88 HEAT OR CALORIC. 



an equal number of degrees above and below ebullition, but fixed 

 oils, sulphuric acid and mercury, afford no readily appreciable vapor 

 under the boiling point. 



(j.) By being converted into steam, a cubic inch* of water be- 

 comes nearly a cubic foot or 1728 cubic inches. Dr. Black and Mr, 

 Watt estimated the enlargement at nearly 1800 times. According to 

 Gay Lussac it is 1698 times. Alcohol, in vapor, under the common 

 pressure, occupies 659 times the volume that it did when liquid, and 

 ether 443 times. The specific gravity of steam is 623, air being 

 1000, but the vapor of alcohol is half as heavy again as air, and that 

 of ether more than twice and a half as heavy, and generally with a 

 few exceptions, the lower the boiling point of a fluid, the more dense 

 is the vapor formed from it. 



(K.) THE PRESSURE OF THE ATMOSPHERE, AND PRESSURE IN 



GENERAL, EXERTS AN IMPORTANT INFLUENCE ON VAPORIZATION. 



As already observed, no correct conclusions respecting aeriform 

 bodies can be formed, without taking this subject into view. 



(/.) The pressure of the atmosphere is measured by the column of 

 mercury which it is capable of sustaining. A glass tube not less than 

 32 inches long nor over half an inch in diameter, closed at one end, 

 being filled with mercury, and having its mouth first closed by the 

 finger, and then inverted and opened under the surface of mercury, 

 exhibits die amount of atmospheric pressure, vibrating on both 

 sides of 29 or 30 inches, which is about the medium of different cli- 

 mates, seasons and countries. f 



(m.) At the, medium pressure, pure water boils at 212 ; if the 

 pressure be diminished, water and all fluids boil at a lower tempera- 

 ture. This is shewn by the air pump, and by the Torricellian 

 vacuum. J Natural variations of atmospheric pressure vary the boil- 

 ing point about 5. 



(n.) According to Dr. Slack, fluids boil in vacuo ivith 124 less 

 of heat than under the pressure of the atmosphere ; others say with 

 145 less, if estimated in the Torricellian vacuum.^ 



As we ascend, it requires less heat to make water boil ; on the 

 top of Mount Blanc, it boils at 187, || and on the range of Pasco, 

 Peru, at 180.1T In the Rev. Mr. Wollaston's thermometer, each 



* Weight 252 grains. The specific gravity of steam at 212 and of the force of 30 

 inches of mercury, in pressure, is to dry air as 10 to 16. Henry. 



t See Dr. Hare's experiments, in his Compendium. 



t For a table, see Henry, Vol. I, p. 116, Lon. Ed. 10. 



The space above the mercury in a barometer tube : it was called after its discov- 

 erer Evangelista Torricelli. 



|| The monks at one of the highest monasteries on the Alps, complain that they 

 cannot make good Bouillie, (milk porridge,) because the water boils so soon. Paris' 

 Pharmacologia. A digester would remove the difficulty. 



1F Am. Jour. Vol. XVII. p. 50. 



