Page:The New International Encyclopædia 1st ed. v. 04.djvu/77

CALORIC ENGINE. 55 the ashes on metal pistons and cylinders, he used oil pistons. Following Cayley and Arnott, a number of inventors worked on the idea, among them being the Americans Stephen Wilcox, S. H. Koper, and Philander Shaw, each of whom built and sold a number of engines. Broadly speak- ing, the difficulties of operating these engines were so great that they recorded a general fail- ure. These difficulties were caused by ttuedust and the grit in the cylinders, the rapid destruc- tion of the working surfaces and valves by the intense heat, and the practical impossibility of lubrication. They were also more bulky than other t-pes of hot-air engines in proportion to the power developed. The possibilities of the hot-air engine as a competitor of the steam-engine are often urged, but so far it has never reached any practical suc- cess which warranted much hope that the compe- tition would prove serious. The two sides of the question are fairly and concisely summarized by Prof. F. R. Hutton as follows: '"The hot-air engine in small sizes is more eco- nomical than the steam-engine of the same capac- ity. In larger sizes it has about the same economy as the less economical steam-engine, measured in coal consumed i>er liorse-power. It has the advantage of avoiding the steam-boiler as a magazine or reservoir of energj' which may be liberated by accident so suddenly as to be explosive. It can be run by less skilled and expensive labor, and no steam-runner's license is demanded. It is safe and odorless. The objec- tions to the hot-air engine are the greater bulk and greater weight for the same power than is required with the steam-engine; the low mean pressure with high initial pressure, which latter comijels great strength of structure; the de- terioration of heating surfaces exposed to high heats and consequent oxidation; the difficulties of packing and lubricating at high temperatures; the difficulty of regulation closely to varying resistances. If there is any danger to the present supremacy of the steam-engine, it will be in rela- tively small plants that a hot-air engine can be a substitute; the gas or internal-combustion en- gine is more to he feared than the hot-air engine proper." A theoretical and practical discussion of hot- air engines and heat-engines generally is con- tained in Hutton, Heat and Heat-Engines (Xew York, 189')). CAL ORIMrETRY (from Lat. color, heat + Gk. ^Tpo^, (nefroH, measure) . The science of the measurement of quantities of energy when mani- fested by heat effects. By the name 'heat effects' is meant the changes produced in material bodies when they are exposed to what is called a 'source vi heat,' e.g. a llame or the rays of the sun. Among these changes which may take place are expansion, fusion, evaporation, alteration in elec- trical and magnetic i)roperties, etc. It is now believed that these changes are occasioned by increase in the energy of the smallest portions of the bodies. When a body is 'heated' or 'warmed,' we mean that its minute parts gain energy; and opposite changes, e.g. freezing, condensation, cooling, etc., take place when these parts lose energy. It is the province of calorimetry to measure these amounts of energy gained or lost. The erg (see Mecua.nic.l Units) is the unit of energy and work, and therefore all quantities of energy should be measured in terras of it ; but CALORIMETRY. it rarely happens that heat effects are due direct- ly to mechanical work except in case of friction. Consequently the erg is not a convenient unit. Heat effects and the energy required to produce them are almost invariably compared with one definite heat effect, viz. rise in temperature of water; and the practical unit emplo3'ed for measuring thermal energy may be defined as the quantity of energy required to raise the tempera- ture of one gram of water from 15° to 16° C. on the thermometric scale of the constant-pres- sure hydrogen thermometer. (Other definitions of a practical unit have been pr(ji)osed. e.g. bv the substitution of 20° to 21° in place of 15° to 16° C. ; or the one-hundredth portion of the quan- tity of energy required to raise the temperature of one gram of water from the freezing-point to the boiling-point under normal pressure.) This practical unit is called the calorie, and its value is very nearly 4.187 joules, or 4.187 X 10' ergs. See He.t. By the 'specific heat' of a substance at a given temperature and under definite conditions is meant the number of calories required to raise the temperature of one gram of the substance one degree by the hydrogen scale (see Ttiekuome- ter), at that temperature, and under those con- ditions. By the 'latent heat' of a substance for a definite change of state (e.g. fusion, evapora- tion, sublimation, dissociation ) , under definite conditions, is meant the number of calories re- quired to produce the particular change of state in one gram of the substance under the specified conditions. Thus we speak of the 'specific heat of air at constant pressure,' or the "latent heat of evaporation of water at normal atmospheric pressure.' In general, however, we can learn simply the average specific heat, i.e. the number of calories required to raise the temperature of one gi-am through t degrees, divided by t. Calo- rimetry is. then, chiefly, the science of measuring specific and latent heats. There are two general methods for the meas- urement of specific heats, which may be regarded as satisfactory — the method of mixtures and the use of an ice or a steam calorimeter. In the method of mixtures a known quantity of the substance at a known temperature is mixed with a known quantity of some liquid at a different k-nown temperature and the temperature of the mixture is observed. The specific heat of the liquid for the given range of temperature being known, and allowance being made for losses by radiation and conduction, and for the calories spent in changing the temperature of the vessel containing the liquid, the specific heat of the substance may be at once deduced. The most improved form of apparatus for use in this method is that of Prof. T. A. Waterman, a full description of which is given in the Physical Review, Vol. IV., p. 161 (18!)6). In the ice-calorimeter, the substance whose specific heat is desired is inti'oduced into an apparatus which allows the heat energy with- drawn from the body to be spent entirely in melt- ing ice. The change in temperature of the sub- stance and the quantity of ice melted may be observed; and thus, assuming that the latent heat of ice is known, the specific heat of the sub- stance may be calculated. This method is due to Black; and the most improved apparatus is that designed by the late Professor Bunsen, of Heidelberg. The most accurate method of using