246 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 
surface soil indicates unsuitability for cacao growing. The limits of shrinkage 
range from 2 per cent, in an open sandy soil to 16 per cent, in an exceptionally 
heavy clay. 
The author has made corresponding and further investigations of soils in 
the Leeward Islands. He attributes shrinkage to the action of a gel. At the 
point of maximum plasticity all the water present in the experimental blocks 
of soil is probably in union with the colloid matter present as a gel occupying 
the whole of the interstitial spaces of the soil and forming a network which, 
as water is lost, draws the soil particles together. The cubical contraction 
observed is equal to the volume of water evaporated from the block until the 
point is reached where internal friction begins to exert an influence. Further, 
the author finds that there is a roughly constant ratio between the percentage 
of soil particles less than 001 mm. in diameter and the percentage linear 
shrinkage. 
Pore space is related to linear shrinkage, and is determined by measuring the 
true density (D x ) of the soil, and the apparent density D 2 . The pore space (P) 
is then given as a percentage by P = — — - x 100. D 2 is determined by 
1 
weighing blocks of soil coated with a thin layer of wax of known specific gravity. 
When the percentage pore space is plotted on the #-axis against linear shrinkage 
(L) on the y-axis a straight line curve is obtained, of which the extreme points 
are given by L = 1, P = 27-7 and L = 13, P = 12-5. If this straight line is 
continued both ways it cuts the axes at x — 28 and y = 2 3 "5. It is known 
that the figure 28 approximates to that of the pore space found to exist in un- 
contracted coarse sand of uniform texture. What is the significance of the 
figure 23-5 ? It is the point where the linear shrinkage is 23*5 per cent, and the 
pore space zero. That is, it is a limit representing an imaginary soil entirely 
composed of pure dry colloidal clay, the dimensions of whose colloidal particles 
approach to the molecular order of magnitude. Since the distance through 
which the curve is extrapolated is considerable, the above figure can only be 
accepted as moderately approximate ; the curve does nevertheless enable us 
to calculate, to the same degree of approximation, the true colloidal content 
of a soil from a measurement of its percentage linear shrinkage, and thus to 
measure easily what it has not been possible hitherto to measure with any 
accuracy even by the best methods of physical analysis. Full experimental 
details with illustrations of the apparatus employed are given in the paper. 
J. E. W. E. H. 
Spruce and Balsam Fir Trees of the Rocky Mountain Region. By George B. 
Sufiworth (U.S.A. Dep. Agr., Bull. 327, Feb. 19, 1916). — The spruces are 
important forest trees, and some are much planted for ornament. They yield 
superior saw timber, the straight and even -grained wood being used for a great 
many commercial purposes, including paper pulp, for which it is unsurpassed. 
Seven species are indigenous to North America, and all occur within the 
United States. Four species occur over the western half of the United States, 
and three through the north-eastern States and Canada, two extending from the 
Great Lake region into Alaska. 
Black spruce is mainly an eastern and far northern species, but occurring 
in the Canadian Rocky Mountain region. There it is from 25 to 40 feet high 
and from 4 to 8 inches in diameter. In its wider eastern range this tree exception- 
ally attains a height of from 50 to 75 feet and a diameter of about 1 foot ; very 
occasionally it grows to 100 feet in height and 2 to 3 feet in diameter. 
White spruce has its main range in north-eastern United States and Canada. 
It varies in height, according to situation, from 15 to 75 feet and in diameter 
from 12 to 20 inches. The largest trees occur in the East, where the height is 
from 80 to 100 feet or more and the diameter from 24 to 36 inches. Trees 3 or 
4 feet in diameter and over 100 feet are rather rare. 
Engelmann — Picea Engelmanni — is known to lumber men mostly as ' spruce,' 
while some call it ' white spruce,' probably because of its resemblance to the true 
white spruce (P. canadensis), with which they may have become acquainted in the 
East. It is, however, commonly known to foresters and botanists as Engelmann 
spruce, a name which it is hoped may be generally adopted, both because of its 
distinctness and the fact also that it commemorates the name of one of the ablest 
students of western trees. It is cut extensively for lumber, which is used for 
general construction and to some extent locally for interior finish. The timber 
is also much used for temporary or light traffic ties, telephone and telegraph 
poles, mine props, fuel, house logs, and corral poles. 
Abies balsamea (Linn.) Miller is found only within the Canadian Rocky 
Mountain region. It extends southwards into the United States only from the 
