62 Proceedings of the Royal Society of Edinburgh. [Sess. 
rain, snow, frost, a dry surface mulch, etc., on the soil temperatures at 
various depths ; * in this present paper I propose to show that soil 
temperatures are also affected by soil colloids, since the properties of these 
colloids bring about changes in the conductivity of the soil. 
II. Scope of the Investigation and Effect of Change of 
Period on Values of 
For this investigation it was essential that variations in the value of 
R 
due to any cause other than temperature should be eliminated. To this 
K 0 
end hourly readings were made from minimum to maximum only, and no 
observations were taken on days of precipitation or when the ground was 
frozen or covered with a dry surface mulch : moreover, to ensure the 
more or less uniform water content of the soil, observations in summer were 
made on those days only when rain has fallen in the previous twenty-four 
R 
hours, so that any observed variations in the values of p- 4 were due 
apparently either to variations in the length of the interval from minimum 
to maximum or to temperature changes in the soil. 
It was assumed that the temperature curves from minimum to 
maximum could be regarded as half a sine curve of period double the 
interval from minimum to maximum ; we were thus able to use the formula 
10 
R 
where ~ is the ratio of the ranges of temperature at the 
-tC 
4-inch depth and the surface from minimum to maximum, h 2 is the 
diffusivity of the soil, and T is the period, i.e. double the interval from 
minimum to maximum, reckoned in seconds.^ 
From chosen observations in June 1920, when the mean surface 
R 
temperature was 10° C. and the period 24 hours, — in sand and clay loam 
were found to be ‘52 and *42 respectively. 
Thus ^ 4 =52 = e 
X -006 
for sand, giving a value of *09 for h in sand ; 
in clay loam the corresponding value for h was *07. 
Assuming that for the rest of the observations these values for h 
remained constant for the two soils, and this seemed probable since 
* “ The Effect of Weather Changes on Soil Temperature,” T. B. Franklin, Proc. Roy . 
Soc. Edin ., vol. xl, part i, No. 8, 1920. 
t Mathematical Theory of Heat Conduction , Ingersoll and Zobel. 
