THE- APPROXIMATE SIZE OF SOIL PARTICLES AT
WHICH THE HEAT OF WETTING IS MANIFESTED
AND ITS USE IN DIFFERENTIATING
COLLOIDAL FROM NON-COLIOIDAL
SOIL MATERIAL.

by
Lawrence GJCjyaP

A THESIS
PRESENTED TO THE COMMITTEE ON ADVANCED
DEGREES OF THE MICHIGAN STATE COLLEGE OF
AGRICULTURE AND APPLIED SCIENCE IN
PARTIAL FULFILLMENT OF REQUIREMENTS FOR
THE DEGREE OF DOCTOR OF PHILOSOPHY.
Soils Department
East Lansing, Michigan
19

2 9

ProQuest Number: 10008345

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ACKNOWLEDGMENT

The writer wishes to express his appreciation to
Dr. M. M. McCool and Dr. G. J. Bouyoueos for their
helpful suggestions and assistance during the period
of graduate study and in planning and working on this
problem.

71299

1

THE APPROXIMATE SIZE OP SOIL PARTICLES AT WHICH THE
HEAT OP WETTING IS MANIFESTED AND ITS USE
IN DIPPERENTIATING COLLOIDAL PROM
NON-COLLOIDAL SOIL MATERIAL
INTRODUCTION
Recently there has been a great deal of controversy
concerning the basis of distinguishing and defining soil
colloids from non-colloids.

Even in the text books on

colloids there is a difference in size attributed to
colloidal systems.

It is true that size is one of the

most dependable criteria for colloids but there are wide
variations in the diameter of the particles of different
elements and compounds at the point where the exhibiting of
the generally accepted properties of colloids cease.

The

Brownian movement is given as one of the properties of collpidal
systems, although it is now definitely known that an absolute
size limit cannot be set where this movement ceases for all
colloidal systems.

Therefore, why should we consider this

Brownian movement as a true colloidal property when we
realize that the larger size particles give us other colloidal
properties and not this one?

This has been considered by most

workers as only one property of a colloid smaller in size than
the maximum limit which we attribute to soil colloidal
material.
Anderson et al (2) selected one micron as the upper limit
for colloidal particles in the study of soil colloidal mater­
ial absorption.

Davis (8) in the study of the plasticity

and coherence of different soils used one-tenth of a micron

as the upper limit.

Most workers state that one micron is

taken as the diameter of the largest size soil colloidal particle
as this is the upper limit of Brownian, movement. Anderson
(1) in the work on the heat of wetting of different minerals
takes as the upper limit of non-colloidal material that which
passes through a 130 mesh sieve but does not set a definite
lower limit.

Due to the difficulties and labor involved in

a complete mechanical separation of a soil and finely ground
minerals, the lower limit of size that most workers were
dealing with is not known.
It must be admitted that at present the classification
of soil particles into colloidal and non-colloidal material
is not based on any fundamental grounds, but is merely an
arbitrary classification.

Sjpllema (15) considers that

practically all the constituents of soils, except quartz
grains and undecomposed mineral fragments, are colloids,
because they are colored by dyes.

Gile et al (9) states

that the data on the heat of wetting of minerals closely
parallel the results obtained in a more extensive study of
soil minerals with respect to absorption, indicating that
the heat of wetting is almost a function of the colloidal
material.

Bouyoucos (6) states that recent studies on heat

of wetting indicates that the finer silt separates manifest
the heat of wetting.

In the studies of soils by the

hydrometer method he includes the fine silt in the colloidal
content of a soil.

He believes that the material which,

exhibits the property of heat of wetting should be spoken of
as colloidal material, and that which does not possess this

property should be spoken of as non-colloidal material,
regardless of the size particles.

Since this property is

an energy manifestation, rather than arbitrary size limit, it
appears most logical to adopt it for distinguishing colloidal
from non-colloidal material.
Since the absorptive power and heat of wetting become
more pronounced as the particle, size decreases, it was con­
sidered advisable to separate different soils into different
size separates as accurately as possible, and to ascertain the
size at which the heat of wetting ceases to be manifested.

To

secure soil separates suitable for heat of wetting determinations,
it was necessary to free the soils from calcium and magnesium
carbonates, organic matter, and coating of gels around the soil
particles.
The calcium and magnesium carbonates, if present, would
lower the heat of wetting of the separates by decreasing the
effective weight of the soil, as carbonates according to
Bouyoucos do not possess the heat of wetting property.

