STEAM. 68 FEB 1965 march of the density and other chief properties of satu- Temperature Fah renheit. +10. 39. Pressure in inchesof mercury. The same in atmo- Temperatures. Volume of the water. Volume of the same in steam. Expansion in evapo rating. Density to air of the same compared temperature, 3.2 65. 100. 2. 0-054 1-555th 1.0036 635000. 632722. 1-888th 7439. 1-8th 3136. .3125 way. That a law doubtless as simple as other natural | and elasticity, when the density for one particular value It is conceivable, too, that the relation between the temperature, truly measured, and the density of the saturated steam, may be simpler than between the temperature and the elasticity. As doubts are now thrown on the applicability of Dalton's and Gay Lussac's law of gaseous expansion to this vapour, we can no longer, as heretofore, look on the densities as calculable from the other two elements, temperature VOL. II. 486.6 1200. 510-6 1500. Fifty 1.155 42-77 31-25 The corresponding temperatures and elasticities are here taken from Dalton for low-pressure steam, from Dulong and Arago for high-pressure. The fourth column is then calculated thus. The volume of an unit of water at 39° (its point of greatest density) being taken as unity, the space which it would occupy at any other temperature is taken from the observations of Kirwan and Gilpin, up to 212°. But as these measures were made under the constant pressure of one atmosphere, while the water is here supposed subject to the varying pressure of its own vapour only, the correction for this is reckoned from Perkins's experiments on the compression of water, and found not to affect the last decimal here given. Although we have no measures of water at the two temperatures below freezing, it is known under these circumstances to go on expanding by cold, and these volumes have been reckoned by the empirical formula that Dr. Young found to represent very closely the expansions both ways from 39° as far as they had been measured. But in applying Dr. Young's rule above 212°, we find it give a maximum volume at about 360°, above which it makes the water contract by heat as fast as it had previously expanded. So anomalous an effect could only be admitted on experimental proof. Until further observations, therefore, had been made, we could only, as a more probable supposition, reckon these volumes by a regular expansion at the rate Gay Lussac measured in the last few degrees below boiling; and then reduce Y Y the proportion that air under a constant pressure would expand if heated from 212° to 2504°; and by increasing the 850 in this proportion we obtain very nearly 900 for the smallest space in which 1,700 measures of boiling steam (or one measure of water in its densest state) could be wholly converted into steam of two atmospheres, or of 2504. them for the compression caused by the steam, which, I than the half of this,-more than 850 measures, in however, only affects the last decimal in the last two numbers. This shows that at 510° we are still far short of the temperature at which water would cease to expand, being stopped by the pressure of its own steam, balancing its own tendency to dilate. The next column, headed Volume of Steam," is based on the number 1,700, which, according to Gay Lussac, represents the space that one measure of water at its The numbers in the sixth column are found by greatest density will occupy when converted into dividing each of these volumes of steam by the boiling steam. To obtain the space occupied at any volume which it occupies as water at the same tempeother temperature, we first increase or diminish this rature; and those in the last are all ğths of those in inversely as the pressure in the second column, and the third, because we have seen that all steam is prothen increase or diminish the resulting number, by bably ths as dense as air of the same temperature ath of itself for every degree above or below 212°. and pressure. This column shows us, that while For instance, if the 1,700 measures of steam of one steam of the high pressures observed by Dulong atmosphere were to expand without change of tempe- and Arago is by far the densest aeriform matter rature, till its elasticity were reduced one-half, it known, the steam of low and common temperatures would, by the Boylean law, occupy twice its former presents the lightest form of matter of which we have bulk, or 3,400 measures; but it would not then be any evidence (if that of comets be excepted); that saturated steam, but subsaturated, having only half which depressed the mercury of Gay Lussac's barothe density and elasticity its temperature could sup-meter in his coldest observation being probably 888 port, and capable of taking up its own weight of times thinner than air, while even at summer tempeadditional water. In other words it would be over-ratures, in which he contrived with exquisite ingenuity heated steam, having at 2129 only the pressure to submit it to actual weighing, it is the lightest belonging to saturated steam of 180°. It would be body ever weighed, hydrogen having only been weighed like the steam of the atmosphere, a dry or drying at the common pressure, under which it is much steam, one in which water could evaporate, or wet denser than saturated steam of 100° Fahr. bodies of its own temperature be dried, and so could any bodies hotter than 180°. But 180° would be its dew-point, down to which it must be cooled before any