1887 .] 
Mr John Aitken on Hoar-Frost. 
125 
It will be observed that these thick and foggy nights, when heavy 
deposits of hoar-frost are formed, are the very nights on which little 
or no dew would he deposited, because the radiation would he checked 
by the fog. These, however, are the very nights most favourable 
for the deposit of hoar-frost, because the air has a large amount of 
vapour in it, — is in fact saturated. And further, while dew re- 
quires that the surface on which it is deposited be cooled by radia- 
tion, this is not so necessary, and indeed may he absent, in the 
formation of hoar-frost ; because the fog particles radiate and cool 
the air to the saturated temperature of vapour at a water surface, 
and the passing air discharges part of its vapour on all ice crystals 
or other nuclei with which it comes in contact ; the passing air at 
the same time absorbs the heat of crystallisation, while the heat of 
condensation is balanced by the heat absorbed by the evaporation 
from the water particles. 
But, further, it will he observed that not only are those nights 
on which hoar-frost is most abundant not similar to the nights on 
which heavy dews are formed, but they are generally nights on 
which there would be no dew at all if the temperature was above 
the freezing-point ; these hoar-frosty nights do not therefore corre- 
spond to dewy nights, when the temperature is higher, but rather 
to those nights when every object is wet and dripping, not with 
dew, but wet with deposited fog particles. 
7. On the Quotient of a Simple Alternant by the Difference- 
Product of the Variables. By Dr Muir. 
8. Investigations on the Influence of certain Bays of the 
Solar Spectrum on Root-Absorption and on the Growth 
of Plants. By Dr A. B. Griffiths, F.R.S.E., F.C.S. 
(Bond. & Paris), and Mrs A. B. Griffiths. 
This paper details an investigation undertaken to see the influence 
of certain rays of white light on root-absorption and assimilation 
in the vegetable kingdom. One of us has been for some years in- 
vestigating a problem as to the use of ferrous sulphate as a plant 
food (see Dr Griffiths’ memoirs in Journal of Chemical Society 
