Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/589

HEAT 555 referred to above, depends on the change produced in the hand in respect to this property by a change of circum stances which preceded the sensation. We now call heat the property of matter concerned in these sensations, and temperature a certain variable quality of matter varying according to its temporary condition in respect to heat. In the strictest modern scientific language (compare 3 below) the word heat is used to denote something com municable from one body or piece of matter to another, and temperature a definite variable quality of matter, vary ing generally in any particular piece of matter when heat is communicated to it or taken from it, varying also as we shall see ( 8 and 9 bslow ; also THERMODYNAMICS) in consequence of operations which can take place within the body itself, or which may be performed upon it from without, but which cannot be described as communication of heat to it or drawing off of heat from it. 2. Latent Heat. There are exceptional cases in which temperature does not vary in a mass of matter when heat is communicated to it from, or taken from it to, external mitter. For instance, when the body is ice at the melting point, heat communicated to it does not raise its tempera ture ; or if the body be water at the freezing point with ever so small a piece of ice in it (see MATTER, PRO PERTIES OF), heat taken from it does not cause its temperature to fall ; or if the whole mass considered be ice and water well mixed, heat may be either communi cated to it or taken from it without altering its tempera- tare; or if the body be water at the boiling point in the open air, heat very slowly communicated to it in however great quantities does not raise its temperature sensibly, but causes it to disappear by evaporation from its surface ; or if the body be steam in a cylinder with a little water in the bottom and with a frictionless piston above it for roof (fig. 1), under atmospheric pressure, heat taken from it very slowly does not cool it until the whole steam has become condensed into water, and heat communi cated to it very slowly does not warm it until the whole water has become evaporated into steam; or if the body be ice (or frozen water), in place of the liquid water of the last case, and if the pressure on the upper side of the piston, instead of atmospheric pressure of about 1033 grammes per square centimetre (14 - 7 Ib per square inch), be any thing less than -^th of a gramme per square centimetre, the same statement will still apply with " ice " substituted for water. Black s celebrated doctrine of latent heat is merely the declaration of a class of phanomana of which the preceding illustrations sufficiently indicate the character. Modern mysticism has been much exercised in respect to the terms sensible heat and latent heat, whether in decrying them, or in continuing to use them, but with aggravating haziness, instead of the clear wrongness of the old doctrine. It has become of late years samewhat the fashion to decry the designation of latent heat, because it had been very often stated in language involving the assumption of the materiality of heat. 1 Now that we know heat to be a mode of motion, and not a 1 A hundred years ago those deeper philosophers who in their judg ment anticipated, or tended to anticipate, what we now know to be the true theory of the nature of heat, had indeed good grounds to be jeal ous of even the phrase latent heat. Maxwell says "It is worthy of remark that Cavendish, though one of the greatest chemical dis coverers of his time, would not accept the phrase latent heat. He prefers to speak of the generation of heat when steam is condensed, a phrase inconsistent with the notion that heat is matter, and objects to Black s term as relating to an hypothesis depending on the suppo sition that the heat of bodies is owing to their containing more or less material substance, the old " impressive, clear, and wrong " statements regarding latent heat, evolution and absorption of heat by compression, specific heats of bodies and quantities of heat possessed by them, are summarily dis carded. But they have not yet been generally enough followed by equally clear and concise statements of what we now know to be the truth. A combination of impres sions surviving from the old erroneous notions regarding the nature of heat with imperfectly developed apprehension of the new theory has somewhat liberally perplexed the modern student of thermodynamics with questions unanswerable by theory or experiment, and propositions which escape the merit of being false by having no assignable meaning. There is no occasion to give up either " sensible heat " or Sensible "latent heat"; and there is a positive need to retain the lieat term latent heat, because if it were given up a term would be needed to replace it, and it seems impossible to invent a better. Heat given to a substance and warming it is said to be sensible in the substance. Heat given to a sub stance and not warming it is said to become latent. These designations express with perfect clearness the relation of certain material phenomena to our sensory perception of them. Thus when heat given to a quantity of water warms it, the heat becomes sensible to a hand held in the water. When a basin of warm water and a basin of water and ice are placed side by side, a hand dipped first in one and then in the other perceives the heat. If now the warm water be poured into the basin of ice and water, and stirred for a few seconds of time (unless there is enough of warm water to melt all the ice), the hand perceives no warmth ; on the contrary, it perceives that the temperature is the same as it was in the basin of ice and water at the begin ning. Thus the heat which was sensible in the basin of w r arm water has ceased to be sensible in the water that was in that basin, and has not become sensible in the other. It is therefore well said to have become latent. CALORIMETRY. 3. Calorimetry by Latent Heat. The doctrine of latent heat leads us very smoothly to a most important measure ment in thermal science, the measurement of quantities of this wonderful property of matter which we call heat ; and this without our knowing anything of what the nature of heat is, whether it be a subtle elastic fluid, or a state of motion, or possibly some modification of matter related to action of force. Without, in the first place, admitting into our minds any definite idea as to the nature of heat, we may agree to measure quantities of heat by quantities of ice melted into water without change of temperature. Thus if a kilo gramme of ice is melted by a large quantity of water at a lukewarm temperature, or by a comparatively small quantity of very hot water, the same quantity of heat has certainly gone from the warm water to the ice in each case, supposing that the result in each case is the ice and warm water left all in a state of ice-cold water. The measurement of quantities of heat, whether thus by the melting of ice, or by any other means, received the name of " Calorimetry," when the essence of heat was supposed to be a fluid, and this fluid called caloric. The name calorimetry is still by general consent retained to designate measurement of quantities of heat, as distinguished from thermometry, or of a substance called the matter of heat ; and, as I think Sir Isaac Newton s opinion that heat consists in the internal motion of the par ticles of bodies much the most probable, I chose to use the expression heat is generated (Phil. Trans., 1783, quoted by Forbes). We shall not now be in danger of any error if we use latent heat as an expres sion meaning neither more nor less than this : "DEFINITION. Latent heat is the quantity of heat which must be communicated to a body in a given state in order to convert it into another state without changing its temperature? Maxwell s Theory of

Heat, pp. 72, 7-3.