would condense into water, or at which point it would become saturated steam. Hence the same quantity that as steam of one atmosphere filled 1,700 measures, plainly cannot, as saturated steam of half an atmosphere, fill 3,400 measures, but to find its new volume we must diminish this 3,400 in the proportion that airs under a constant pressure contract in cooling from 212° to 180°, that is (if the Daltonian law apply here) in the proportion of 448 +212 to 418180, the volumes being, according to this law, proportional to the temperatures reckoned from 448° below Fahrenheit's zero. We thus diminish it to 3,235, which expresses the smallest space in which 1,700 measures of boiling steam would cool to 180° without depositing any moisture, or the smallest in which one measure of water at 39° could pass wholly into steam at 180°. Or, again, if 1,700 measures of boiling steam could be compressed by an additional atmosphere without any being liquefied, they would, like air or gas, occupy under this doubled pressure only half their former bulk. But it is impossible to compress this or any saturated steam ever so little, without liquefying a part, unless we add at the same time heat enough to raise its temperature to the point at which water boils under this increased pressure. Thus we cannot possibly have steam of one atmosphere cooler than 212°, nor steam of two atmospheres below 2504, (though as much above these temperatures as we please.) It follows, then, that the water which at 212 requires 1,700 measures to exist in as steam, must at 250°, though twice as elastic, require more As we see that with increase of temperature the liquid and its vapour both tend to approach each other in density, the water becoming expanded and the steam at the same time rapidly denser, so that while ice at zero (supposing it one-tenth rarer than water at zero) requires more than half-a-million times its own space to evaporate in, water at 510° requires only 43 times its space; a question naturally arises whether there be not some higher fixed temperature at which the two forms of this substance would be equally dense, and thus merge into one, the distinction of steam and water no longer existing; or rather a temperature above which water could not exist, but steam, however much compressed, even to the density of water, would remain permanently gaseous, as air and hydrogen seem to do at common temperatures. On this point, M. Cagnard De la Tour made some curious experiments, and is said to have heated alcohol and ether, enclosed in glass tubes, to temperatures at which they became wholly vaporous and invisible, in little more than the space they occupied as liquids, and water in about four times that space, at a temperature about that of melting zinc, above which the experiments were prevented by the solvent power of this liquid becoming so increased as to attack any kind of glass. Supposing the temperature 700°, this would make steam of that temperature 425 times as dense as boiling steam, and, of course, if the Boylean law extend so far, 425 times as elastic as boiling steam (or atmospheric air) would be if heated from 212° to 700°, without change of density, or with neither access of water nor room for expansion. An aeriform body so heated would, by the Daltonian law, have its elasticity increased as 448 +212 to 448700, or as 660 to 1,148, and by increasing | the middle of the last century that one of our great 425 in this ratio we get 739 atmospheres for the thinkers, Dr. Black, began to reason precisely on so pressure probably borne by M. De la Tour's glass common a matter, and to deduce from it some highly tube. This is the nearest approach to the yet un-important natural laws. [See HEAT.] He proved solved problem of "making water red-hot." It is that this disappearance or apparent loss of heat took well-known that in red-hot metallic vessels, it does place whenever a solid melts, as well as when a liquid not touch them, nor become much hotter than 210°, evaporates; and on the contrary, a precisely equal nor produce steam nearly so elastic as the atmo- apparent production or restoration of heat when the sphere. [See EBULLITION.] The luminous film of same liquid solidifies or the same vapour liquefics. every hydrogenous flame may be regarded as red or For instance, if a quantity of ice, some degrees colder white-hot steam, and shows that the rarity or transpa- than freezing, say at 20°, be placed in ever so warm rency of aeriform bodies prevents their giving, at the a situation, even in a vessel over a fire, although its intensest degrees of incandescence, so much light as temperature will rise to 32° just as quickly as an solids give at far lower degrees. In ordinary flames equivalent mass of any other body (i. e. a mass having it is the light of incandescent carbon only that we the same heat-capacity) would rise 12°, yet its temuse, and it entirely drowns that of the equally hot perature will then remain at 32° during the whole newly formed steam or carbonic acid. time occupied in melting, which of course will be proportioned to the quantity melted; and not until the whole has become water will a thermometer in it again begin to rise as before. During all this time heat enters the ice, and yet neither that nor the icewater, nor anything that we can observe, becomes hotter. We see that heating power is expended just as if the contents of the vessel were being warmed, and yet they are