12 INTRODUCTION TO GENERAL SCIENCE 



container is free to move, we can change the force of the ex- 

 plosion into rotary motion, as in the gas engine. Here a 

 mixture of gas and air, in the right proportions for complete 

 combustion, is drawn into a cylinder by the forward stroke 

 of the piston, compressed by the return stroke, and finally 

 exploded, thus pushing the piston forward with great energy. 

 It is then forced out upon the return of the piston. This is 

 the so-called four-cycle gas engine. 



In the two-cycle engine the back stroke of the piston causes 

 a partial vacuum in the crank case, which is inclosed, and a 

 mixture of gas and air is drawn in. The forward stroke of 

 the piston forces this mixture up into the cylinder, the fresh 

 mixture pushing out the burnt gases and being compressed 

 at the return of the piston. The explosion then takes place 

 and forces the piston forward. The incoming fresh mixture 

 forces out the old burnt mixture. 



References : 



1. 1803 : 191-194. The Principle of the Gas Engine. 



2. Farmers' Bulletin No. 277 : The Use of Alcohol and Gasoline 



in Farm Engines. 



a. 1607 : 522-530. Gasoline Engines. 



b. 1801 : 299-300. The Gas Engine. 



c. 1802 : 322. The Gas Engine. 



d. 1809 : 208-210. International Combustion, or Gas Engines. 



e. 1810 : 193-196. The Gas Engine. 



9. ANIMAL HEAT 



We call the heat produced by slow combustion of food 

 within the body, animal heat. This combustion, while slow, 

 is perfect. All of the carbon in food is completely combined 

 with oxygen, and its combustion causes no smoke. Com- 

 bustion likewise takes place within the lungs, and moreover, 



