136 THE THERMOMETER. 



glass at the extremity is then suddenly melted by the blowpipe, so as to close 

 the aperture immediately above the surface of the mercury, leaving no space 

 between them. In this state the sealed instrument is completely filled with 

 mercury to the exclusion of air. The instrument being now removed from the 

 source of heat, the mercury again contracts, leaving the space between the top 

 of the column and the extremity of the tube a vacuum. 



So far as the formation of the tube and the preparation of the mercury is con- 

 cerned, the thermometer is now complete, and by exposure to any variations 

 of temperature, the column of mercury in the tube may be seen to rise and fall ; 

 but it is necessary to provide an accurate and easy means of measuring the 

 variations of this column. As we suppose the tube to be uniformly cylindrical, 

 a scale of equal divisions attached to it would accomplish this purpose ; but 

 such a scale would merely give the variations of temperature relative to one 

 thermometer, and would not be capable of indications by which observations at 

 different times and places might be compared when taken with instruments 

 similarly constructed. To render the results of different thermometers, thus 

 constructed, capable of being compared one with another, it will be necessary 

 to select some points of temperature, by reference to which all thermometers 

 may be graduated. 



Let us suppose that the instrument, as already described, is plunged in a 

 vessel containing melting snow or ice. It will be observed that the mercury 

 in the tube will gradually descend until it arrives at a certain point, at which 

 it will remain stationary, neither ascending nor descending, so long as any 

 portion of the snow or ice remains to be dissolved. When, however, the 

 whole of the ice or snow is liquefied, and the contents of the vessel become 

 pure water, then the thermometer will be observed gradually to rise until it 

 attains that elevation at which it would stand if it were placed in the atmo- 

 sphere of the apartment in which the experiment takes place. The inference 

 from this experiment is, that so long as the process of liquefaction continues, 

 the temperature remains constant, but after the liquefaction is complete the 

 superior temperature of the apartment causes the water to become hotter ; and 

 this increase of temperature continues until the water in the vessel and the air 

 in the apartment acquire the same temperature. Now it is found that the point 

 at which the column of mercury fixes itself, when immersed in the melting ice, 

 is invariable under all circumstances. In whatever part of the world the ex- 

 periment be tried, and at whatever season, and whatever be the temperature of 

 the apartment, still the column will stand at the same height. This, therefore, 

 furnishes a fixed point of temperature, which can be ascertained in all coun- 

 tries, and under all circumstances. This fixed point of temperature, being 

 marked in the scale attached to the tube, is called the freezing point, or the tem- 

 perature of melting ice. 



( Let a vessel of pure water be now placed on a fire, and let the thermometer 

 ) be immersed in it. It will be observed that the column of mercury in the tube 

 < will gradually rise, according as the water receives heat from the fire, and this 

 } ascent will continue until ebullition takes place. It will be then observed that 

 ( however long a time the fire continues to act on the vessel, the mercury will 

 J no longer rise, nor will the intensity of the fire cause any difference in this 

 <. effect. The mercury will remain steadily at the same point until the whole of 

 the water escapes in steam, and the vessel remains empty. From this experi- 

 ment we infer that there is a temperature beyond which water is incapable of 

 rising, so long as it remains in the liquid state ; and that *he whole of the heat 

 communicated to it, after it has attained this pint, is carried off by the vapor 

 into which the water is converted. If this experiment be repeated under like 

 circumstances, it is invariably found that in all countries, and at all seasons. 



