324 



HEAT. 



tion, will be observed to be different. In the thermometer which has a tube P 

 with a smaller bore in proportion to its bulb, the distance will be greater than 

 in the other, because the same volume of mercury which forms the dilatation 

 of that liquid from the freezing to the boiling point fills a greater length of the 

 smaller than of the large tube. It is plain, therefore, that since this given dif- 

 ference of temperature causes the column of mercury to rise through a greater 

 space in the one than in the other, the one instrument is properly said to pos- 

 sess a greater sensibility than the other. 



Let the intervals on the scale between the freezing and boiling points be 

 now divided into 180 equal parts ; and let this division be similarly continued 

 below the freezing point to the place ; and let each division upward from that 

 be marked with the successive numbers, 1, 2, 3, &c. The freezing point will 

 now be the 32d division, and the boiling point will be the 212th division. 

 These divisions are called degrees, and the freezing point is, therefore, 32, 

 and the boiling temperature 212. 



It is evident, that although the degrees on these two instruments are differ- 

 ent in magnitude, still the same temperature is marked by the same degree on 

 each, and therefore their indications will correspond. 



The manner of dividing and numbering the scale here described, is that 

 which is commonly adopted in England, and is called Fahrenheit's scale. 

 Other methods have been adopted in France and elsewhere, which will hereaf- 

 ter be described. 



Let a mass of snow at the temperature of 0, having a thermometer im- 

 mersed in it, be exposed to an atmosphere of the temperature of 80. As the 

 snow gradually receives heat from the surrounding air, the thermometer im- 

 mersed in it will be observed to rise until it attain the temperature of 32. 

 The snow will then immediately begin to be converted into water, and the 

 thermometer will become stationary. During the process of liquefaction, and 

 while the snow constantly receives heat from the surrounding air, the ther- 

 mometer will still be fixed, nor will it begin to rise until the process of lique- 

 faction is completed. Then, however, the thermometer will again begin to 

 rise, and will continue to rise until it attain the same temperature as the sur- 

 rounding air. 



Heat, therefore, when supplied to the snow in a sufficient quantity, has the 

 effect of causing it to pass from the solid to the liquid state, and while so em- 

 ployed, becomes incapable of affecting the thermometer. The heat thus con- 

 sumed or absorbed in the process of liquefaction, is said to become latent, the 

 meaning of which is, that it is in a state incapable of affecting the ther- 

 mometer. 



The property here described, with respect to snow is common to all solids. 

 Every body in the solid state, if heat be imparted to it, will at length attain a 

 temperature at which it will pass into the liquid state. This temperature is called 

 its point of fusion, its melting point or its fusing point ; and in passing into the 

 liquid state, the thermometer will be maintained at the fixed temperature of 

 fusion, and will not be affected by that heat which the body receives while un- 

 dergoing the transition from the solid to the liquid state. 



If water, at the temperature of 60, be placed in a vessel on a fire having a 

 thermometer immersed in it, the thermometer will be observed gradually to 

 rise, and the water will become hotter, until the thermometer arrives at the 

 temperature of 212. 



Other liquids are found to undergo a like effect. If exposed to heat, their 

 temperatures will constantly rise, until they attain a certain limit, which is dif- 

 ferent in different liquid ; but having attained this limit they will enter into a 

 state of ebullition, and no addition of heat can impart to them a higher temper- 



