140 HEAT. 



equal additions of heat ; and water boils with less heat 

 than mercury ; and for the same reason, alcohol (spirits 

 of wine) expands more than water. 



The nearer a liquid is to boiling, the more it expands, 

 and of course, the further it is from boiling, the less it 

 expands by the addition' of a certain quantity of heat. 

 The expansion of liquids therefore, does not depend on 

 their density, but upon the quantity of heal necessary to 

 make them boil ; and no reason can be given why differ- 

 ent liquids require different degrees of heat to make them 

 boil. 



The expansion of liquids by heat, and contraction by 

 cold, has furnished us with the means of measuring the 

 relative heat or temperature of all other bodies. This 

 is done with an instrument called a Thermometer. It is 

 almost unnecessary for us to describe a Thermometer. It 

 consists essentially of a glass tube, with a bulb or hollow 

 ball at one end, which is filled with mercury which, after 

 the air has been expelled by making the mercury boil, is 

 sealed up air tight. When this instrument is exposed to 

 heat, the mercury expands, and of course, rises in the 

 tube ; and when exposed to cold, the contrary takes place. 

 To this tube a scale is attached for the purpose of com- 

 parison ; this scale is graduated and marked, by plunging 

 the tube and bulb into melting ice ; the point at which 

 the mercury then stands, is called the freezing point of 

 water. It is then plunged into boiling water, the point 

 at which the mercury stands is called the boiling point 

 of water. However often we perform these operations 

 we shall find that the mercury will always stand at the 

 same point ; hence we learn, that snow or ice always be- 

 gins to melt at the same temperature, and that water boils 

 always at the same degree of heat. Mercury is used for 

 thermometers, because it expands more equally than any 

 other liquid, owing to the great distance between its freez- 

 ing and boiling points ; quicksilver boils at 660 of 

 Fahrenheit, and freezes at 40 below zero ; so that there 

 is 700 between the two points. 



Theie are many kinds of thermometers, which derive 

 their names from their inventors. Thus Fahrenheit's* 

 thermometer, which is used in Britain and her posses- 



Wherever, the degrees of heat are mentioned in this tract, it 

 is intended to refer to Fahrenheit's scale. 



