1887.] 
Mr John Aitken on Hoar-Frost. 
123 
A considerable time ago it was suggested by Professor James 
Thomson and by Kirchhoff that the vapour pressure of ice might 
be less than that of water at the same temperature ; Professor 
Ramsay and Dr Young have shown that this is the case,* and they 
have experimentally measured the comparative temperatures of ice 
and water under the same vapour pressure. This they have done to 
a temperature of 9 degrees below the freezing-point ; lower they 
could not go, as the water always froze when its temperature was 
reduced to that point. They found that the ice and the water had 
the same temperature at 32°, and under a pressure of 4*6 mm. But 
when the pressure was still further reduced, the water became colder 
than the ice ; and when the pressure was about 3*20 mm. the water 
was at a temperature of about 23°, while the ice was about 24°. 
The water was thus about a degree colder than the ice. 
It is evident, therefore, that if by any means the ice had been 
cooled to the same temperature as the water, its vapour-pressure 
would have been less than that of the water. So that if we have a 
water-surface and an ice one at the same temperature and near each 
other, vapour will tend to pass from the water to the ice, because, 
the vapour pressure of the water being higher than that of ice, 
the air which is saturated to a water surface is supersaturated to 
an ice one. 
Something like this seems to take place when hoar-frost is form- 
ing. When the air is cooled, condensation takes place on the dust 
nuclei, resulting in a foggy condensation. This moisture condensed 
in the air seems always to keep the liquid form ; at least we do not 
see during frosty weather any indications of the particles being 
frozen. In the fogs formed low down in our atmosphere there are 
no optical or other phenomena such as we might expect to find if 
they were frozen. That the temperature of the air is far below the 
freezing-point is no evidence that the fog particles will be solid, as it 
is well known that water, even when in contact with solid surfaces, 
and with what seem favourable nuclei for forming freezing centres, 
may yet remain liquid at a temperature far below the freezing- 
point. Thin films and small drops seem difficult to freeze ; I have 
frequently seen my night-radiation thermometer cooled many degrees 
below the freezing-point, and yet the film condensed on its surface 
* Phil. Trans. Roy. Soc., part ii. , 1884. 
