Engineers' and Mechanics' Pocket-book ...

2. The specific heat of a body varies with its form. A solid has a less capacity for caloric than the same substance when in the state of a liquid; the specific heat of water, for instance, being 9 in the solid state, and 10 in the liquid.

3. The specific heat of equal weights of the same gas increases as the density decreases; the exact rate of increase is not known, but the ratio is less rapid than the diminution in density.

4. Change of capacity for caloric always occasions a change of temperature. In crease in the former is attended by diminution of the latter, and vice versa.

TABLES of the Specific Heat of various Substances.

The specific heat of the foregoing compared with that of an equal

quantity of water.

ILLUSTRATION.-If 1 lb. of coal will heat 1 lb. of water to 1000,

lb. will heat 1 lb. of mercury to 1000.

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The term Capacity for heat means the relative powers of bodies, in receiving and retaining heat, in being raised to any given temperature; while Specific applies to the actual quantity of heat so received and retained.

When a body has its density increased, its capacity for heat is diminished. The rapid reduction of air to of its volume evolves heat sufficient to inflame tinder.

TABLE showing the relative Capacity for Heat of various Bodies.

LATENT CALORIC is that which is insensible to the touch, or incapable of being detected by the thermometer. The quantity of heat necessary to enable ice to assume the fluid state is equal to that which would raise the temperature of the same weight of water 1400; and an equal quantity of heat is set free from water when it assumes the solid form.

If 54 lbs. of water, at the temperature of 320, be placed in a vessel, communicating with another one (in which water is kept constantly boiling at the tempera

ture of 2120,, until the former reaches this temperature of the latter quantity, then let it be weighed, and it will be found to weigh 64 lbs., showing that 1 lb. of water has been received in the form of steam through the communication, and reconverted into water by the lower temperature in the vessel.

Now this pound of water, received in the form of steam, had, when in that form, a temperature of 2120. It is now converted into the liquid form, and still retains the same temperature of 2120; but it has caused 51⁄2 lbs. of water to rise from the temperature of 320 to 2120, and this without losing any temperature of itself. It follows, then, that in returning to the liquid state, it has parted with 5 times the number of degrees of temperature between 320 and 2120, which are equal 1800, and 1800X59900. Now this heat was combined with the steam; but as it is not sensible to a thermometer, it is called Latent.

It is shown, then, that a pound of water, in passing from a liquid at 2120 to steam at 2120, receives as much heat as would be sufficient to raise it through 990 thermometric degrees, if that heat, instead of becoming latent, had been Sensible.

The sum of the Sensible and Latent heat of Steam is always the same at any one temperature; thus, 9900+212° =1202°.

If to a pound of newly-fallen snow were added a pound of water at 172°, the snow would be melted, and 32° will be the resulting temperature.

SENSIBLE CALORIC is free and uncombined, passing from one substance to another, affecting the senses in its passage, determining the height of the thermometer, and giving rise to all the results which are attributed to this active principle.

To reduce the Degrees of a Fahrenheit Thermometer to those of Reaumur and the Centigrade.

FAHRENHEIT TO REAUMUR.

RULE.-Multiply the number of degrees above or below the freezing point by 4, and divide by 9.

Thus, 2120-32 =180X4720÷÷980, Ans.

+240-32

8X4 3293.5, Ans.

FAHRENHEIT TO CENTIGRADE.

RULE.-Multiply the number of degrees above or below the freezing point by 5, and divide by 9.

Thus, 212°-32=180x59009100, Ans.

MEDIUM HEAT of the globe is placed at 50°; at the torrid zone, 75°; at moderate climates, 50°; near the polar regions, 36°.

The extremes of natural heat are from -70° to 120°; of artificial heat, from -91° to 36000°.

EVAPORATION.

Evaporation produces cold, because heat must be absorbed to form vapour.

Evaporation proceeds only from the surface of the fluids, and therefore other things equal must depend upon the extent of surface exposed.

When a liquid is covered by a stratum of dry air, evaporation is rapid, even when the temperature is low.

As a large quantity of caloric passes from a sensible to a latent state during the formation of vapour, it follows that cold is generated by evaporation.

CONGELATION AND LIQUEFACTION.

Freezing water gives out 1400 of heat. Water may be cooled to 200. All solids absorb heat when becoming fluid.

The particular quantity of heat which renders a substance fluid is called its caloric of fluidity, or latent heat.

The heat absorbed in liquefaction is given out again in freezing.

Fluids boil in vacuo with 1240 less of heat than when under the pressure of the atmosphere. On Mont Blanc water boils at 1870.

DISTILLATION.

Distillation is the depriving vapour of its latent heat, and, though it may be effected in vacuo with very little heat, no advantage in regard to a saving of fuel is gained, as the latent heat of vapour is increased in proportion to the diminution of sensible heat.