324 



VAP ORIZA TION. 



are obliged to boil meats and vegetables (if at all) in closed vessels 

 and under a pressure greater than that of the atmosphere. In the 

 arts, a higher temperature than 100 C. is sometimes required for 

 water, as, for example, in the extraction of gelatine from bones. In 

 a closed vessel, water may be raised to a much higher temperature 

 than in the open air, but, for reasons now obvious, water cannot be 

 kept boiling in such a vessel. Papin's Digester consists of a metal 

 vessel of great strength covered with a lid pressed down by a 

 powerful screw. That the joint may be more perfect, a ring of 

 sheet lead is placed between the edges of the cover and of the vessel. 

 It is provided with a safety valve, pressed close by a loaded lever. 

 When the tension of the steam reaches a dangerous point, it opens 

 the valve, lifting the weight, and thus allows some of the steam 

 to escape. 



5O7. Marcet's Globe. Marcet's globe is represented 

 in Fig. 249. It consists of a spherical metallic boiler, five 

 or six inches in diameter, provided with three openings, 

 through one of which a thermometer, T, passes ; through 

 the second of which a glass manometer tube, M, passes ; the 

 third opening being provided with a stop-cock, S. The 

 thermometer and manometer tubes fit their openings so 

 closely that no steam can escape at those points. The 

 thermometer bulb is exposed directly to 

 the steam. The lower end of the manometer 

 tube dips into mercury placed in the lowei 

 part of the globe. The boiler is to be half 

 filled with water and heated until the 

 water boils, the stop-cock being open. As 

 long as the stop-cock is open, the ther- 

 mometer will not rise above 100 C. When 

 the stop-cock is closed, the steam accumu- 

 lates, the pressure on the water increases, 

 the thermometer shows a rise of temperature 

 beyond 100 C. higher and higher as the 

 FIG 2 mercury rises in the manometer tube. 