not warmed, but only melted. A change is effected, more and more of the solid becomes liquid, and we see that a supply of heat is necessary to effect this, but when all is over we have no more apparent heat, nothing hotter, only a vessel of water at 32° instead of ice at 32°. Dr. Black showed, and it is perfectly established and demonstrable in a variety of ways, that to melt ice simply without any rise of temperature, it must always thus receive and absorb, or render latent, a certain precise amount of the heating power or principle, viz. about as much as would heat an equal weight already melted no less than 140°. So also, in the melting of all other solids, there is a fixed quantity of heat, different for each, and generally greater than this, rendered latent; and in the conversion of liquids into vapour, a generally greater quantity still; which in the case of steam is greater than in any other case known, being little, if at all less than would have heated the water, if it could remain water, 1,000°,-(or ten times as much water, 100°; or a thousand times as much, 1°). The various modes of determining this number are exactly analogous to those by which the 140° absorbed in the liquefaction of ice are ascertained. Thus, as the vessel in which ice is melting, so also that in which water is boiling, can be made no hotter by any heat we may apply, until the whole has boiled away. The only effect of a fierce fire is to melt the ice more quickly, or to evaporate the water with greater rapidity, but not to render either hotter. Now Dr. Black, by contriving to keep a plate of iron at a constantly equable red-heat, and placing on it a small tin dish of water, compared the time occupied in heating it from 50° to 212°, which was 4 minutes, with the time afterwards required to boil it all away. This, however, is the least accurate method of any, and gave only an approximation to the relative quantities of heat expended. A better method is that called the method We have now to explain how it is known, that although steam and water can thus co-exist at almost every measurable temperature, the steam has always, as stated at the outset, more heat than water has. We know that the visible cloud commonly called steam is never so hot as the liquid whence it comes, and will never even scald the hand, although the steam from a boiling kettle will do so, while in its invisible state, or before it becomes cloud. But a thermometer plunged into even this invisible part of the current will never mark a higher degree than in the water, rarely so high; and never in any case higher than 212° in the open atmosphere. Hotter than this, no steam can be made without forcible confinement, and then the water also is made equally hot, a thermometer placed in a high-pressure boiler marking exactly the same degree whether in the water or the steam. And in the jet issuing from such a boiler, it will never even rise to 212°, as in that from a tea-kettle, however hot the steam immediately within may be; for so universally is all visible cloud colder than this, that the jet from such a boiler, which, unlike that from a tea-kettle, becomes visible immediately on passing the aperture, will not scald a hand however close it may be held; which has given rise to the absurd paradox that "high-pressure steam will not scald;" whereas high-pressure steam has never been touched, nor can be, unless by making the hand a valve to help confine it; for when it has issued from a jet, it has ceased to be high-pressure steam. Notwithstanding all this it has been usual, from the time even of Hero, the Alexandrian writer on Pneumatics, 2,000 years ago, to speak of water being converted into steam by the addition of heat, or, as he expressed it," converted into an air by fire," the four popular "elements" having then been understood as simply the names of four states of matter, solid, liquid, gaseous, and imponderable, in strict accordance with our latest science. It was impossible to overlook this absorption, or expenditure of fire or heat, in converting water into an air; a portion of both constituents, the water and the heating power or principle, being evidently lost as such, or ceasing to show their characteristic qualities, in uniting to form the new substance. Yet it was not until about site results, have, until very lately, if they do not still, appeared in the same books, or almost the same pages, without a remark on their incompatibility, and are alternately and indifferently taken for granted in the same calculations. The first, which is attributed to Watt, and seems, at one time at least, to have been accepted by Dalton, is to the effect that the higher the temperature, the smaller, by just so many degrees, is the difference of latent heat between steam and water at that temperature. The other rule, founded on three experiments by Southern on steam of 1⁄2, 1, and 2 atmospheres, or 180°, 212o, and 2504° only, assumes this difference to be constantly equal at all temperatures, being always such as would raise water 942° or 950°, for to this extent did even those three experiments vary. of mixtures. In melting ice by water, it will be found | Two rules leading, if followed out, to the most oppothat if the weights of water and ice be equal, the resulting water is 70° colder than the mean of their temperature before mixing. Thus, no ice can be melted by its own weight of water of a lower temperature than 172°, or as