ON THE NATURE OF LIFE 201 



Q units of heat, then, have flowed from a mass at 293 (or 273+20) 

 to another mass at 283 C. (or 273+10). When heat flows from 

 a body at high temperature to another body at low temperature, 

 the entropy of the former is diminished and that of the latter is 

 increased. Change of entropy is measured by the simple ex- 

 pression -, Q being the quantity oJ^e^^wMcluDJBes-aiid JT_ 

 being the absolute temperature_. The entropy of the gas at 293 



is therefore diminished by the amount 



293 



But at the same time a mass of gas. originally at 10 C. (or 

 283 Abs.) becomes raised in temperature to 15 C. (or 288 Abs.) 

 as the result of Q units of heat entering it, and so we get the 

 change due to the receipt of Q units of heat by the colder gas as 



. Now the total entropy change due to the Q units of heat 



288 



lost by the hot gas, and the Q units gained by the cold gas is 

 + - . Obviously this expression is positive, and so we 



get our result that entropy increases as the result of the mixture 

 of a hot with a cold gas. 



Let, then, all this occur in the case of our specified decilitre 

 of hydrogen. The mixture of gases takes place in a few seconds, 

 and the resulting gas, at a temperature of 15 C., is stable and 

 homogeneous as regards temperature. No further work (with 

 regard to its temperature) can be done by it. 



But while that is the case, it is still possible that the state of 

 all the molecules in the gas may become such that at a given 

 moment the latter become disposed, purely by " chance," in pairs, 

 the members of which are approaching each other with equal 

 velocity in the same straight line. When they collide their 

 velocities will be the same as before, but the directions in which 

 all the molecules are moving will become simultaneously reversed. 



A very important result flows from this. It can be shown 

 (though the proof cannot be given here) that when every mole- 

 cule completely reverses its direction of motion, the velocity 

 remaining the same, the gas will retrace its past history. That 

 means that the total quantity of gas at a temperature of 15 C. 

 (or 288 Abs.) will separate into two portions one at a tem- 

 perature of 10 (or 283 Abs.), and the other at a temperature of 

 20 C. (or 293 Abs.). Therefore Q units of heat will flow from 



