Introduction, 1228. Heat and temperature, 1230. Thermoscopic instruments, 1232. Construction of the mercurial thermometer, 1235. Testing of a thermometer, 1252. Formula for the conversion of degrees of one scale into those of another, 1254. Exercises. Boiling points, 1255. Kopp's law of the boiling points, 1261. Exercises. Fractional distillation, 1279. Specific heat, 1286. Methods employed for determining the specific heat of solids and liquids, 1290. Measure of the sensible heat absorbed by a body, 1291. Exercises. Description of Regnault's method, 1302. Specific heat of a body not a constant quantity, 1316. Specific heat of gases, 1320. Table of specific heat of gases, 1322. Atomic heat of bodies, 1323. Relation between the specific heats and atomic weights of the elementary bodies, 1327. Tables of the specific and atomic heats of the elements. Atomic heats of compound bodies, 1332. Development of heat by chemical action, 1335. Description of Dr. Andrew's method, 1343. Combination of gaseous bodies with oxygen, 1349. Exercises. Combination of solid and fluid bodies with oxygen, 1354. Exercises. Table of the heat developed during the combination of bodies with oxygen, 1361. Tables of the heat developed during the combination of bodies with chlorine, bromine, and iodine, 1368. Influence of dimorphism on the heat evolved, 1372. Heat developed on the union of acids and bases, 1373. Table of heat evolved during metallic precipitations, 1378. Heat evolved on the combination of an acid with water, 1382. Heat evolved on the combination of a salt with water, 1384. Heat evolved on the combustion of polymeric compounds, 1385. Cold produced by chemical decomposition, 1391. Exceptions, 1396. Calorific intensity, 1397. Exercises. Calorific intensity of fuel, 1404. Exercises. Lewis Thomson's apparatus for determining the absolute heating power of fuel, 1407. On the burning of fuel, 1409.

1228. What heat is, is still unknown. Two different

theories have been proposed to explain its nature, but it does not come within our province to notice them.

1229. Whenever, as in the case of heat, we have to speak of any agent whose nature is unknown, the expres sions and allusions we employ in regard to it must necessarily possess a degree of vagueness; and to those students who require exact definitions, they must appear unsatisfactory. This vagueness is further increased if different views have prevailed at different times as to the nature of the agent; because the terms which have been employed under one theory are frequently retained, when other and totally different views prevail. These few general observations on the terms employed we have thought it necessary to make, in order to show the student that the terms which may appear vague and unsatisfactory to him are equally so to all who study the subject.

1230. Heat and temperature. The term heat we apply to the agent; the term temperature we apply to that portion of heat in a body which we can perceive by the senses. The temperature of a body can be increased or diminished. If two bodies of different temperatures are brought together, the one containing the most sensible heat-in other words, having the highest temperature-will lose heat, which the body with the lower temperature will absorb. This imparting of heat by the one body, and the receiving of it by the other, will only cease when they have both arrived at the same temperature. If, therefore, we bring the hand, or any other part of the body, in contact with a substance having a higher temperature, heat is imparted by that substance to the hand, and we experience the sensation we term heat. If, on the contrary, the hand is warmer than the substance, it imparts some of its heat to the substance, and we then experience the sensation we term cold. The student will observe, from these illustrations, that heat and cold are merely relative terms.

1231. Our sense of touch cannot be employed to ascertain the correct temperature of substances; for that will only tell us whether the substance has a higher or a lower temperature at the time, than that part of our body which comes in contact with it. The same body might even be made to feel both hot and cold at the same moment, if we made one hand hotter and the other colder than the body,

before we brought them in contact with it. For the same reasons, we pronounce a cave hot in summer, cold in winter, although the temperature of the cave may have remained the same. To measure the correct temperature of bodies, we have recourse to the physical action of heat on bodies. These actions are of various kinds. Expansions and contractions have been adopted, as being the easiest to observe. But heat also produces electrical phenomena in bodies, on which very delicate methods of observing temperatures have been based.

1232. Instruments termed thermometers (from 0ɛpμòc, hot, μέrpov, a measure) are employed to measure temperatures up to about 600° F. Instruments termed pyrometers (from up, fire), are employed to measure temperatures higher than 600° F.

