VELOCITY OF POLYMORPHIC CHANGES BETWEEN SOLIDS. 



59 



of pressure might consume a little time. I could not make a measure- 

 ment of pressure, if all the adjustments liad to be made, in much less 

 than 15 seconds, but if I knew approximately what the pressure was, 

 the measurement could be made in considerably less time. It was 

 not possible, therefore, to make accurate measurements when the 

 pressure was rapidly changing, and if the transition were very rapid, 

 as that betAveen ice I and III near the triple point with the liquid,^ 

 it was not possible to make any measurements at all. On the other 

 hand, if the reaction is slow, it is possible to make the readings as 

 accurately as the time can be read from the second hand of an ordinary 

 watch, that is, to within perhaps one second. This is accomplished 

 by setting the slider at a definite mark and keeping the key pressed 

 tlown. As the pressure changes with time the galvanometer swings. 



9200 



^8800 



</) 



1^8400 



8O0O 



\i^J 



_^ .-0 — »- 



5 10 15 20 25 30 

 Minutes 

 Silver Nitrate 



Figure 1. The rate at which pressure rises or falls toward equilibrium for 

 Silver Nitrate at 75°. 



and the null position is indicated when the cross hair passes the zero 

 mark. Furthermore, by setting the slider at a mark and reading when 

 the galvanometer passes zero, the accuracy of the readings is greatly 

 improved. With a magnifying glass the slider may be set on a 

 division within perhaps 1/50 mm., whereas the position of the slider, 

 if between divisions, cannot be estimated to better than 1/10 mm. The 

 sensitiveness of the galvanometer was such that the mirror magnified 

 the motion of the slider four times. Of course the battery current 

 was chosen so small that the circuit could be closed indefinitely with no 

 perceptible alteration of resistance due to heating by the current. 



Such measurements give immediately data for two curves at each 

 of several temperatures, showing pressure as a function of time as it 

 rises from below or drops from above. An example of such a curve is 



2 A, p. 534. 



