temperature, the divisions on the sliding scale expand with rise in temperature and in a different ratio for the three metals specified, thus necessitating a special sliding scale for each metal of which the cylinders are composed. Instructions.-The temperature of a furnace, &c., is ascertained in the following manner : A pint (o 568 litre or 34'66 cubic inches) of clean water is placed in the pyrometer vessel, and, after this has stood for a few minutes, the zero point of the sliding scale is set at the temperature indicated by the thermometer. It One of the metal cylinders, d, is then exposed from two to ten minutes to the heat to be measured, and allowed to remain in it until it has acquired its temperature. is then quickly withdrawn and dropped into the water, the temperature of which rises gradually until a maximum is reached. This rise of temperature, as indicated by the sliding scale, added to the temperature of the water at the end of the experiment, gives that of the furnace. The range of the pyrometer with copper and iron cylinders extends to 1000 C. or 1800° F., but with platinum cylinders to 1500° C. or 2700° F. Cylinders of copper are found to be most suitable for general use in ascertaining the temperature of ordinary furnaces, while for very high temperatures those of platinum are alone available. Wrought-iron cylinders are sometimes employed, as they offer an advantage over those of copper in having a higher melting point, and being less liable to alteration in weight through loss by incrustation when plunged at a high temperature into water. This advantage may, however, be considered as counterbalanced by the fact that copper is much less affected by the corrosive action of the gaseous products of combustion. For the reasons just given, platinum cylinders offer still further advantages. 3. The third method of estimating high temperatures, viz., that based on the melting-points of metals or metallic alloys, is applied by simply suspending in the heated chamber or furnace a small piece of metal, the melting point of which is known, and, if necessary, two or more pieces of different melting-points, so as to ascertain the temperature within certain limits, according to the pieces which are melted and those which remain unmelted. This is the rough and ready method most frequently adopted by workmen in iron, steel, and other smelting works. LECTURE IV.-QUESTIONS. 1. Define the temperature of a body. What two natural phenomena have been employed to determine two points of reference in the scale of thermometers? And why? 2. State what is meant by " temperature." Describe how ordinary temperatures are measured. How many scales of temperature are there in common use in Europe? A thermometer registers 400° on the Fah. ; find what it would indicate on a Cent. one. Ans. 204°4 C. 3. Compare the Fah., Cent., and Réau. scales. A Cent. thermometer indicates 15°; show by proportion (in full) how you find what are the corresponding readings in the Fah. and Réau. scales. Ans. 59° F.; 12° R. 4. Zinc boils at 1204° F., mercury at 676° F.; change these readings to Cent. (show your work in full). Ans. 651° C., and 358° C. 5. Sketch and describe concisely the construction and action of Siemens' water pyrometer. 6. State the principles upon which four distinct classes of pyrometers are based. LECTURE V. CONTENTS.-Effects of Heat-Unit of Heat-Quantity of Heat-Capacity for Heat-Specific Heat-Table of Specific Heats of Substances. Effects of Heat.-When heat is applied to a body it produces various effects. For example, in most instances it raises the temperature of the body, it generally alters its volume or its pressure, and in certain cases it changes the state of the body from solid to liquid, or liquid to gaseous. Unit of Heat.-The standard unit now adopted in this country is called The British Thermal Unit, and is the quantity of heat required to raise 1 lb. of water by 1° Fah., when at its maximum density, i.e., from 39°1 to 40°•1 Fah. Quantity of Heat.-To be able to compare different quantities of heat, we must first fix on a standard or unit of heat. This is done by selecting a standard body, and noting the effects of heat upon it. For example, we might take a pound or other known weight of ice at its freezing or melting point, 32° F. or o° C., and apply heat to it, until it all melts into water at the same temperature. This would give a definite standard, by which to compare other quantities of heat applied in the same way. Or, we might take a known weight of water at its boiling point, and apply heat to it until it all becomes converted into steam at the same temperature as the boiling water. This method is used in determining the quantity of heat obtained from different kinds of coal. We often find it stated, in connection with trials of steam boilers, that so many pounds of water were converted into steam at a certain pressure per pound of coal of a certain quality. Capacity for Heat.-The capacity for heat of a body (or its thermal capacity) is the number of units of heat required to raise that body one degree in temperature. Ι Specific Heat.-The specific heat of a body is the ratio of the quantity of heat required to raise that body one degree, to the quantity required to raise an equal weight of water one degree in temperature. To illustrate the above, take the case of lead. Weigh out I lb. of sheet lead, roll it into an open spiral, and attach it to a string. Now, dip the lead into a pot of freely boiling water until it has attained the temperature of the water. While this is going on T weigh out a pound of cold water, and ascertain its temperature with a thermometer; say it is 47° F. Then lift the lead from the boiling water, and, while holding it by the string in the steam rising from the water, allow all water to drop from it, and immerse it quickly in the cold water vessel, keeping it moving by means of the string, so as to bring it intimately into contact with every portion of the water, as shown in the annexed figure, where, L, is the lead, and, T, the thermometer. Observe the gradual rise in temperature of the water due to the heat passing from the lead, note the point at which it ceases to rise, and suppose that to be 52° F. We have thus ascertained data, from which we may calculate the relative capacities for heat of lead and water, if none of the heat from the lead was given to any other body than to the water. Thus-The diminution in temperature of the lead from 212° to 52° 160°; the increase in temperature of the water from 47° to 52° = 5°. Now, since The Loss of Heat from the one substance by the other. .. The heat from 1 lb. of lead falling 160° = the heat imparted to 1 lb. of water raised 5°; The units of heat in 1 lb. of lead = 5 = I 160 32 1 In other words, the capacity for heat of lead is only 2 part that of water, or the same quantity of heat would raise 1 lb. of lead through 32 times as many degrees as it would 1 lb. of water. We may now apply the knowledge we have gained in this lecture to prove the rule for using Wilson's pyrometer, as given in our last lecture. Observe, Wilson plunges a known weight of platinum (for the sake of illustration, assume it to be 1 lb.) at an unknown temperature, t,, into double its weight of water (say 2 lbs.), and notes the rise in temperature, t, to to, from which he calculates the original temperature, t,, of the platinum, and, therefore, of the furnace from which it had been taken. Thus |