This

effect would be more pronounced in the fine separates, such
as colloid and clay, as they are in most cases evaporated to
dryness and the carbonates from the entire soil are thus
concentrated in these separates.
The organic matter and coating of gels must be removed,
as organic material and gels give a high heat of wetting.

The

coatings exist in natural field conditions as sols around the
soil particles, and no doubt are hard to remove completely.
It was thought that pure minerals and rocks being free
from organic matter and coating of gels should be ground for

comparison with the soil separates.

In this manner a few

interesting facts may be secured on the important question
of the approximate size soil particles at which the heat of
wetting property cease to be manifested.

5.

EXPERIMENTAL PROCEDURE
The heat of wetting was measured according to the
method described by Bouyoucos (3).

It consisted of placing

twenty-five to fifty grams of air-dried material in a wide
glass tube and drying in an electrically heated oven at a
temperature of 110° Centigrade for twenty-four hours.

The

tube was removed, closed with a rubber stopper, and allowed
to attain the room temperature.

After exact readings of weight

and temperature were observed, the soil was quickly and
carefully poured into a calorimeter containing 100 grams of
water.

Extreme care was exercised before mixing to have both

the soil and water at exactly room temperature.

In order to

be able to convert, if necessary, the rise of temperature into
heat calories, the water equivalent of the calorimeter was
determined.

This was found to be twenty-five grams of water.

Eor the specific heat of the material used in this work, the
value of 0.2 was employed throughout.

This value was used for

\*

the mineral separates as well as for the soil separates as
Regnault, Joly, Batholi and pionchon (13) found this value
for different mineral powders several years ago.
The temperature of a gray soil was used as a standard
for obtaining the temperature of the materials used in each
determination.

This gray soil had been subjected to the same

conditions of heating and cooling as the other materials.
Suppose that the temperature of the gray soil was 0.05° C.
lower than that of the other materials used.

As the specific

heat is 0.2 calorie per gram, the water would be increased
0.01 calsrie for each gram ad.ded to the calorimeter. Therefore,

by using 20 grams of material the increase heat transferred
to the water v/ould be only 0.2 calories.

It is impossible

to read this increase in 100 grams of water.

Therefore, the

small difference in temperature between the standard and the
materials used, would not effect the results to a measurable
extent.
The thermometers used were checked against each other.
They could be read within 0.025° C. and slightly less with
the aid of a reading glass.

Therefore, if the heat of wetting

was only 0.2 calorie per gram and fifteen grams were used,
the increase in temperature would be only 0.024° C. on the
determination thermometer.

This increase in temperature .

could be read quite easily.
If extreme care is used, the temperature of the water
in the calorimeter would never be higher than that of the
separates.

In every case it would be lower, due to the

cooling effeot by evaporation from the water surface and
wet parts of the calorimeter.
After considering the above sources of error, it appears
logical to believe that 0.2 calorie per gram of material is
sufficient to consider as an experimental error for each
determination.
The soils were treated with dilute hydrochloric acid,
washed with suction until free from chlorides, and then
treated with dilute hydrogen peroxide.

The hydrogen peroxide

used was Mercks Superoxyl, a commercial product containing
thirty per cent hydrogen peroxide.

The dilute hydrogen

peroxide treated soils were heated on a steam bath until the
effervesence ceased and more concentrated solution could be
used.

finally the thirty per cent solution was used.

The

soils were then transferred to a Buchner funnel and washed
free ffom the formed organic compounds.

They were then

dispersed every day for two weeks in a special dispersion
machine described by Bouyoucos (5).

After each dispersion,

the soil was placed in a liter beaker which was filled with
distilled water to a depth of 13 cm.

The temperature of the

water was taken and after a certain interval of time, read
from Table 1, the particles in sa.spension were siphoned off.
In order to obtain the particles, this suspension was placed
on a steam bath to evaporate the water present or the supernated liquid siphoned off.
The soils were treated with dilute hydrochloric acid
to decompose the carbonates present and remove the bases in
the form of chlorides.

The carbonates are known to have a

decided effect upon the heat of wetting produced by soil as
their presence reduces the weight of soil and thus gives
a low heat of wetting determination per gram of soil material.
Pate (12) has shown that monovalent bases in a soil decreased
the heat of wetting as much as 46$ in some cases.

