365 
of Edinburgh, Session 1873 - 74 . 
It only remains now to explain the first experimental fact, i.e., 
the expansion of the wire at the critical instant. This follows 
from what has already been said, when we consider certain experi- 
ments made by Colonel Clarke, communicated to the Royal Society 
of London in 1863, and the explanation of them which was given by 
Professor Stokes. A hollow cylinder of iron was heated in a 
furnace, and plunged into water, so that half of it was buried in 
the water, the axis of the cylinder being vertical. After cooling, 
the cylinder was found to be permanently indented at the water- 
level, so that its diameter was there diminished. The explanation 
is as follows : — When plunged in water the lower part immediately 
contracts and cools. The upper part remains expanded. At this 
instant there is at the water-line a conflict between the upper, 
hot, expanded portion and the lower, cool, contracted portion. 
Now iron is much stronger when cool than when hot. Hence the 
cool iron has the advantage, and at the water-line the iron is at first 
forcibly shrank, and afterwards cooled, and hence at that line 
the cylinder is contracted. 
Now, exactly the same thing may happen in the cooling wire. 
Before cooling down to the dull-red heat, the hot inner part is 
expanded, and the cooler outer part contracted, and owing to the 
greater strength of the cooler iron, the wire is on the whole unduly 
contracted. But at the moment of after-glow the internal heat 
is driven out, and the contraction is no longer maintained. Hence 
the expansion at that temperature. 
The hypothesis I have now given explains all the facts observed ; 
but it cannot be stated to be proved. An alternative, and only 
one remains, which is to consider that when iron is heated to an 
intense white heat it becomes different in its nature from cold iron , 
and that the iron in the hot state has a certain amount of latent heat , 
which is given out when , by cooling , the iron changes its nature . 
In the absence of any data for determining between these two, 
I prefer the former hypothesis, as it does not involve a new pro- 
perty of iron quite unlike that of any other substance yet examined. 
The apparently opposite phenomena observed when the iron is 
massive can be explained equally well on either hypothesis. But 
the second hypothesis is favoured by certain experiments made by 
Professor Barrett while heating the iron. 
