50 THE ATMOSPHERE. 



atmosphere, in contradistinction to the chemical quali- 

 ties of which our first number treated. 



The first inquiry into the mechanical properties of 

 the air, was made in Italy about the year 1640. And 

 we may conclude, that the brilliant display this science 

 now makes, had its origin in the experiments of Torri- 

 cclli, which were caused by the failure of the mechan- 

 ics of the Duke of Florence, to raise water in a pump 

 above thirty feet. 



That we may fully understand this substance as a 

 mechanical agent, we must examine it a little in detail, 

 and observe its relations to common matter. 



It has been already stated that the atmosphere * con- 

 sists of two distinct gases or airs, in the proportion of 

 79 parts of nitrogen to 21 of oxygen, and a small quan- 

 tity of carbonic acid. 



All substances in nature considered mechanically, 

 are divided into solids and fluids. By a solid, we are 

 to understand that body or substance whose particles 

 are so related to one another, as not to admit of motion 

 among each other, when any force disturbs the inertia 

 or rest of the whole. 



As a fluid we recognise that substance whose parti- 

 cles have a less cohesive attraction, and are easily set 

 in motion among each other by a slight disturbing cause. 

 Besides, when at rest, its surface is uniformly level. 

 This forms a peculiar character of a fluid; while a solid 

 is disposed to continue in that position which followed 

 the impulse. 



The philosophical difference of these two conditions 

 of matter, depends on the quantity of heat which each 

 possesses. A familiar illustration is exhibited of the 

 various conditions of the same material, in ice, water, 

 and vapor. Ice at 32 Fahrenheit requires many degrees 

 of heat to liquify it, and when melted, the thermometer 

 shows no evidence of the addition. Water requires 

 many degrees to convert it into vapor, yet this vapor 

 in passing off, exhibits no greater heat than the water 

 from which it is escaping. 



* The word aim. sphere is derived from t : ,e Greek, and signifies 

 cplierc of vapor. 



