USED IN CHEMISTRY. 11 



provement has been made upon this instrument, in 

 Scotland, within these few years. This improved 

 apparatus is known by the name of the Scotch 

 still, a section of which is represented, Fig. 6. The 

 principle of the improvement consists in exposing 

 a great quantity of the surface of the fluid to the 

 action of the fire, and affording a more ready means 

 for the escape of the vapour or gas. 



A, is the body of the still, made very shallow 

 and concave at the bottom, in order that the fire 

 may act better upon it ; bb, are a number of tubes 

 opening into the still, and communicating with the 

 neck of the still B, in order to convey the vapour 

 off as soon as it is formed ; cc 9 is a cover that shuts 

 down over the pipes and top of the still, to keep it 

 warm, by preventing the loss of heat which would 

 be occasioned by the contact of the cold air. This 

 is effected by the quantity of air that is confined 

 between the cover and the top of the still ; for it 

 is a fact which is now well known, that confined air 

 is a non-conductor of heat. In general, the heads 

 of stills are kept warm by laying blankets upon 

 them, at least when this is attended to, as it ought 

 always to be ; but this metallic covering, by sur- 

 rounding the still with a quantity of confined air, 

 answers the purpose still better. 



When the materials which are evaporated con- 

 crete in a solid form, within the neck of the dis- 

 tilling vessels, then the distillation is more properly 

 called sublimation. 



By the above means, one fluid may be separated 

 from other materials; but it often happens, that in 

 distillation the substances which are subjected to 

 this process have a chemical action upon each 

 other ; new combinations take place, and perma- 



