LATENT HEAT OF STEAM. 153 water, at the temperature of 50°. The water will be raised 11°. Hence, 1 gallon of water, condensed from steam, raises the temperature of 100 gallons of cold water 91° more than 1 gallon of boiling water; and, by an easy calculation, it appears that the caloric imparted. to 100 gallons of cold water by 8 pounds of steam, if it could be condensed in 1 gallon of water, would raise it to 950°;* and 1 gallon of water converted into steam of ordinary density, contains as much heat as would bring 5 gallons of ice-cold water to the boiling point. The quantity of ice, which is melted by steam of mean density, is invariably 7 times the weight of the steam. For exhibiting the latent heat of steam, by means of a small apparatus, which may be placed on a table, and with the assistance only of a lamp, the boiler represented in the annexed woodcut will be found extremely well adapted. The right angled pipe e must be screwed, however, into its place, and must be made to terminate at the bottom of a jar, containing a known quantity of water of a given temperature. This conducting pipe and the jar should be wrapped round with a few folds of flannel. The apparatus being thus disposed, let the water in the boiler be heated by an Argand's lamp, with double concentric wicks, or any other convenient source of heat, till steam issues in considerable quantity through the cock c, which is then to be closed. The steam will now pass through the right angled pipe into the water contained in the jar, which will condense the steam, and will have its temperature very considerably raised. Ascertain the augmentation of temperature and weight; and the result will show how much a given weight of water has had its temperature raised by a certain weight of condensed steam. To another quantity of water, equal in weight and temperature to that contained in the jar at the outset of the experiment, add a quantity of water at 212°, equal in weight to the condensed steam; it will be found, on comparison of the "Black's Lectures," i. 169. two resulting temperatures, that a given weight of steam has produced, by its condensation, a much greater elevation of temperature, than the same quantity of boiling water. This will be better understood by the following example, taken from actual experiment. Into eight ounces of water, at 50' Fah., contained in the glass jar, f, steam was passed from the boiler, till the temperature of the water in the jar rose to 173°. On weighing the water, it was found to have gained 81 drachms; that is, precisely 8 drachms of steam had been condensed, and had imparted its heat to the water. To facilitate the explanation of this experiment, it is necessary to premise the following remarks, To measure the whole quantities of caloric contained in different bodies, is a problem in chemistry which has not yet been solved. But the quantities of caloric added to, or subtracted from, different bodies (setting out from a given temperature) may, in many cases, be measured and compared with considerable accuracy. Thus if, as has been already stated, 2 pounds of water at 120° be mixed with 2 pounds at 60°, half the excess of caloric in the hot water will pass to the colder portion; that is, the hot water will be cooled 30°, and the cold will receive 30° of temperature; and, if the experiment be conducted with proper precautions, 90°, the arithmetical mean of the temperature of the separate parts, will be the temperature of the mixture. If 3 pounds of water at 100° be mixed with 1 pound at 60°, we shall have the same quantity of heat as before, viz., 4 pounds at 90°. Hence, if the quantity of water be multiplied by the temperature, the product will be a comparative measure of the quantity of calorie which the water contains, exceeding the zero of the thermometer employed. Thus, in the last example, 3 x 100 300 60 the caloric above zero in the first portion. The sum, 360 the caloric above zero in the mixture. Dividing 360 by 4, the whole quantity of water, we obtain 90', the temperature of the mixture. This method of computation may be conveniently applied to a variety of cases. Thus, in the foregoing experiment, 8 drachms of steam at 212°, added to 64 drachms of water at 50°, produced 72 drachms of water at 173°. Now, 72 X 17312542 64 X 50 3200 9342 whole heat of the mixture. heat of 64 drachms, one of the component parts. heat of 8 drachms, the other component part. Therefore 93421 divided by 8 = 1099, should have been the temperature of the latter portion (viz. 84 drachms), had none of its heat been latent; and 1099 - 212-887 gives the latent heat of the steam. This result does not differ more than might be expected, owing to the unavoidable inaccuracies of the experiment, from Mr. Watt's determination, which states the latent heat of steam, on an average of nine experiments, at 949-9°, say 950°.* Lavoisier, with the aid of the calorimeter, makes it 1000°, or a little more;† Mr. Southern, 945; and Dr. Ure, 967. XIII. The same weight of steam contains, whatever may be its density, (nearly) the same quantity of caloric; its latent heat being increased, (nearly) in proportion as its sensible heat is diminished; and the reverse.