HEAT OR CALORIC. 89 



degree near the boiling point is divided into 1000 parts. Each de- 

 gree of Fahr. is equivalent to 0.689 of an inch of the barometer, in- 

 dicating an elevation of 530 feet. The 1000th part of a degree in 

 Wollaston's thermometer is therefore equivalent to about six inches, 

 and the height of a common table produces a manifest difference 

 in the boiling point of water.* 



This delicate instrument therefore answers the purpose of a bar- 

 ometer, it being necessary only to make water boil in order to deter- 

 mine the elevation of the place. 



The boiling point of water is raised by having salt dissolved in it, 

 and the steam has the temperature of the boiling fluid, and so in other 

 cases.f 



(o.) Slight variations of pressure may be exhibited in glass vessels. 

 Boil water in a flask until the air is all expelled by the steam ; 

 cork it while boiling ; if tight, it will continue to boil, and the more 

 rapidly, if it be cooled, as by touching it with or immersing it in cold 

 water, and the boiling will be repressed or stopped by hot water. 



In a retort corked in the same manner, the same phenomena are 

 still more strikingly exhibited ; the water, if shaken after all is cold, 

 falls like lead, thus illustrating the principle of the water hammer.f 



Water, boiled in a flask, furnished with a stop cock, has its ebulli- 

 tion repressed by closing the key for a very short time ; on opening 

 it, it boils violently again, and so vice versa. This must be done 

 with caution, the operator avoiding exposure both to the mouth and 

 bottom of the vessel. All these effects depend on variations of pressure. 



(P.) GREAT VARIATIONS OF PRESSURE ARE SAFELY EXHIBITED 



IN STRONG METALLIC VESSELS. 



In Papin's digester, or any strong boiler, fitted with a cover, stop- 

 cocks and valve, the vapor of boiling water or other fluids may 

 be confined ; then the temperature of the fluid will rise as the pres- 

 sure increases, and the ebullition will be repressed or stopped. Wa- 

 ter may be heated in this manner to 400 of Fahr. or more ; the 

 danger of explosion is of course greater in proportion to the heat ; 

 the machine being suddenly opened, a jet of steam rushes out with 

 great violence, and the temperature of the water falls. 



Mr. Southern's table of pressure and temperature is copied from 

 Henry. || 



Henry, and Phil. Trans. t Eng. Quar. Jour. Vol. XVIII. 



t This is owing to the want of atmospheric resistance, and shews that rain would 

 fall like shot if it were not resisted hy the air. 



As formerly helieved, although now controverted by Mr. Perkins ; see Jones' 

 Journal, and American Jour. Vol. XIII, p. 52. Mr. Perkins thinks that the pres- 

 sure of steam will not be in proportion to the temperature, unless there be an abund- 

 ant supply of water to generate new steam and thus add to the quantity. Aside 

 from this, the steam is no more expanded "by increased heat, than air or any other 

 elastic fluid would be. || Vol. I, p. 122, Lond. Ed. 10. 



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