Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/593

Rh HEAT 559 perature than that of the hand before contact ; and thus, if the sensation could be perfectly remembered, it would be perceived that the wood was first felt to be cold, and afterwards to be warm. This latter warmth is rendered very perceptible by first holding the hand in contact with a piece of wood, as for instance a mahogany table, for a considerable time, half a minute or more, and then suddenly removing it ; a sense of cold is immediately perceived in consequence of the exposure of the hand to the air. The foot is similarly sensitive, If, after holding a bare foot for some time in the air, it be placed on a varnished wooden floor, the floor is perceived to be cold, and if, after standing some time with it pressed to the floor, the foot be suddenly lifted, the air now seems cold by contrast. If a person walks with bare feet on a wooden floor, a continued sense of cold is experienced ; and if, immediately after doing so, he sits down, and holds his feet in the air, the air seems to be warm by contrast. The same sensations are perceived even on a carpeted floor, but much less markedly than on a plain wooden floor, and much less markedly on a plain wooden floor than on a varnished wooden floor, and much less markedly on a varnished wooden floor than on a stone floor. In the case of touching soft wool, or finely carded cotton-wool, or eider down, the first instantaneous sensation of cold is scarcely if at all perceived, and that which first provokes consciousness is the subsequent heating ; and it is very startling to find a body which we know to be ice- cold on a frosty day feeling positively warm to the first consciously perceptible sensation after it is touched. In this casa the small thermal conductivity cr great thermal resistance of the substance is such that heat is carried off by it from the hand slower than it was carried off by radiation and aerial convection ( 70, 71) before contact; and thus, after the first momentary cooling of the hand by the initial cataract of heat from it to the cold body touched, in a small fraction of a second of time a higher temperature is attained by the hand than it had before contact. 11. Sense of Temperature. The sense of heat is in reality a somewhat delicate thermal test when properly used. Even an unskilled hand alternately dipped into two basins of water will, as we have found by experiment, detect a differ ence of temperature of less than a quarter of a degree centigrade ; and there can be no doubt that bath and hospital attendants, and persons occupied with hot liquors in various manufactures, such as dyeing, can detect much smaller dif ferences of temperature than that, and, what is still more re markable, can remember permanently sensations of absolute temperature sufficiently to tell within less than a degree centigrade that the temperature of a bath, or a poultice, or dyeing liquor is &quot;blood heat,&quot; or &quot;fever heat,&quot; or some other definite temperature to which they havo been accustomed. 1 2. Thermometry by Sense of Heat with arbitrary Centigrade Scale deduced from Mixtures of Hot and Cold Water. Without knowing anything of the nature of heat we might found a complete system of thermometry on the mixing of hot and cold water with no other thermoscope ( 13) than our sense of heat, if we had but two definite constant temperatures of reference. These in practical thermometry are supplied by the melting-point of ice and the temperature of steam from water boiling in air at a definite pressure (the &quot;atmo&quot; or standard atmosphere, 5). Thus, suppose perfectly abundant supplies of iced water and of water at the^boiling temperature to be available, and suppose it to be desired to measure the temperature of a river, or lake, or sea. Take measured quantities of the boiling and of the ice-cold water, and mix them by trial until, tested by the hand, the mixture is found to have the same temperature as that of the mass of water of which the temperature is to be determined. Suppose, for example, the mixture giving the required temperature to consist of 86 -6 parts by weight of ice-cold water, and 13 -4 parts by weight of boiling water ; An the required temperature is 13 4 on a perfectly definite arbitrary scale of thermometry in which the temperature of ice-cold centi &quot; water is called zero, that of boiling water 100, and other [ e e temperatures are reckoned according to the law of proper- founded tion of mixtures of water in the manner indicated by the on as - example, and defined generally in 31 below. For tern- peratures within the range of sensibility of the hand this method would give more accurate results than many (68) common thermometers sold by instrument makers for of -water ordinary popular purposes. It may be relied upon for to ^ absolute accuracy within -j^ths of a degree centigrade, pro- vided the mixing of hot and cold water is performed with sufficiently large quantities of water, and with all proper precautions to obtain in that part of the process all the accuracy obtainable by the living thermoscope. We shall see ( 25) that with the most accurate mercury or air thermometers, made for scientific investigation and carefully tested, absolute determinations can scarcely bo depended upon within -j-^th of a degree centigrade. The method of mixtures with only the sensory thermoscope is not limited to the range of temperature directly perceptible with unimpaired sensibility ; but when the temperature to be tested is beyond this range an indirect method must bo followed, as thus: A large quantity of water too warm for the hand is to be tested. Mix it with say twice its weight of ice-cold water, this giving a convenient temperature for the hand ; then find by trial what proportions of ice-cold and boiling water give a mixture of the same temperature as tested by the hand; suppose these proportions to be 2G 2 of boiling water and 73 - 8 of ice-cold water. The temperature of the mixture is by definition 26 2, and on the same principle the required temperature is three times this, or 7 8 6. This system of thermometry is, however, strictly limited to the range between the. freezing and boiling points of water, for we do not at present consider the possibilities (see articles STEAM ; MATTER, PROPERTIES OF ; THERMODY NAMICS) of obtaining and using thermometrically quantities of water below the freezing point and above the boiling point. It is described here, not only because it is very instructive in respect to the principles of thermometry, but because it is in point of fact the thermometric method used through a large range of processes not only in the arts but in scientific investigation. In many cases the hand is a more convenient and easy test than a common mercury thermo meter, and it has just about the same sensibility, the commonest thermometers in popular use being in fact scarcely to be read to a quarter of a degree centigrade. Tn respect to accuracy a common cheap thermometer, though perhaps a degree or two wrong in its absolute indications, may still be used as an accurate indicator of equality of tem peratures just as is the hand in the method of mixtures. In many cases the hand is more convenient than the thermometer, in other cases the thermometer is more con venient than the hand, but in many cases the thermometer is applicable when the hand is not. When the quantities of water tested are abundant, the hand is always the quicker test, but there must be abundance of water to allow it to be satisfactorily and accurately applicable. THERMOSCOPES DIFFERENTIAL AND INTRINSIC. Differential Thermoscopes essentially continuous. Intrinsic Thermoscopes discontinuous and continuous. Single and Multiple Intrinsic Thermoscopes (discontinuous). Continuous Intrinsic Thermoscopes. 13. A thermoscope is an indicator of temperature. Differ- A differential thermoscope is a thermoscope which shows difference, or tests equality, of simultaneous temperatures in two places. Its action is essentially continuous, de-