Page. — Molecular Association and Dissociation. 513 



librium until its original velocity is doubled ; for '^i\ 2 to be equal 

 to mv 2 , v/ must be equal to 2v 2 . In other words, beat will be 

 absorbed by the dissociation. Each molecule that dissociates 

 is thereby reduced to half its original absolute temperature, if 

 the term " temperature " can be applied to individual molecules. 



The internal energy neglected above may be a disturbing 

 factor. If the sum of the internal energies of the two new 

 molecules is less than that of the original molecule, the difference 

 will be added to the external translation energy, and the fall of 

 temperature may be less than given above. 



As for the quantity of heat which appears or disappears in any 

 given case, this will obviously depend upon the initial tempera- 

 ture, increasing and decreasing with it. For instance, in the 

 oxidation of CO into C0 2 the number of molecules decreases 

 by one-third, the kinetic energy of the vanished molecules is 

 divided amongst those which are left, and hence, neglecting any 

 heat produced by chemical attraction, or absorbed as internal 

 energy in the more complex new molecules, the temperature will 

 rise one-third. Supposing the initial temperature to be 600° C. 

 the final temperature will thus be 800°, a rise of 200° which does 

 not represent any chemical or physical work done in the reaction. 

 If by the use of catalytic agents or otherwise the reaction could 

 be carried out at a temperature of 60° the rise would be 20° 

 only. If, on the other hand, the reaction could take place in an 

 arc lamp at, say, a temperature of 3,000° C, the rise of tempera- 

 ture would be 1,000°. The quantities of heat disengaged would 

 be proportional to the temperature-rise. Hence it would appear 

 that heats of combustion of gases, in all cases where the number 

 of molecules alters, should vary with the initial temperature of 

 the experiment, and should be affected also by any rise of tem- 

 perature during the experiment. A difficulty in calculating the 

 amount of this effect in any given case may arise in this way : 

 In reactions accelerated by heat those molecules with velocities 

 above the average may suffer change more readily than the 

 rest, introducing an uncertainty as to the true initial temperature. 



If a gaseous molecule could be split up into parts small 

 enough, the temperature of these parts might be reduced nearly 

 to absolute zero without any reduction of molecular velocity. 

 Supposing, for example, the alpha particle separated from a mole- 

 cule of radium at 0° C. to have a mass of j±-q of the original mole- 

 cule, and to split off without gaining or losing velocity, its kinetic 

 energy would be T ws °^ that of the original molecule, and would 

 correspond to a temperature of 2 73° absolute. It would appear, 

 then, that if association or dissociation could be carried on suffi- 

 ciently far in a given mass, the temperature of that mass may 

 alter to any extent, its kinetic energy remaining the same. As, 



17— Trans. 



