fc'4 Chemistty. [Lecture 26. 



shadow of smoke. This circumstance makes ob- 

 jects seem to change place and tremble, when 

 viewed through the rarefied atmosphere of a 

 heated field when the sun shines upon it ; the 

 fluid contained in the earth and plants constantly 

 evaporating, and the vapours having a tendency 

 upwards. It is owing to this tendency upwards 

 that iron or any other body will heat a body held 

 over it sooner than under it. Hence the cold 

 observed at sea upon approaching mountains of 

 ice ; the cold air being condensed falls down the 

 sides of the ice and floats along the surface of 

 the sea. A vessel of water being placed over the 

 fire, the heat expands and renders lighter the 

 bottom of the water, which of course ascends 

 to the top, while another part, denser and colder, 

 falls to the bottom ; and thus there is a constant 

 circulation from the under to the upper part of 

 the vessel. When we have occasion to heal 

 glass all over it is very apt to break, if the fire 

 is not equally applied. For this end we immerse 

 the vessel in water, using the operation of the 

 balneum maria?, as it is improperly called. 



From the nature of fluids, deep lakes of water 

 do not freeze in the winter : the cold air rushing 



O 



over the surface, a portion of the water at the 

 top being thus rendered heavier will sink, and 

 its place be supplied with a warmer portion from 

 below, which in its turn must also be cooled. 

 Thus the cold air has the whole heat of the 

 water to carry away, which is frequently not 



