16 



CHEMISTRY. 



Stance; the alcohol will rise much higher in the tube than the mer. 

 cury. Liquids are not equally expansible; in being heated from 32° 

 to 202°, alcohol expands -^^th of its bulk; water ^^^d; mercury 

 ^jth. The rate of expansion in liquids is not uniform, — it increases 

 with equal increments of heat. Exception to the law that liquids 

 expand by heat, in the case of water near the freezing point, which 

 expands on being cooled below 39° — hence ice swims on water; this 

 expansion is due to a new arrangement of its particles in the act of 

 freezing. 



Expansion of Gases, — Gases are much more expansible than 

 either liquids or solids; — their rate of expansion is 

 uniform, and is the same for all gases, being about 

 480th of the whole volume for every degree of 

 Fahrenheit. 



Thermometers are instruments for measuring sensi- 

 ble heat by means of the expansion and contractions 

 of some fluid. Sanctorid's thermometer was the first 

 invented; — it consisted of a glass tube terminating in a 

 bulb, partially filled with air, and the open end plunged 

 into some coloured liquid ; the expansion or contrac- 

 tion of the contained air was marked by the depression 

 or rise of the liquid. This thermometer is liable to 

 two objections : the expansibility of air is too great to 

 mark very considerable changes of temperature ; and 

 it is liable to be influenced by atmospheric pressure. 

 Leslie^s differential thermometer is a modification of Sanctorio's 

 air thermometer ; it consists of a glass tube 

 bent at right angles, and terminating in two 



O,- -X bulbs. Both bulbs contain air, but the greater 

 \^ part of the tube is filled with a coloured fluid. 

 So long as the same temperature acts upon 

 both bulbs, no change can take place ; but the 

 slightest difference between the temperature of 

 the two is detected by the movement of the 

 liquid under the pressure of the air. 



Liquids are much better suited than gases for 

 thermometers; and of liquids, mercury is the 

 best adapted, on account of the great range be- 

 tween its boiling point, 656°, and its freezing 

 point, — 40°; it is also very sensible to the action of heat; and its di- 

 latations between 32° and 212° are nearly uniform. The essential 

 parts of a thermometer consist of a tube of a uniform small bore, 

 terminating in a bulb; the ball and part of the tube are filled with 

 mercury, and the air expelled by boiling the mercury, and then her- 

 metically sealing the tube. The boiling point is ascertained by im- 



Fig. 2. 



