Chemical Apparatus. 113 



cury in the bulb of the thermometer; yet we 

 have reason to believe that the cold is limited. 

 To retard the entrance of heat, the mixture ought 

 to be wrapped in wool : the cold remains at its 

 greatest degree while any of the ice or snow re- 

 mains unmelted. 



V. I may mention the heat produced in Fer- 

 mentation, though this mode of applying heat 

 is chiefly employed in promoting vegetation, and 

 in Egypt in hatching eggs. In chemistry it is 

 used where a long and gentle heat is to be ap- 

 plied, and was much in repute with the old che- 

 mists. Horse dung, and tanner's bark after it 

 has been usefl, have a degree of heat equal to 120 

 degrees or more. The horse dung rises soonest 

 to this degree, but it flies off in about a month ; 

 the bark retains it for some months. It is ne- 

 cessary that there should be a proper quantity of 

 moisture, and that it should also be in a compact 

 heap ; otherwise it has too much surface for its- 

 bulk, and the heat is dissipated. 



VI. The Rays of the Sun. When a subject is 

 exposed to them, a degree of heat sometimes 

 higher than that by fermentation is produced, 

 but it is not so constant. It is often used for 

 the promotion of the action of fluids upon so- 

 lids, and is then called Insolation. It is used 

 for the dissipation of moisture ; and thus vege- 

 tables are sometimes dried. It has been erro- 

 neously thought that it was better to dry them in 

 the shade; but by experience it is found that 

 they are always better cured by a quick heat than 



