6. Heat may be transmitted by radiation, by conduction, and by- convection.

Radiation. The tendency of heat is to pass away from a warm body instantaneously, and with equal energy in all directions. This manner of transit is called radiation. Strictly speaking, all heat is radiant heat, because it invariably proceeds by radiation when it is not obstructed or retarded by the medium or material through which it passes. All known materials retard the transmission of heat to a greater or less degree. Thus, dry air permits heat to pass through it with very little obstruction, but wood offers great resistance.

If a person attempts to warm himself at a blazing fire, out of doors, on a cold day, he may be scorched on one side by the radiant heat of the fire at the same time that he is almost frozen on the other side by the cold air which surrounds him. The air which is between him and the fire thus permits the heat to pass through it without raising its temperature to any considerable degree.

## 8. Intensity

Intensity. The law which governs the intensity of radiant heat is as follows:

The temperature is inversely proportional to the square of the distance from the source of heat.

Thus, the heat which is received upon a surface 1 foot square at a distance of 5 feet will diverge and cover a space of twice that width and height, having four times the area at a distance of 10 feet; the heat, being spread over four times as much surface, can have only one-fourth the intensity.

## 9. Reflection

Reflection. Heat may be reflected, dispersed, or concentrated by means of mirrors or lenses, in the same manner as light. The following table shows the reflecting power of various substances when the heat rays fall upon them at an angle of 90°, in percentage of the total radiant heat received:

 Per Cent. Polished silver.... 97 Polished brass...... 93 Polished copper..... 93 Polished steel..... 83
 Per Cent. Polished zinc...... 81 Polished iron.... 77 Bright tin...... 85 Glass........ 10

Thus, polished silver will reflect 97 per cent. and will absorb 3 per cent. of the radiant heat falling upon it. The metal will slowly become warmed by the heat absorbed.

## 10. Conduction

Conduction. When the transmission of heat through any certain substance requires a measurable amount of time, the manner of transmission is called conduction. Thus, it becomes clear that the distinction between radiation and conduction has regard to the rapidity of the transmission, radiation being instantaneous transmission, and conduction being merely retarded transmission. All known substances will conduct heat to a measurable extent, but the rapidity of the conduction varies greatly in different materials. Thus, substances are classed as good conductors, or bad conductors, according to the rapidity with which they will conduct heat.