DUST, FOGS, AND CLOUDS. 339 
saturated” in both cases, but in the one case it condenses out and forms a 
cloudiness, while in the other it remains in its invisible vaporous form. It 
will be necessary to diverge here a little from our immediate subject, to say a 
few words on the conditions under which water changes from one of its forms 
to another. 
We have what are called the “ freezing-point ” and the “ boiling-point” of 
water. These are, of course, the same as the melting-point and the condensing- 
points of water. Water at 0° C. will freeze if cooled, or melt if heated. It 
will pass into vapour if heated above 100° C., and will pass from vapour to 
liquid if cooled below 100° C., that is, at standard pressure. But something 
more than mere temperature is required to bring about these changes. Before 
the change can take place, a “free surface” must be present, at which the change 
can take place. I may here say that what I mean by a ‘free surface” is a 
surface at which the water is free to change its condition. For instance, the 
surface of a piece of ice in water is a “free surface” at which the ice may 
change to water, or the water change to ice. Again, a surface of water 
bounded by its own vapour is a “free surface,” at which the water may 
vaporize, or vapour condense. What are called the “freezing” and. “ boiling 
points” of water are the temperatures at which these changes take place at 
such “free surfaces.” When there is no “ free surface” in the water, we have 
at present no knowledge whatever as to the temperature at which these 
changes will take place. 
It is well known that water may be cooled in the absence of “free surfaces ” 
far below the “ freezing-point” without becoming solid. Some years ago* I 
showed reason for believing that ice in the absence of ‘‘ free surfaces ” could be 
heated to a temperature above the “ freezing-point” without melting. Pro- 
fessor Carnelly has quite lately shown this to be possible, and has succeeded in 
raising the temperature of ice to 180° C.t Further, I have shown in the paper 
above referred to, that if water be deprived of all “free surfaces,” it may be 
heated in metal vessels while under atmospheric pressure to a temperature far 
above the “ boiling-point,” when it passes into vapour with explosive violence. 
From this we see that it requires a lower temperature to cause a molecule 
of water to adhere to another molecule of water to form ice, than for a molecule 
of water to adhere to a molecule of ice. Also that it requires a much higher 
temperature to cause a molecule of water surrounded on every side by other 
water molecules to pass into vapour, than for a water molecule bounded on one 
side by a gas or vapour molecule to pass into a state of vapour ; and that a 
necessary condition for water changing its state is the presence of a “free 
surface” or “ surfaces,” at which the change can take place, if these changes are 
* “Transactions Royal Scottish Society of Arts,” 1874-75. 
+ “Nature,” vol, xxii. p. 435. 
