64 
Proceedings of the Royal Society of Edinburgh. [Sess. 
as being sufficiently trustworthy out of the whole year 1920; of these 32 
were arranged according to their length of period : — 
11 for a period of 12 hours — Min. to Max. 6 hours. 
14 „ 18 „ „ 9 „ 
7 „ 22 „ „ 11 „ 
These values for the two soils are shown plotted according to their length 
of period against the mean surface temperature in figs. 1 and 2 ; in both 
figures it will be noticed that the distribution of the plotted values of 
R, 
R„ 
at low mean surface temperatures is wider than at high temperatures : 
R 
this is due to the difficulty of computing exactly the value of p 4 in winter 
1C 0 
when both R 4 and R 0 are small, and the least error in either makes a 
R 
considerable difference in the value of 
R 0 
The vertical dotted lines on the graph are the lines on which the values 
R 
of -p 4 in sand should lie at periods of 12, 18, 22 hours respectively if there 
R 
R, 
was no variation in value of with a change of mean surface 
temperature. 
The plotted points do fall so nearly on these lines that it seems justifi- 
able to assume that there is no change in conductivity of sand with change 
of temperature. 
(b) In Clay Loam. 
R 
Here we have quite a different picture, as the groups of points for ^ lie 
diagonally and not vertically as in sand, and it seems that there is a 
distinct change of conductivity in clay loam with a change in the mean 
surface temperature. The chief difference between clay loam and sand- — 
apart from the size of the soil grains — is that clay loam contains organic 
matter and soil colloids, while sand contains neither; therefore it would 
appear that the change in conductivity of clay loam with temperature is 
due to one of both of these constituents. 
R 
(c) in Clay Loam, Ignited Clay Loam, and Sand. 
tv 0 
R 
During the autumn of 1920 I compared the values of ^ in clay 
R 0 
loam, ignited clay loam, and sand ; the interval from minimum to maximum 
was about 8 hours — period 16 hours — for most of the time during which 
the observations were made. 