many degrees above 172° as the ice is below 32"; and in this case the resulting water will be only just above freezing. It will be the same when ice is melted by 10 times its weight of water 14° warmer than itself; or 5 times its weight 28° warmer; or 28 times its weight 5° warmer; these being the smallest quantities that can melt it in those cases. So in condensing steam by passing it through water, one of Watt's first experiments, he was astonished, till Dr. Black explained to him his theory, at the large quantity of water that a little steam could raise to nearly its own temperature. That steam at 212°, may become water at 212°, it must give out heat enough to raise, according to Rumford, 10 times its own weight of water 102°, or produce an equivalent effect, such as heating 102 times its weight 10°, or 1,020 times its weight 1°; and though this is the highest measurement, the lowest, that of Southern, makes the number 942. Another mode of proof by Watt is very remarkable. We have seen that, under forced confinement, water may be heated much above 212° without boiling, that is to say, any boiling will cease as soon as the confined steam acquires the den- | sity of saturated steam for the particular temperature maintained. Papin, who, about the year 1700, invented the safety-valve, [see DIGESTER,] applied it chiefly to a small culinary vessel for the highly useful purpose of extracting nourishment from bones, &c. by water of higher temperatures than boiling, and it is called after him Papin's digester. Now, however hot the water in one of these may be made, it is found that, on opening the valve, only a part of the water rushes out in steam, (instantly becoming a dense and cold cloud,) and what remains as water instantly cools to 212°. The hotter the original temperature, the greater portion indeed rushes out, but nothing remains hotter than 212°. Moreover, if the whole were previously 95° above boiling, just a tenth of it will thus rush out, so that this tenth, in changing from water at 307° to steam at 212o, takes nevertheless as much additional heat as raised all the other 9 parts 95°; or in simple conversion into steam it absorbs as much as raised all 10 parts 95o. If all were heated at first 190° above boiling, then the portion flying out will be th, so that it requires for vaporization, independently of all change of temperature, 5 times the heat that would raise it 190° without evaporation. And in every case the same rule will apply, the part evaporated bearing that ratio to the whole, which the degrees it was raised above boiling bear to 950o. Important as this quantity, called the latent heat of steam, and all laws relating to it, must evidently be to the economy of the steam-engine, affecting fundamentally all questions as to the best form of engine or mode of working, it cannot be said to be either more precisely known, or the least better understood, as to its laws now, than in the time of Dr. Black. The former, or Watt's rule, is commonly expressed by saying that the total heat of saturated steam (i.e. the latent and apparent heat together) is a constant quantity, viz. in round numbers 1,200° on Fahrenheit's scale, so that when the temperature or apparent heat is 212°, the latent will be 9889,-when the former is 250o, the latter is 950o, &c. In Southern's rule the latent instead of the total heat is constant. But as the quantities are expressed in degrees that, as we have seen, measure no causal force, only an effect whose laws are unknown, it is not at all likely that either these or any other modes of reckoning founded on our present thermometric scale, will agree with the phenomena. Some experiments of Regnault, made by the direction of the French government, have rendered it certain that the truth lies between these two hypotheses, or that saturated steam contains more heat the hotter it may be, but less latent heat. The rule he has deduced, however, as agreeing best with his experiments, is very arbitrary, and unlike the simplicity of a natural law. It amounts to this, that for every increase in the density of saturated steam, there must be an increase of the total heat equivalent to 30.5 per cent. of the rise of temperature; or, in other words, the difference between the heat latent in the steam and in the water, is neither constant, as Southern supposed, nor does it diminish as much as the temperature increases, as Watt supposed, but about 7-tenths as much, viz. 69.5 per cent. Thus the truth would lie nearer to Watt's supposition than to Southern's, as 7 to 3 nearly. The latent heat, in saturated steam of the following different temperatures, will be, according to the three hypotheses, as follows: Tempera ture. Elasticity. WATT. REGNAULT. SOUTHERN. Latent Total Latent Total Latent Total deg. deg. deg. deg. 1020 1200 988.4 1168-4 988 1200 966.2 1178.2 966 1200 950.9 1184.9 950 1184 949 1200 939 1190 950 1201 924 1200 921.7 1197.7 950 1226 906 1200 909-2 1203-2 879 1200 890.3 1211-3 840 1200 863.3 1223.3 deg. deg. deg. 950 1130 950 1162 950 1244 950 1271 950 1310 What is here called the total heat, however, should | as it is rarefied or condensed. Ice and water are be called the excess above that of water at 0° Fah. an exception to this rule, but an instance of a more It has been justly observed that the greatest incon- general one, viz. that the same substance has always venience of this scale is its not having the zero at a greater capacity or