-That portion of the statement which appears in italics, is commonly known as Watt's law. Recent investigations have demonstrated that it is "Black's Lectures," i., 174, and "Letter to Dr. Brewster," p. 7, n. +Ibid. i. 175. slightly incorrect, and must now be received, in a philosophical sense, as limited in the text. For all practical purposes, however, the original law of Watt may be considered as unaffected.* This principle, though scarcely admitting of illustration by an easy experiment, is one of considerable importance; and an ignorance of it has been the occasion of many fruitless attempts to improve the economy of fuel in the steamengine. The fact, so far as respects steam of lower density than that of thirty inches of mercury, was long ago determined experimentally by Mr. Watt.t As the boiling point of liquids is known to be considerably reduced by a diminished pressure, it seemed reasonable to suppose that, under these circumstances, steam might be obtained from them with a less expenditure of fuel. Water, Mr. Watt found, might easily be distilled in vacuo when at the temperature of only 70° Fahrenheit. But, by condensing steam formed at this temperature, and observing the quantity of heat which it communicated to a given weight of water, he determined that its latent heat, instead of being only 955°, was between 1200° and 1300°. The same principle may be explained also by the following illustration, which was suggested by Mr. Ewart. Let us suppose that in a cylinder, furnished with a piston, we have a certain quantity of steam, and that it is suddenly compressed, by a stroke of the piston, into half its bulk. None of the steam will in this case be condensed; but it will acquire double elasticity, and its temperature will be considerably increased. Now, if we either suppose the cylinder incapable of transmitting heat, or take the moment instantly following the compression before any heat has had time to escape, it must be evident that the sensible and latent heat of steam, taken together before compression, are precisely equal to the sensible and latent heat taken together of the denser steam. But in the dense steam the sensible heat is increased, and the latent heat proportionally diminished. Reversing this imaginary experiment, if we suppose only half the cylinder to contain steam at 212°, and the piston to be suddenly raised to the top of the cylinder, the steam will be expanded to twice its volume, its temperature will fall, but its sensible and latent heat taken together will still remain unchanged. The explanation of these facts will be furnished by a principle to be hereafter explained, viz., that the capacities of elastic fluids for caloric are uniformly diminished by increasing their density, and the reverse. Direct experiments to ascertain the latent heat of steam, formed under higher pressures than that of the atmosphere, have been made by Mr. Southern, of Soho, and by Mr. Sharpe, then of Manchester.‡ Those of the latter were first published, and were to him quite original. They were conceived and executed very ably, but were subsequent, in point of time, to the experiments of Mr. Southern, which, though only lately made public, § were instituted many years ago. The latter consisted in ascertaining the augmentation of weight and increase of temperature, gained by given quantities of water, from the condensation of known volumes of aqueous vapour of different densities. The results presented differences in the latent heat of steam of different densities, but of so very small an amount as to arise probably from unavoidable sources of error in manipulation. The following table exhibits the principal results obtained by Mr. Southern : * See M. Regnault on "Relation des Expériences enterprises pour déterminer les Principales lois et les données Numériques qui entrent dans le Calcul des Machines à Vapeur," Paris, 1847; or a translation of the same in Vol. I. of the Works of the Cavendish Society. +"Black's Lectures," i. 190. "Manchester Society's Memoirs," Vol. II., New Series. Brewster's Edition of Prof. Robinson's Works. The experiments of Mr. Sharpe, and also a recent series by Clement and Desormes (of which an abstract is given in the Appendix to Thenard's "Traité de Chimie,” vol. iv., p. 262, 3me edition), and more recently M. Regnault, establish the same general law, at least within certain limits. This law is of great importance in practice, since it shows that no essential saving of fuel can be reasonably expected from using, as a moving power, steam formed under high pressures: for as a pound of steam, of whatever density, gives out by condensation the same quantity of caloric, it is obvious that, to convert a pound of water into steam, will always require the same quantity of fuel, applied under equal circumstances, whatever may be the density of the steam which is produced. So far, indeed, from tending to economize fuel, it seems probable that the higher thè temperature of the water in the boiler, the greater will be the loss of heat by the escape of hot air through the chimney. Nevertheless, there are certain cases in which highpressure steam may be applied with great advantage to various manufacturing processes, as a means of communicating heat, when the temperature is required to exceed 212° Fah. A remarkable fact has been observed respecting steam of great elasticity, viz., that when suffered to escape suddenly from a cock or small aperture in the boiler, the hand may be held close to the orifice from which the steam is issuing violently, without