206 Messrs. Mendenhall fy Ingersoll on Phenomena 
general effect is observed with gold, palladium, platinum, 
silicon, rhodium, and iridium. 
The only record we can find o£ the previous observation 
of such an effect is that given in Winkelmann's Handbucli 
( II. 1, p. 487, 1st. ed.), where the flash is considered to be 
a luminescent phenomenon associated with the giving off of 
absorbed hydrogen by silver, no mention being made of 
undercooling. The article refers to a paper by Dr. T. L. 
Phipson (B. A. Report, 1859, p. 76), who noted such a flash 
in cooling a globule of silver, just at its melting-point. 
It seemed to us worth while to determine more closely 
the conditions which made possible the retention of the fluid 
state at temperatures as much as say 300° below the normal 
melting-point — especially to see if the occlusion of gases 
played any great part in it, and also to see whether the 
brightening of the drop was a luminescent effect connected 
directly with molecular rearrangement — or simply a tem- 
perature effect due to the liberation of the latent heat of 
fusion. Microscopic examination of the particle shows con- 
clusively that the flash accompanies a change of state, for in 
general there is a marked change from a smooth surface often 
showing decided motion, to a rough, corrugated, t and very 
evidently solid surface. 
Bearing upon the problem as stated above we have noted 
the following : — 
The temperature at which " flashing " occurs is extremely 
variable. In general the smaller the drop and the higher 
its melting-point, the more it can be undercooled before 
flashing occurs ; the difference in this respect between a drop 
•1 mm. in diameter and one *2 mm. in diameter is quite 
noticeable. The more the globule works into and imbeds 
itself in the glower material, i. e. the more it departs from a 
spherical form, the less it can be undercooled. The possible 
undercooling is also slightly less if the cooling occurs rapidly. 
The melting must be thorough and complete or else no under- 
cooling whatever will occur. Granted thorough melting, 
further heating above the melting-point seems to be without 
influence *. 
The behaviour is essentially the same if the glower is 
placed in an atmosphere of C0 2 . Flashing also occurs in the 
same way if the globule is heated on a non-conducting clay 
surface, i. e. a Nernst-glower heater. The phenomenon was 
readily obtained with gold particles on such a heater, and 
* Tapping the support of the glower did not affect the undercooling 
which, considering the minute particles involved, is only what might be 
expected. 
