THE THERMOMETER. 



not only in physical science, but in the arts and in domestic 

 economy. 



2. But to comprehend clearly the principle and application of 

 these instruments, it is first necessary to obtain some acquaintance 

 with the principal effects of heat, upon which their indications 

 depend, and the degrees of which they are applied to measure. 



One of the most familiar of these effects is the sense of more or 

 less warmth which a body, when it receives or loses heat, produces 

 upon our organs. 



When the heat received or lost by a body is attended with 

 this sense of increased or diminished warmth, it is called sensible 

 heat. 



3. But it will occur in certain cases that a body may receive a 

 very large accession of heat without any increased sense of 

 warmth being produced by it, and may, on the other hand, lose a 

 considerable quantity of heat without exciting any diminished 

 sense of warmth. The heat which a body would thus receive or 

 lose without affecting the senses, is called latent heat. 



4. When a body receives or loses heat, it generally suffers a 

 change in its dimensions, the increase of heat being usually 

 attended with an increase, and the diminution of heat with a 

 diminution of volume. 



This enlargement of volume due to the accession of heat is 

 called dilatation, and the diminution of volume attending the loss 

 of heat is called contraction. 



There are, however, certain exceptional cases in which heat, 

 whether received or lost, is attended by no change of volume, and 

 others in which changes take place the reverse of those just 

 mentioned ; that is to say, where an accession of heat is accom- 

 panied by a diminution, and a loss of heat by an increase of 

 volume. 



5. If heat be imparted in sufficient quantity to a solid body, it 

 will pass into the liquid state. Thus, ice or lead, being solid, will 

 become liquid by receiving a sufficient accession of heat. This 

 change is called fusion or liquefaction. 



If heat be abstracted in sufficient quantity from a body in the 

 liquid state, it will pass into the solid state. Thus, water or 

 molten lead losing heat in sufficient quantity will become solid. 

 This change is called congelation or solidification ; the former 

 term being applied to substances which are usually liquid, and 

 the latter to those which are usually solid. 



6. If heat be imparted in sufficient quantity to a body in the 

 liquid state, it will pass into the state of vapour. Thus, water 

 being heated sufficiently, will pass into the form of steam. This 

 change is called vaporisation. . 



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