being scalded by it, though every one knows that steam of ordinary density scalds severely under the same circumstances. On applying a thermometer to the steam escaping from the boiler, its temperature is found to be considerably reduced: for instance, steam, which, within the boiler, was at 290° Fah., falls suddenly when let out to 160°. That it should fall to 212° would not be surprising, since that is the temperature of steam under the ordinary pressure of the atmosphere; but it is difficult to conceive why it should descend 52° below 212°. Of this fact, Mr. Ewart has given an ingenious, and, it appears to me, a satisfactory explanation, founded on the results of experiments, which he has described in the "Annals of Philosophy" for April 1829. He supposes that the particles of elastic fluids have a tendency, when they are forced near to each other, to fly asunder, not only to their original distance, but beyond it. To illustrate this, let us suppose two equal balls of lead, A B, to be connected by an elastic steel spring, in a perfectly neutral state, so that they have no tendency either to collapse or expand. Compress them nearer to each other, as C and D, and, suddenly setting them at liberty, they will separate, not merely to their original distance, but beyond it, as at E and F. Now, if we suppose A B to represent two atoms of steam, of air, or of any elastic fluid, compressed as CD, and suddenly liberated, they will, by the com A mo B bined action of their elasticity and momentum, separate to E F, or thereabouts. Thus VAPORIZATION UNDER PRESSURE. 157 high-pressure steam, on suddenly taking off all pressure beyond that of the atmosphere, is converted into low-pressure steam, and its temperature falls, in consequence of the law which connects the rarefaction of elastic fluids with an absorption of caloric. It is quite in accordance with this theory that the more strongly steam is compressed, the more dilated and the colder it is on being suddenly released from pressure. XIV. Conversion of Liquids into Vapours under strong Pressure.-It is well known that by means of a Papin's digester we are enabled to raise the temperature of liquids considerably above the points at which they boil under the mean pressure of the atmosphere; and it seemed probable that the internal pressure, augmenting with the temperature, must effectually prevent the total volatilization of the liquid, especially if the space, left above the liquid, is not of a certain extent. But provided a sufficient space is allowed for the generated vapour, it appeared to M. Cagniard de la Tour, a necessary consequence that there should be a limit beyond which these liquids ought, notwithstanding the pressure, to be completely volatilized; and to verify this opionion he was led to make some interesting experiments. A strong glass tube, containing about two-fifths its capacity of alcohol, sp. gr. 837, being carefully heated, the alcohol continued to expand, till, after having attained nearly double its original volume, it was converted into vapour so transparent, that the tube appeared completely empty. Allowing it to cool, the alcohol was again condensed into a liquid. When the proportion of alcohol to the capacity of the tube was increased, the consequence was the bursting of the tube. Similar results were obtained with naphtha and ether, the latter requiring less space than the former for being converted into vapour without breaking the tube; and naphtha less space than alcohol. No difference was occasioned by the presence of atmospheric air in the tubes, or its exclusion from them, except that the ebullition of the liquid was then much more moderate. The same success did not attend the first attempt to convert water into vapour; for when a tube, about one-third filled with water, was similarly heated, it burst with an explosion. The inner surface of the glass tube appeared also to have been acted upon, its transparency being impaired. M. de la Tour afterwards determined the densities of these vapours, by means of a gauge which measured the bulk of a confined portion of air, subjected, through the intervention of a column of quicksilver, to the pressure of the generated vapour. Alcohol converted into vapour, and occupying a space a little exceeding three times the volume of the original liquid, he found to exert a pressure = 119 atmospheres, and to require a temperature of 404.6' Fah. Ether, under the same circumstances, required a temperature of 369' Fah., and the force of its vapour was equivalent to 37 or 38 atmospheres; bisulphuret of carbon required 527° Fah., and the pressure of its vapour was equal to 78 atmospheres. Water, to which a minute quantity of carbonate of soda had been added, ceased to act upon glass tubes; and, though several tubes were broken, it was ascertained that water itself may be converted into vapour, provided the vacant space exceed its volume about four times.* XV. The evaporation of water is carried on much more rapidly under a diminished pressure, especially if the vapour, which is formed, be condensed as soon as it is produced, so as to maintain the vacuum. On this principle depends Mr. Leslie's ingenious mode of freezing water, in an atmosphere of any common temperature, by producing a rapid evaporation from the "Ann. de Chim. et de Phys.," xxi. 127, 178; xxii. 410; or "Ann. of Philos.," v. 299. |