144 GENERAL SCIENCE 



in the basement sets up convection currents in water that 

 fills pipes within the furnace, and radiators in the rooms 

 to be heated, and the connecting pipes. The heated water 

 goes from furnace to radiator, and then as cooled water from 

 the radiators back to the furnace to be warmed anew. In 

 steam-heating plants and in vapor systems the essential 

 difference from the hot- water system is in having the pipes 

 filled with steam or water vapor, the heat liberated at the 

 radiators being largely heat of vaporization (latent heat) 

 from the steam as it condenses to water in the radiators. 

 Where there are pipes to carry the steam to the radiators, 

 and other pipes to carry away the water of condensation, 

 there is little or none of the disagreeable " bumping" that 

 often occurs when steam is entering a radiator from which 

 water is draining back to the boiler through the same pipe. 



Conduction and convection are modes of transmission of 

 heat by reason of molecular activity in matter. But heat 

 can pass through space destitute of matter, even through a 

 vacuum. Heat from the sun reaches the earth through vast 

 spaces supposed to be lacking in any material medium for its 

 transmission. Scientists assume that heat requires some 

 medium in which it can pass through space. It is assumed 

 to be the case that heat, light, and other forms of energy 

 (see pages 162 and 343) are transmitted through space as 

 pulsations (wave motions) in a so-called "ether" medium, 

 and that these pulses move outward or radiate in every 

 direction and always in straight lines. Such a supposition 

 has sufficient experimental evidence supporting it to make 

 it the most reasonable of all explanations of heat radiation. 



However, this " radiant energy" is not heat until the ether 

 waves fall upon some body whose molecules acquire a quick- 

 ened motion by reason of the absorption of the motion of the 

 ether medium. In this way the temperature of the body is 

 raised as ether energy is transformed into molecular motion 