specific heat when in the liquid that great natural standing point, the congelation of than the solid state, and it is generally supposed to water. In the thermometric systems of the conti- have a greater still in a gascous state. In expressing, nent, where this is the case, the number in question therefore, the quantities of heat that disappear, or represents the excess of heat in the steam above that that seem produced in any process, it is necessary to of freezing water, which is an actual and well-known | say on what substance their effect is reckoned. This standard; whereas, water at 0° Fah., which we are is generally water, and from it we can of course obliged to refer to, has never been seen. We might estimate the number of degrees change (usually add 140° to the above numbers, and call them the greater) that would be produced in any other body excess of heat above that of ice at 0°; or what is whose specific heat is known. much better, diminish them by 32° and call them the excess above freezing water; but this would lead to continual mistakes unless we also reckoned the temperatures from the freezing point. Again, it is necessary to remember that the latent heat is measured by the number of degrees it would warm an equal weight of water, (or rather the number of such weights of water that it would warm 1°,) and that its effect on any other body, or even on the same body in the state of steam or of ice, would be quite different. From observations of the warming and cooling effects of different substances on each other at different temperatures, as well as from the different expenditures of fuel to heat them equally, it is clearly proved that the same quantity of heat which warms a pound of water 1°, will warm a pound of most other bodies much more than 1o, and a pound of mercury no less that 30°, or at least 30 pounds of it 1°. A pound of water and 30 pounds of mercury are therefore equivalents in regard to heat. Both have the same heating or cooling effect on the same body, when their temperatures are equal, and this is commonly expressed by saying they have the same capacity for heat. But if so, a pound of water must plainly have 30 times the capacity of a pound of mercury, and must (when their temperatures are equal) contain 30 times as much of that heat which is employed in maintaining their temperature, leaving latent heat out of the question. For when an addition of heat is so shared between these two bodies as to have no tendency to leave one and enter the other, the water will have 30 times more of it than the mercury, though both will warm a thermometer equally, or have the same effect on any body touching them, in short the same temperature. Hence, if we call the capacity of any quantity of water 1 or 1,000, that of an equal weight of any other substance is called the specific heat of that substance, and is, for almost all known solids and liquids, considerably less than this unit of comparison. If these specific heats were expressed by the numbers representing the capacity of equal bulks of the different bodies, instead of equal weights, they would be much less widely unequal, though the numbers would still be less for almost all other bodies than for water. Again, when a given weight of any substance is expanded into a larger space, whether by heat or any other cause, its capacity is increased, so that the specific heat of the same substance is greater or less according Dr. Dalton contrived the following elegant mechanical illustration, or rather way of better conceiving these changes and their curious effects in liquefaction and vaporization. Let there be three concentric cylinders with open tops, of unequal diameters and heights, placed one within another, and the outer rising higher than the second, and the second than the innermost; and let a slender tube proceed from the bottom of the innermost, through the sides of the others, and turn up vertically as high as the outer vessel, B A Fig. 2035. may to serve as a gauge of the height a fluid attain within them. Now if water be poured into the inner vessel, its rise will be accurately indicated by this gauge, till it arrives at the level of the inner vessel's top, and begins to overflow into the next larger vessel. No addition will now be at all indicated by the gauge, till the space surrounding the inner vessel has been filled up to the same level. Then, indeed, the gauge will again begin to show the rise of liquid, but will risc more slowly than before, if the supply continue uniform, because the area of the second vessel is larger than that of the first. This represents the thermometer indicating the addition of heat to ice till it arrives at 32°, then remaining stationary till all is melted, and again rising more slowly in the water, for an equal supply of heat, because its heat-capacity is greater than that of ice. So when the liquid arrives at the top of the middle vessel and again begins to overflow, the level will remain stationary a longer time, because a larger space has to be filled up than before, and we may make this space exceed the former in the ratio of 76 to 1, (in which ratio the heat absorbed or rendered latent in boiling water into steam, exceeds that in melting ice into water,) and at the same time make the horizontal sections of the three vessels as the numbers 900, 1,000, and 1,550, the relative capacitics of ice, water, and steam; and again make the space around the |