268 



SCIENTIFIC AMUSEMENTS. 



will answer) is provided with a tightly fitting piston like a syringe ; 

 on heating the bulb the air expands and presses on the piston, 

 forcing it outwards ; on cooling the bulb again the air contracts, 

 and the pressure is relieved, so that the atmospheric pressure out- 

 side forces the piston down again. 



Motive power is obtained by means of " gas engines " in this 

 way. A chamber containing a mixture of coal gas (or other com- 

 bustible gas) and air is fired, so that much heat is developed by 

 the chemical action taking place ; this heat largely expands the 

 products of combustion, producing pressure upon a piston working 

 in a cylinder, and so causing it to move; by means of rods, cranks, 



Fig. 122. Maximum Density of Water. 



Fig. 123. Expansion of 

 Air moves Piston. 



shafts, &c., this motion is communicated to the machinery to be 

 driven by the engine. A steam engine works after somewhat the 

 same fashion, only here the power is produced by the expansion 

 of steam produced in a pressure boiler. 



Expt. 317. Air Thermometer. Into the mouth of a 6-ounce 

 flask fix a cork air-tight, with a piece of quill tubing passing through 

 the cork, like the arrangement used in Expt. 311. Hold the tube 

 and flask with the latter upwards, and place the lower end of the 

 tube in a bottle half full of water. On warming the flask the air 

 inside will become expanded and will partly bubble out through 

 the water in the bottle (like the air from the diffusion apparatus, 



