534 PROCEEDINGS OF THE AMERICAN ACADEMY. 



without the expenditure of a great deal of time. These disturbing 

 factors are such as the size and shape and material of the containing 

 vessel, the rapidity of heat conduction, and the distance from the equi- 

 librium line. But just as for the question of subcooling, so here, every 

 measurement ever made, whether of the equilibrium pressure alone or 

 of the change of volume, involved this matter of reaction velocity. 

 One had to be sure before making a reading that the reaction had 

 stopped running, and in the endeavor to waste no more time than was 

 necessary the progress of the reaction was constantly watched, so as 

 to make the reading as soon as possible after sensible equilibrium had 

 been reached. 



There are two distinct types of behavior of the reaction velocity, 

 according as the reaction is between a solid and a liquid or between 

 two solids. In general the reaction between solid and liquid was slower 

 than between solid and solid. The time for the completion of the re- 

 action liquid-solid was about two hours on the I-L, V-L, and lower end 

 of the VI-L curve. No particular variation in this time was noticed 

 from one end to the other of the I and V curves, but at the upper end 

 of the VI-L curve, the velocity had been very appreciably accelerated, 

 the time for completion of the reaction at the upper end being about 

 one hour. On the III-L curve, as already mentioned, the reaction was 

 very much slower. It was not practicable here to wait for the reaction 

 to run to completion, but the equilibrium points were taken as the 

 mean of values approached from above and below. This shows that 

 the velocity depends on the form into which the water is transformed 

 as well as on the heat of reaction, for the heat IlI-L is of the same 

 magnitude as that of the neighboring modifications I and V. On all 

 the curves the velocity seemed the same for melting as for freezing. 



The most striking behavior of the reaction velocity is shown by the 

 reaction solid-solid. The velocity ranges from explosive rapidity at 

 the end near the triple point to such sluggishness 20° further down as 

 to make further prolongation of the equilibrium curve out of the ques- 

 tion. Of course the very low heat of reaction would lead one to expect, 

 other things being equal, a high rate of transformation, but that the 

 heat of reaction has practically nothing to do with it is shown by the 

 enormous temperature coefficient of reaction velocity, while the heat 

 of reaction is practically independent of temperature. The slowness 

 of the reaction does not depend so much on the actual temperature as 

 on the nearness of the triple point. Thus I-II is explosive at its triple 

 point, —35°, while at —35° III-V is almost impossible ; III-V is ex- 

 plosive at its own triple point, —17°, but at this temperature V-VI 

 may take a couple of hours to run to completion. 



