it gave corresponded very nearly 

 with those obtained by actual observation, when the vapour was given 

 off from water heated beyond 212, but that below that temperature 

 the results no longer agreed. Regnault adopted, therefore, another 

 formula proposed by Biot, and discussed at length by that philosopher 

 in his ' Trait<5 de Physique,' 1816 : but, as this formula contains no 

 less than five constants, and the indications given by the simpler 

 formula of Arago are admitted to be correct for all the pressures dealt 

 with in practice, there can be little reason for adopting the former. 



It is important, in calculations with respect to the use of steam as a 

 motive power, to ascertain whether the latent heat of the steam 

 remains the same at all temperatures ; and M. Regnault has added to 

 the other services he has rendered to science by going through a 

 series of direct experiments for the purpose of verifying the laws of 



