The Co-Efficient of Humidity. 137 
The concluding set is of special interest because it gives the 
minimum value of the coefficient determined under natural conditions 
as well as values obtained earlier in the season, though from widely 
separated localities. 
Calcareous Woodlands. 
Mercurialis perennis in Ash-Oak Wood on Mountain Limestone, 
North Lancs. 
Air-dry 
Soil. 
Water. 
Water. 
Humus. 
Humus. 
133. 
April, 1911 
66-2% 
27-1% 
2-44 
160. 
August, 1911 
13-3 
10-2 
1-31 
Mercurialis perennis wilting 
in Beech Hangers, on 
the chalk 
escarpment, Hants., September, 
1911. 
Air-dry 
Soil. 
Water. 
Water. 
Humus. 
Humus. 
186 
• •• 
9-7% 
13-6% 
0-72 
187 
- In different woods 
12-8 
17-6 
0-73 
188 
• • • 
10-6 
10-6 
1-00 
Much more information must he collected before the conditions 
of the plant associations on calcareous soils can be compared fully 
with those of siliceous soils. But from what has been already set 
out it is clear there is no great disparity in the humidity of the 
habitats of Deschampsia on the grit and of Mercurialis on the lime¬ 
stone and chalk. Both appear liable to periods of considerable 
dryness and of about the same intensity. 
The Sand-Dune Formation. The principle that the water- 
content is a function of the humus-content, that has so far been the 
sole condition for the determination of the coefficient of humidity, 
proves to be insufficient in the case of certain soils with a very low 
humus-content. In the case of sand-dunes, where the humus is less 
than 1% and the water less than 5%, the ratio water/humus is both 
high and irregular in value. Obviously this is because the water 
held by the sand particles is no longer a negligible fraction of the 
whole water-content. Though allowance might be made for this 
disturbing factor either experimentally, or by the consideration of 
a sufficiently large number of analyses, its influence was first 
detected in another field. As this appears to afford a solution of 
the problem it appears desirable to use this material for a more 
critical examination of the coefficient of humidity and this is done 
from a mathematical standpoint in the succeeding part of this 
paper. 
Heather Moor Association. This habitat is selected for a more 
searching examination of the soil humidity because whilst the 
