66 



proportion between its length and diameter, as lie found that 

 if it were lengthened beyond a certain limit the draught was 

 diminished, and that a wide chimney might be longer than a 

 narrow one. Tliis fact depends on the greater loss of heat in the 

 narrow tube, the cooling surface is as its diameter, but the air 

 to be cooled as its diameter squared, so that the latter dimin- 

 ishes much faster than tlie former. Where the tube was six 

 • inches wide, it could not be used longer than twelve feet, but 

 much heat was last, for it was composed of sheet iron which 

 was heated to redness. But though his reasoning was bad, his 

 practice was excellent, and his furnace was the best which had 

 then been constructed. De Morveau found that it was capable 

 of fusing Manganese and softening Platuia; but it nmst be re- 

 marked, that at that very time a connnittee of the institute, who 

 were appointed to repeat Clouet's experiments on steel, could not 

 fuse iron in it, and found that Wedgewood's Pyrometer marked 

 152. Baume's furnace was a hollow tube witliout any contrac- 

 tion, with a grate at its bottom, and he found it superior to any 

 with which he was acquainted ; but though no other chemist has 

 adopted it, except Saussure, there is no question that it is the best 

 form which has yet been invented. Since that time little has 

 been done by chemists towards the improvement of these instru- 

 ments : the English have acquiesced in the conmion mode of 

 constructing them, and from the great superiority of coke to char- 

 coal, their air furnaces produce, with ease, a higher heat than 

 is necessary for most operations, so that they have not been com- 

 pelled by this consideration to enquire into their defects. What- 

 ever alterations have been made here, regard the convenience of 



