828 



THE PNEUMATIC SYRINGE. 



engine, heat disappears, and for every thermal unit 

 which disappears 424 kilogramme-metres of work is 

 produced. Hence 424 kilogramme-metres is the me- 

 chanical equivalent of 1 kilogramme- degree of heat. 



The production of heat by the compression of gases 

 may be shown by means of the so-called pneumatic 

 syringe, fig. 400. It consists of a glass 

 tube with thick sides, closed at the bottom, 

 and furnished with a leather piston which fits 

 air-tight and is attached to an iron piston 

 rod with a wooden knob. At the bottom 

 of the piston there is a hollow piece of brass 

 in which a piece of tinder may be fixed to 

 a small horizontally projecting brass pin. The 

 tube being full of air, the piston is suddenly 

 pushed downwards as forcibly and rapidly 

 as possible. The air thus compressed dis- 

 engages so much heat as to ignite the tinder, 

 which is seen to burn w r hen the piston is 

 rapidly withdrawn. 



The rapid withdrawal of the piston is necessary, 

 for the small quantity of air in the cylinder is insuffi- 

 cient to maintain the combustion of the tinder for 

 more than an exceedingly short time. 



It is not always possible to withdraw the tinder 

 . burning. Sometimes only a flash is seen just when 

 the piston is pushed down to the lowest point. Before 

 the experiment is repeated, whether the preceding one was success- 

 ful or not, a narrow tube reaching down to the bottom must be 

 introduced into the syringe, and the air must be sucked out, 

 so that the apparatus may be filled with .fresh air for each experi- 

 ment. 



The piston must be well greased with oil, so as to close air-tight 

 and yet to move smoothly. 



A piece of the yellow tinder sold by tobacconists should be used. 



FIG. 400 



