AIR PUMPS AND APPARATUS. 



203 



Pressure Glass. (Fig. 195.) This instrument 

 consists of a glass globe, about three and a half 

 inches in diameter, with a neck inverted in a glass 

 vessel of about the same size, which is to be 

 nearly filled with water, and placed under the re- 

 ceiver of the air pump. As the receiver is ex- 

 hausted, bubbles of air pass out of the globe 

 through the water and escape away, but as soon 

 as the pressure is restored the water is forced out 

 of the lower vessel upwards into the globe. The 

 instrument may now be used to illustrate the expansion of 

 air, by again exhausting the receiver, when the small bubble 

 of air left at the top of the last experiment, will expand and 

 fill the whole globe. Price, 75cts, larger size, $1.00. 



Fig. 196. 



Bolt Head Experiment (Fig. 196) consists 

 of a glass globe of four or five inches in di- 

 ameter, with neck about thirty inches long, 

 having cemented on the neck a plate fitting 

 the open top air pump receiver. To use, the 

 plate is set on the open top receiver with the 

 end of the neck immersed in a jar of water, 

 which, to render the experiment more con- 

 spicuous, is usually colored red or blue ; on 

 exhausting the air from the receiver, the air 

 in the globe is expanded and escapes from 

 the neck, and is seen bubbling through the 

 water. On returning the air to the receiver, 

 it cannot enter the globe, but pressing on the 

 water forces it up the neck into the globe, 

 occupying the place of the air that escaped 

 by its expansion, and showing the quantity. 

 Price, $1.00. Glass jar, 25c. J 



Magdeburg Hemisphere. (Fig. 197, next 



age). This instrument consists of two hol- 



ow hemispheres of brass, which are made to 

 fit upon each other by a ground joint, ren- 

 dered air tight by a little oil. Having screw- 

 ed off the handle, put both the hemispheres together, and 

 screw them into the pump-plate, and turn the cock so that 

 the pipe may be open all the way into the cavity of the 

 hemispheres ; then exhaust the air out of them, and turn 



