62 Experimental Philosophy. [Lecture 6. 



improper at present, since it is of the general 

 properties of air of which I am now to treat, or 

 rather of its mechanical and not its chemical pro- 

 perties. 



Fluids are divided into two classes ; the incom- 

 pressible, and the elastic. That branch of science 

 which is called hydrostatics treats of all the 

 known qualities of the former, and that of pneu- 

 matics embraces all which respects the general 

 properties of the elastic fluids. The elastic fluids 

 are again divided into two classes, those which 

 are condensible, such as vapour, which is easily 

 condensed by cold; and the permanently elastic 

 fluids, of which there are many, such as oxygen 

 air or gas (the word gas being an old German 

 term signifying spirit * ), nitrogen or azotic gas, 

 or phlogisticated air, as it was first called, carbo- 

 nic acid gas or fixable air, hydrogen gas or in- 

 flammable air ( that which is used to inflate bal- 

 loons), nitrous gas, hepatic gas, &e. But of 

 their general or mechanical properties the com- 

 mon air will serve to give a perfect idea. 



The properties of air of which the science of 

 pneumatics particularly treats, are its weight, 

 pressure, and elasticity or spring. 



That air, like all other bodies, is possessed of 

 weight or gravity many obvious facts will serve to 

 convince us ; and, in truth, it may be reduced to 

 the simplest of ah 1 experiments, for air may be ac- 



* Whence our word ghost. 



