half an inch in length and finer than the bore of the thermometer into the tube above the mercury, in a mercurial thermometer. The thermometer is placed with its stem in a horizontal position, and the steel index is brought into contact with the extremity of the column of mercury. Now when the heat increases and the mercury expands the steel wire will be thrust forward; but when the temperature falls and the mercury contracts the index will be left

behind, showing the maximum temperature. For showing minimum temperature a spirit thermometer prepared in a similar manner is used, as the spirits in contracting draw the index with the alcohol because of the capillary adhesion between the alcohol and the glass; but when the alcohol expands it passes by the index, without displacing it, so that its position shows the lowest temperature to which the instrument has been subjected.

II. Use of Thermometers. In the use of thermometers for determining the temperature of the air, they should be exposed to unobstructed circulation in a dry

THERMOMETER.

MOMETER-CUP.

place and in the shade. Any FIG. 5.-STEAM- FIG. 6.-THERdrops of moisture on the bulb of the thermometer tend to evaporate and lower the temperature. For determining the temperature of steam or water under pressure thermometers are set into a brass frame so that they will screw directly into the liquid (Fig. 5) without permitting leakage. In other cases the thermometer can be inserted into a cup made as shown in Fig. 6. Cylinder-oil or mercury is put into the cup, and the reading of the thermometer will then indicate the temperature of the surrounding

fluid. When the thermometer is inserted into a cup some time will be required to obtain the correct temperature. The temperature of steam-pipes or hot-water pipes cannot be obtained accurately by any system of applying the thermometers externally to the pipes, and in case thermometers are used they should be set deep into the current of flowing steam or water, not placed in a pocket where air can gather.

12. Specific Heat.-The capacity which bodies have of absorbing heat when changing temperature varies greatly; for instance, the same amount of heat which would raise one pound of water one degree in temperature would raise about 8 pounds of iron 1 degree in temperature or would raise I pound 8 degrees in temperature. The term used to express this property of bodies is specific heat, which is defined as follows: Specific heat is the quantity of heat required to raise the temperature of a body one degree, expressed in percentage of that required to raise the same amount of water one degree, or in other words with water considered as one. Specific heat can always be found by heating the body to a given temperature, cooling it in water, and noting the increase in temperature of water. Thus if I pound of iron in cooling 8 degrees heats one pound of water one degree, its specific heat is 0.125. A table of specific heats of the principal materials is given in the back of the book, from which it will be seen that the specific heat of water is greater than that of any other known substance.

A knowledge of the specific heat of various materials is of considerable importance in the design of heating apparatus, since it indicates the capacity for absorbing heat without increase of temperature. The heat which is absorbed in raising the temperature of a body is all given out when the body cools, so that although there is a difference in the amount absorbed, there is no difference in the final result due to heating and cooling.

The total heat which a body contains is equivalent to the product obtained by multiplying difference of temperature, specific heat and weight. The results will be expressed in heat-units or in capacity of heating one pound of water.

The specific heat of bodies in general increases slightly with the temperature, the values in the table being true from 32° to 212°.

13. Latent Heat.-When heat is applied to any liquid the temperature will rise until the boiling-point is reached, after which heat will be absorbed; but the temperature will not change until the entire process of evaporation is complete, or until the liquid is all converted into vapor. The heat absorbed during evaporation has been termed latent, since it does not change the temperature and its effects cannot be measured by a thermometer. In the evaporation of water about five and one-half times as much heat is required to evaporate the water when at 212 degrees, into steam at the same temperature, as to heat the water from the freezing to the boiling point. Heat stored during evaporation is given out when the vapor condenses, so that there is no loss or gain in the total operation of evaporating and condensing. A similar storage of heat takes place when bodies pass from the solid to the liquid state, but in a less degree. Although similar in some respects, latent heat differs in nature from specific heat. In both cases, heat not measured by the thermometer is stored; when the temperature is lowered the stored heat is given up in both cases: in the first it represents a change in the physical condition, as from a solid to a liquid or a liquid to a gas; in the second the condition remains unchanged.

14. Radiation.-Heat passes from a warmer body to a colder by three general methods, each of which is of considerable importance in connection with the methods of heating. These methods are radiation, conduction, and convection. The heat which leaves a body by radiation travels directly and in a straight line until it is intercepted or absorbed by some other body. Radiant heat obeys the same laws as light, its amount varying inversely as the square of the distance, and with the sine of the angle of inclination. The amount of radiant heat which is emitted or which is absorbed depends largely, if not altogether, upon the character of the surface of the hot and cold body; it is found by experiment that the power of absorbing radiant heat is exactly the same as that of emitting

it. The relative amount of heat emitted or absorbed by different surfaces is given in the following table.

Radiant heat passes through gases without affecting their temperature or being absorbed to any appreciable extent. It is probably true that a very large body of air, especially air

Q

M

P

P

K

containing watery vapor, does absorb radiant heat, for it is known that the earth's atmosphere intercepts a sensible proportion of the heat radiated from the sun.

15. Reflection and Transmission of Radiant Heat.-Radiant heat, like light, may be reflected and sent in various directions by materials

FIG. 7.-REFLECTION OF HEAT. of various kinds. Thus in Fig. 7 heat radiated from K is reflected to L, and vice versa. The following table shows the proportion of radiant heat which would be reflected by various substances:

Radiant heat also possesses the property of passing through certain substances in very much the same manner that light will pass through glass. This property is called diathermancy. The following table gives the diathermanous value of various substances, the heat being obtained from a lamp. The transmission power varies with the source of heat.

1

SOLIDS.

Colorless Glass 1.88 mm. thick.

PER CENT OF HEAT TRANSMITTED THROUGH DIFFERENT SUBSTANCES.

WHEN RECEIVED FROM AN ARGAND LAMP (DESCHAUD'S PHYSICS).

LIQUIDS 9.21 MM. THICK.
Colorless Liquids.

16. Diffusion of Heat.- Various materials possess the property of reflecting the radiant heat in such a manner as to diffuse it in all directions, instead of concentrating the heat in any one direction. If the heat were all returned, the temperature of the body would not rise, but would remain constant. The diffusive power as determined by Laprovostaye and Desains was found to be as follows for the following substances, the heat received being 100:

White-lead.....

Powdered silver

Chromate of lead..

.82

.76

.66