[ 33 J 



ibrce ; and their joint agencies occasion the motion of 

 the heavenly bodies. Cohesion and chemical attrac- 

 tion are opposed by the repulsive energy of heat, and 

 the harmonious cycle of terrestrial changes is pro- 

 duced by their mutual operations. 



Heat is capable of being communicated from one 

 body to other bodies; and its common effect is to 

 expand them, to enlarge them in all their dimensions. 

 This is easily exemplified. A solid cylinder of metal 

 after being heated will not pass through a ring barely 

 sufficient to receive it when cold. When water is 

 heated in a globe of glass having a long slender neck, 

 it rises in the neck ; and if heat be applied to air con- 

 fined in such a vessel inverted above water, it makes 

 its escape from the vessel and passes through the wa- 

 ter. Thermometers are instruments for measuring 

 degrees of heat by the expansion of fluids in narrow 

 tubes. Mercury is generally used, of which 1OO,OOO 

 parts at the freezing point of water become 101,835 

 parts at the boiling point, and on Fahrenheit's scale 

 these parts are divided into 1 80 degrees. Solids, by 

 a certain increase of heat, become fluids, and fluids 

 gasses, or elastic fluids. Thus ice is converted by heat 

 into water, and by still more heat it becomes steam : 

 and heat disappears, or, as it is called, is rendered 

 latent during the conversion of solids into fluids, or 

 fluids into gasses, and reappears or becomes sensible 

 when gasses become fluids, or fluids solids : hence 

 cold is produced during evaporation, and heat during 

 the condensation of steam. 



p 



