Mat 22, 1908] 



SCIENCE 



831 



between the liquid and vapor phase, the phases 

 Laving become identical." 



Let us take any value M in the region of 

 liquid, and pass to any value N in the region 

 of vapor; this may be done in a variety of 

 ways and may therefore be represented by 

 straight or curved lines in the figure. How- 

 ever, let us choose to pass from If to iV by 

 changing either pressure or temperature, keep- 

 ing the other constant meanwhile. 



First let us change the temperature to that 

 indicated by the value N (which is value L). 

 Now the pressure at L is greater than vapor 

 pressure at constant temperature, therefore at 

 L the substance is a liquid only. Then let 

 us decrease the pressure to that of N. At the 

 pressure and temperature Z where the line 

 LN (LN being any line cutting OA), indi- 

 cating the drop in pressure, intersects OA, 

 the curve of vapor-pressure, there is a dis- 

 continuity in the passage, the phenomenon of 

 " boiling " will ensue, and no further drop in 

 pressure can take place until all of the sub- 

 stance is vaporized. Then the pressure can 

 be lowered till the value N is reached. 



If, on the other hand, we follow the line MPQN, 

 which does not cut the line OA, we can pass from 

 the state of liquid at M to the state of vapor at 

 N, without any discontinuity whatever. We first 

 increase the temperature, following the line MLP 

 to a value above the critical value. This takes us 

 into the region where there is no distinction be- 

 tween liquid and vapor, so that by first reducing 

 the pressure and then lowering the temperature, 

 we pass without any break, to a substance in the 

 truly vaporous state at N, the substance at no 

 time having been in the state of two distinct 

 phases. 



To this last clause we take exception on the 

 ground that we started wdth the substance a 

 liquid at M, and ended with substance as a 

 vapor at N, and therefore there must be a 

 place in the transition where the substance 

 ceases to be a liquid and begins to be a vapor. 

 This follows oust as naturally as it follows 

 that when a ball is thrown into the air it 

 reaches a point where it ceases going up and 

 begins coming down. 



By hypothesis, A was the critical value, 

 which means that any further increase in 



temperature or pressure acting either singly 

 or together can not produce any change in 

 the state of the substance, and it also follows 

 that any decrease of pressure or temperature 

 acting either singly or in unison will place 

 the resultant condition in the liquid or vapor 

 region or on OA. If, as in Fig. 2, we draw 



Fig. 2 



the limits at which the critical condition can 

 exist, we have YA and AX, because, taking 

 the lowest temperature, that of A, and keep- 

 ing it constant, and increasing the pressure, 

 we get AY oi. infinite extent, any value to the 

 left of which, as If, is liquid, and any value 

 to the right, as P, resembles the critical con- 

 dition at A. Now if we take the minimum 

 pressure, that of A, keep it constant, and 

 increase the temperature, we liave AX, any 

 presso -temperature P above AX resembling 

 the substance when at A, while any value 

 below AX, as Q, is vapor. 



It is seen, then, that any value within the 

 region YAX is in a special condition, which, 

 for want of a better name, I propose the name 

 "permanent phase." Wm. P. Hunger 



Vandeebilt Univeesitt, 

 Nashville, Tenn. 



CURBE'NT NOTES ON METEOROLOGY AND 

 CLIMATOLOGY 



BULLETIN OF THE MOUNT WEATHER 

 OBSERVATORY 



Vol. L, Part 1, of a new publication. Bul- 

 letin of the Mount Weather Observatory, haa 



