CHEMISTRY. 169 



chanically mixed at all times with the air, will be de- 

 composed, and certain elastic vapours and fluids 

 produced, of a deleterious quality, and peculiar smell. 

 The matter here alluded to is very visible to the 

 naked eye in a sun-beam let into a dark room. 



Crucibles. 



The most refractory of all crucibles are those made 

 entirely of clay, coarsely powdered, burned clay 

 being used as a substitute for sand. Such a com- 

 pound resists the action of saline fluxes longer than 

 any other, and is therefore used for the pots of glass 

 furnaces. 



Smoke-devouring Furnaces. 



The improved method of constructing furnaces 

 consists in causing the smoke or flame of fresh fuel, 

 on its way to the chimneys, to pass through, over, 

 or among fuel, which, having already been converted 

 into coke or charcoal, had ceased to smoke, by 

 which expedient the grosser parts of the flam6 or 

 smoke are consumed, or converted into pure flame 

 free from smoke. 



Diurnal Heat. 



The hottest time of the day is generally about two 

 o'clock in the afternoon, and the coldest just before 

 sunrise. In this climate the daily heat may be reck- 

 oned from that of eight o'clock in the morning ; and 

 the month of October is found to have nearly the 

 mean temperature of the whole year. 



Cooling. 



The process of cooling is much retarded by sur- 

 rounding the heated vessel (suppose a tea-pot) at 

 the distance of nearly an inch, with a case of pla- 

 nished tin ; and the addition of other cases, following 



