PRESSURE OF THE AIR. 



47 



spheric pressure which maintains the fluid in the bottle, but the milk 

 descends, because fluids are superposed according to their order of density, 

 and the densest liquid falls to the bottom. 



This can be verified by means of the phial of the four elements, which is 

 a plain, long, and narrow bottle, containing equal volumes of metallic mercury, 

 salt water, alcohol, and oil. These four liquids will lie one on the top of the 

 other without ever mixing, even if shaken. 



Another experiment as to the pressure of the air may be made (fig. 42). 

 Take a penny and press it against some oaken bookcase or press, rub the coin 

 against the wood for a few seconds, then press it, and withdraw the fingers. 



Fig. 41. Pressure of the air. 



The coin will continue to adhere to the wood. The reason of this is, because 

 by the rubbing and the pressure you have dispersed the film of air which was 

 between the penny and the wood, and under those conditions the pressure of 

 the atmospheric air was sufficient to keep the penny in its place. 



Or, again, let us now add a water-bottle and a hard-boiled egg to our 

 appliances ; we will make use of the air-pump, and easily perform another 

 experiment. I light a piece of paper, and let it burn, plunging it into 

 a water-bottle full of air. When the paper has been burning a few seconds 

 I close the opening of the water-bottle by means of a hard-boiled egg, which 

 I have previously divested of its shell, so that it forms a hermetic stopper. 

 The burning of the paper has now caused a vacuum of air in the bottle, and 



