76 MOLECULAR VIBRATION AS HEAT. 



dium pervading the particles of the atmosphere, 

 so as to reach the nerves of sight, 

 feeling, and hearing. This is veri- 

 fied by Trevelyan's experiment with 

 a semicylindrical piece of heated 

 brass, B, resting on a piece of cold 

 lead, L, Fig. 15. The lead being 

 a slow conductor of heat, the vibrations of the 

 particles produce a rocking of the brass, B, like a 

 cradle, and at the same time an audible vibration 

 of the air, like that of the glass plate of a harmon- 

 icon. The vibrations produced by combustion of 

 a jet of gas within a glass tube cause a similar 

 musical sound. 



It is the rapid and violent vibration of the 

 molecules of hot bodies that causes the stinging 

 sensations they produce, like those by a blow of a 

 rod on the skin, with the result of similar con- 

 tusions and blisters of the epidermis. Scalding 

 and burning of the skin are wounds produced 

 by the molecular vibration of heated bodies. 



If the vibration of sunshine be intensified by 

 lenses upon mercury to 680 Fahrenheit, it boils 

 like water ; and the particles are separated so far 

 asunder as to occupy more space than an equal 

 weight of particles of surrounding air, and to as- 

 cend buoyantly as mercurial vapor. On the con- 

 trary, if the vibration, or heat, be reduced to 39 

 below zero (Fahrenheit), the terrestrial electro- 

 magnetic currents predominate, polarizing and 



