THE AIR. 



45 



pressure upon an egg in a glass. By blowing in a claret glass containing a 

 hard-boiled egg, it is possible to cause the egg to jump out of the glass; and 

 with practice and strength of lungs it is not impossible to make it pass Vrom 

 one glass to another, as per illustration (fig. 38). 



The force of heated air ascending can also be ascertained by cutting up 

 a card into a spiral, and holding it above the flame of a lamp (fig. 39). The 

 spiral, if lightly poised, will turn round rapidly. 



Now let us turn to a few experiments with the air, which is composed 

 in two gases, Oxygen and Nitrogen, of which we shall hear more when we 

 come to CHEMISTRY. 



Fig. 39. Movement of heated air. 



It is not intended here to prosecute researches, but rather to sketch a 

 programme for instruction, based on amusing experiments in Physics, 

 performed without apparatus. The greater part of these experiments are 

 probably well known, and we desire to say that we merely claim to have 

 collected and arranged them for our descriptions. We must also add that 

 we have performed and verified these experiments ; the reader, therefore, 

 can attempt them with every certainty of success. We will suppose that we 

 are addressing a young auditory, and commence our course of Physics with 

 some facts relating to the pressure of air. A wine glass, a plate, and water, 

 will serve for our first experiments. Pour some water on the plate, light 

 a piece of paper resting on a cork, and cover the flame with the glass 



