84 



Morveau asserts that one of this form performed as well as a 

 cylinder of twice the length. A more certain method of increasing 

 the draught where the height of the chimney is limited might be 

 the application of a small furnace to heat the interior of the chimney 

 by its flame, it should open into it immediately above the flue 

 and be constructed like the fire place of a reverberatory. On the 

 ftu'nace itself little is to be said, it should be cylindrical, formed 

 of a double row of fire bricks moulded according to its figure and 

 arranged in a case of strong sheet iron ; these, from the species of 

 arch which they form, possess great strength and are not loosened by 

 the alternate expansion and contraction to which they are liable, 

 such an instrument has been in my possession for some years and 

 is still uninjured, it was not however constructed on proper prin- 

 ciples as to its flue and I have ceased to use it, its bricks «ere 

 to prevent pyrometric contraction, (or that which Wedgewood 

 observed in clay when heated) baked in a violent heat, and no 

 subsequent shrinkage has taken place. Tlie parabolic and ellip- 

 tic shapes which the old chemists extolled so highly are useless, 

 the reflection and radiation of caloric take place only in trans- 

 parent media, and the crucible which occupies the centre of a 

 furnace is effectually screened by the surrounding charcoal from 

 any influence of its walls, and the only direction which remains 

 is, that its depth from the bottom of the flue to the grate should be 

 at least twice its diameter. If these principles be observed, the 

 chemist will be satisfied with their result : in a furnace whose area 

 was 99 inches and its chimney narrower than it should have been, 

 I have obtained intense heats, on one occasion I obtained a 

 button of manganese which weighed 115 grains, after 2 liours 



