EBULLITION. 



299 



'has continued a short time, the tube C will become gradually heated, and the 

 inner surface of it covered with moisture. After a time, however, this moist- 

 ure disappears, and the tube attains the temperature of 212. In this state 

 it continues until the whole of the water is discharged from the vessel B to the 

 vessel D. 



These effects are easily explained. The water in the vessel B is incapable 

 of receiving any higher temperature than 212, consistently with its retaining 

 the liquid form. Small portions, therefore, are constantly converted into steam 

 by the heat received from the surrounding mercury, and bubbles of steam are 

 formed on the bottom and sides of the vessel B. These bubbles, being very much 

 lighter, bulk for bulk, than water, rise rapidly through the water, just in the 

 same manner as bubbles of air would, and produce that peculiar agitation at 

 its surface which has been taken as the external indication of boiling. They 

 escape from the surface, and collect in the upper part of the vessel. The 

 steam thus collected, when it first enters the tube C, is cooled below the tem- 

 perature of 212 by the surface of the tube ; and consequently, being incapa- 

 ble of remaining in the state of vapor at any lower temperature than 212, it is 

 reconverted into water, and forms the dewy moisture which is observed in the 

 commencement of the process on the interior of the tube C. At length, how- 

 ever, the whole of the tube C is heated to the temperature of 212, and the 

 moisture which was previously collected upon its inner surface is again con- 

 verted into steam. As the quantity of steam evolved from the water in B in- 

 creases, it drives before it the steam previously collected in the tube C, and 

 forces it into the vessel B. Here it encounters the inner surface of this ves- 

 sel, which is kept constantly cold by being surrounded with the cold water in 

 which it is immersed ; and the vapor, being thus immediately reduced below 

 the temperature of 212, is reconverted into water. At first it collects in a 

 dew on the surface of the vessel D ; but as this accumulates, it drops into the 

 bottom of the vessel, and forms a more considerable quantity. As the quantity 

 of water is observed to be gradually diminished in the vessel B, the quantity 

 will be found to be gradually increased in the vessel D ; and if the operation 

 be suspended at any stage of the process, and the water in the two vessels 

 weighed, it will be found that the weight of the water in D is exactly equal to 

 the weight which the water in B has lost. 



The demonstration is, therefore, perfect, that the gradual diminution of the 

 boiling water in the vessel B is produced by the conversion of that water into 

 steam by the heat. In the process first described, when the top of the vessel 

 B was supposed to be open, this steam made its escape into the air, where it 

 was first dispersed, and subsequently cooled in separate particles, and was de- 

 posited in minute globules of moisture on the ground and on surrounding objects. 

 In reviewing this process, we are struck by the fact, that the continued ap- 

 plication of heat to the vessel B is incapable of raising the temperature of the 

 water contained in it above 212. This presents an obvious analogy to the 

 process of liquefaction, and leads to inquiries of a similar nature which are 

 attended with a like result. We must either infer that the water, having ar- 

 rived at 212, received no more heat from the mercury ; or that such heat, if 

 received, is incapable of affecting the thermometer ; or, finally, that the steam 

 which passes off, carries this heat with it. That the water receives heat from 

 the mercury will be proved by the fact, that, if the vessel B be removed from 

 the mercury, other things remaining as before, the temperature of the mercury 

 will rapidly rise, and, if the fire be continued, it will even boil ; but so long as 



i the vessel B remains immersed, it prevents the mercury from increasing in 



| temperature. It therefore receives that heat which would otherwise raise the 



i temperature of the quicksilver. 



