8 THE POPULAR SCIENCE MONTHLY. 



atmosphere move, on an average, nearly four (3.8) times slower than 

 those of hydrogen under the same conditions ; but then they weigh, on 

 an average, fourteen and a half times more than hydrogen-molecules, 

 and therefore strike with as great energy. And do not think that the 

 effect of these blows is insignificant because the molecular projectiles are 

 so small ; they make up by their number for what they want in size. 



Consider, for example, a cubic yard of air, which, if measured at the 

 freezing-point, weighs considerably over two pounds. That cubic yard 

 of material contains over two pounds of molecules, which are moving 

 with an average velocity of 1,605 feet a second, and this motion is 

 equivalent, in every respect, to that of a cannon-ball of equal weight, 

 rushing along its path at the same tremendous rate. Of course, this is 

 true of every cubic yard of air at the same temperature ; and, if the 

 motion of the molecules of the atmosphere around us could by any 

 means be turned into one and the same direction, the result would be a 

 hurricane sweeping over the earth with this velocity that is, at the 

 rate of 1,094 miles an hour whose destructive violence not even the 

 Pyramids could withstand. 



Living as we do in the midst of a molecular tornado capable of such 

 effects, our safety lies wholly in the circumstance that the storm beats 

 equally in all directions at the same time, and the force is thus so 

 exactly balanced that we are wholly unconscious of the tumult. Not 

 even the aspen-leaf is stirred, nor the most delicate membrane broken ; 

 but let us remove the air from one of the surfaces of such a membrane, 

 and then the power of the molecular storm becomes evident, as in the 

 familiar experiments with an air-pump. 



As has already been intimated, the values of the velocities both of 

 hydrogen and of air molecules given above were measured at a definite 

 temperature, 32 of our Fahrenheit thermometer, the freezing-point of 

 water ; and this introduces a very important point bearing on our sub- 

 ject, namely, that the molecular velocities vary very greatly with the 

 temperature. Indeed, according to our theory, this very molecular 

 motion constitutes that state or condition of matter which we call tem- 

 perature. A hot body is one whose molecules are moving compara- 

 tively rapidly, and a cold body one in which they are moving compara- 

 tively slowly. Without, however, entering into further details, which 

 would involve the whole mechanical theorj' of heat, let me call your 

 attention to a single consequence of the principle I have stated. 



When we heat hydrogen, air, or any mass of gas, we simply in- 

 crease the velocity of its moving molecules. When we cool the gas, we 

 simply lessen the velocity of the same molecules. Take a current of 

 air which enters a room through a furnace. In passing it comes in con- 

 tact with heated iron, and, as we say, is heated. But, as we view the 

 process, the molecules of the air, while in contact with the hot iron, 

 collide with the very rapidly-oscillating metallic molecules, and fly back 

 as a billiard-ball would under similar circumstances, with a greatly-in- 