1233. Although all bodies whatever are susceptible of dilatation and contraction, they are not equally convenient for thermoscopic agents. Mercury is the best thermoscopic agent for general purposes, as the range between its solidifying and boiling point is the widest of any liquid, and it has also the most uniform expansion. Alcohol is the only other liquid employed as a thermoscopic agent. It is employed to measure very low temperatures, as it does not solidify at the greatest known cold.

1234. We must refer the student to the general works on chemistry and physics for descriptions of the differential thermometer, the thermo-electric multiplier, the self-registering thermometer, the air thermometer, the pyrometer, -in fact, all the varieties of heat measurers but the ordinary mercurial thermometer. We shall give a brief outline of the plan of making a mercurial thermometer, in order that the student may know how it is made, -not with a view that the student should attempt to make his own thermometer, as he will be able to purchase better thermometers than any he could construct, and at a less cost. We shall also describe how a thermometer ought to be tested as to its accuracy.

1235. Construction.-The first essential in the manufacture of a mercurial thermometer, is a capillary tube of uniform calibre throughout its entire length, so that equal lengths of the tube will contain equal volumes of mercury. To ascertain this, a small drop of mercury is introduced

into the tube, sufficient to fill rather more than a third of an inch of the tube. The mercury is gradually moved from one end of the tube to the other, and its length is measured in each successive position. The length, of course, should be the same in every part of the tube; if it be not, the tube must be rejected. Having selected the tube, the next thing to be done is to blow a bulb upon one end of it. The bulb may be either spherical or cylindrical; the last form, from exposing a large surface, is more readily affected by changes of temperature. The relative dimensions of the bore of the tube and bulb have to be taken into consideration. If the thermometer is intended to have a very open scale-in other words, to be greatly affected by comparatively small variations of temperature, the bore of the tube must be very small, and the bulb comparatively large; but if a less degree of

Fig. 20.

sensibility is required, the bore of

the tube is not required so small, and the bulb is made smaller than in the first case.

1236. Filling the thermometer.— To facilitate the introduction of the mercury, a small funnel is frequently formed upon the other end of the tube; this is filled with mercury, the tube is slightly inclined, and the bulb is warmed by means of a spirit-lamp (see Fig. 20), in order to expand the air in the thermometer; some of the air escapes, the spirit-lamp is removed to allow the tube to cool, the air contracts, and, as a consequence, some of the mercury enters the tube. The bulb is again warmed, so as to expel a further quantity of air, the lamp is again removed, and more mercury enters from the contraction of the air; this warming and cooling the tube is continued until the bulb and part of the tube are filled with mercury. Instead of forming a funnel on the

open end of the tube, and proceeding in the way we have stated, the thermometer may be filled in the following way-The open end of the tube is plunged, after the bulb has been heated, into the mercury; on the contraction of the air some of the metal enters, the mercury in the bulb is then boiled, and the heat is continued until the tube becomes filled with mercurial vapour; the open end is then plunged again into the mercury, and as the vapour contracts the mercury enters, and so the instrument becomes filled. Whichever way the mercury has been introduced, the next thing to be determined is where, on the tube, we desire to fix some point of the thermometric scale. Suppose we desire to have the freezing point of water about 1 inch from the bulb, and the boiling point of water about the same distance from the other end of the tube, we first plunge the instrument in melting ice, and if, when it has attained the temperature of the ice, the mercury stands too high in the tube, some of the metal is expelled by heat; and if it stands too low, some more is introduced, as at the first. When the mercury stands at the proper height in the tube for the freezing point, the thermometer is introduced into boiling water, and when the mercury ceases to rise, the blowpipe flame is directed about an inch above, and the glass is then drawn out into a fine capillary. All traces of air have now to be got rid of; for this purpose the bulb is warmed until the mercury ascends through the capillary, the blowpipe flame is then directed upon the capillary, and the tube is closed and sealed. The thermometer is now tested, to see that all the air has been removed; the mode of doing this will be given when the method for testing is described. If this has been effected, the end is fashioned into a knob or ring, according as it is intended to be fixed in a wooden scale, or graduated upon the stem.

1237. Graduation of the thermometer.-The thermometer requires to be graduated, that is, provided with a scale to which variations of temperature can be referred; and, in order to be able to compare one thermometer with another, it is necessary, in the graduation of thermometers, to select two standard temperatures as fixed points; the temperatures, which have been universally chosen, are those of melting ice and boiling water. The