69. Air cannot be heated by radiation; it can only be heated by conduction-that is, by direct contact with heated surfaces; or by currents of hot air-that is, by convection.

The number of heat units transmitted per hour from 1 square foot of surface, for each degree of difference in temperature between the fluids upon the opposite sides of the heating surface, is called the coefficient of transmission of that heating surface.

70. In experimenting with the condition of heating surfaces, it is found that, with surfaces of various kinds, the rate of emission is about as follows, the total emission from a new cast-iron plate having a natural surface, as cast, being taken as 100:

Cast iron, new...................

100

Cast iron, rusty...................

102

Wrought iron, ordinary or "black".........

93

Wrought iron, bright, but not polished..................................

72

Surface covered with lampblack, dull.........

106

Surface covered with white-lead powder, dull.....

106

It is found, also, that the rate of emission is affected, by painting or bronzing, about as follows, the amount given off without paint being taken as 100:

Two coats of asphaltum paint.........................................

106

Two coats of white-lead paint, dull.....................................

109

Rough bronzing ..................................................

106

One coat of glossy, white paint ......................................

90

This last item shows the effect of a glossy or polished surface in reducing the emission of heat.