The divalent

bases did not exert such an influence.
As organic matter gives a high heat of wetting, as much
as possible was removed by the hydrogen peroxide to secure
samples suitable for the determinations.

Many workers have

estimated the amount of organic matter that can be removed by
the use of this compound.

Other chemicals or heat Could not

be used because this would influence or destroy the heat
of wetting property of the soils.

Robinson (14) in his

recent work on the hydrogen peroxide method of oxidizing
organic matter in soils, states that the variety of soils and
colloids containing 0*42 to 95.85$ of organic matter show
that practically all the organic matter can be decomposed in
some soils and that in other soils considerable organic matter
is unattacked.

If the organic matter is not destroyed, one

may expect it to have an effect on the heat of wetting deter­
mination.
The intensive dispersion treatment should rid the
particles of most of their gel coatings and give particles
above 0.005 mm. in diameter of pure crystalline minerals.
However, it is doubtful if all the colloidal gel coatings can
be removed, especially from the small silt particles, since
the gels, adhere very strongly to the particles.
The minerals used were ground to a fine state with an
agate mortar and pestle.

It was realized that as quartz has the

same hardness as agate which is a quartz given this name
because of its color, the powder would not be contaminated
with any foreign matter as it might be if another apparatus
were used for grinding.

The other materials used will not

grind away the agate mortar and pestle as their hardness is
below that of agate.
After grinding, the different sized mineral particles
and treated soils were separated by the application of
Stokes' law.

Although there may be many objections to the

employment of this law, still it has been adopted by the
National Soil Science Congress (17) as an aid in the mechanical
analysis of soils.

The formula used in calculating the size

of the particles at various rates of falling is as follows:
v *

8 g<3-2(s“s~i)

9 nt .4

v - distance in cm. of a particle d size to fall
in one minute
d - diameter of particle in mm.
s - specific gravity of material used.
s^- specific gravity of water.
nt - coefficient of viscosity of the water at given
temperature.
g - force of gravity or 980 dynes.
2/9 - factor expressed in Stokes original law.
6/40 - factor used to express v in terms of centimeters
per minute when diameter is in millimeters.
_______cm________
as well as the formula
t in minutes
above, and as we.are interested in finding the time, the
As v =

equation becomes (A) t in minutes - cm depth of liquid.9 .nt.40
2.g.d.8 .{s-s£).6
The value of (s) changes for different minerals but for the
soil separates 2.65 was always used.
Table 1 was calculated by the above general formula (A)
for a depth of 13 cm. of water for both kinds of separates.
In the process of separation by the application of
Stokes1 law, the concentration of the material in water was
never allowed to exceed 5% of weight.

By keeping the con­

centration below 5$, the errors due to the friction of the
solid particles in suspension would not be encountered and

TABLE

1 - TIME

III'MINUTES ^OR TOLLO’TING
TO SETTLE 15 CM.

SIZE

(mm.) PARTICLES

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the results would be more accurate.

In making the separations,

the samples were thoroughly stirred with the water, the
temperature was taken, and after the mixture had been allowed
to stand for the length of time read from the table for the
specific sample, the water and its suspended particles were
siphoned off by using a glass tube having a side hole at the
lower end.

This was used to avoid drawing into the tube the

particles which had already settled to the bottom.

12.

EXPERIMENTAL

results

Table 2 presents the data obtained on the heat of
wetting of different soil separates expressed as calories
per gram of material.

In examining the table, it is noticed

that the heat of wetting ceases entirely to be manifested
for the separate 0.035 to 0.05 mm. in diameter for each soil
except one.

In soil separate 0.02 to 0.035 only four soils

gave a heat of wetting above the considered experimental
error.

The Missouri soil shows a heat of wetting of 1.11 calories

per gram in the 0.005 to 0.02 separates.

The Florida soil

separate gave a heat of wetting of 3.65 calories for the 0.005
to 0.02 mm. in diameter separate.

In the 0.005 to 0.01 separate

column, it is noticed at once that all the soil give a decided
heat of wetting.

In this separate the Florida and Missouri

soils would be expected to give high heat of wettings as their
clay separates give a high heat of wetting as well as their
next finer separate size.

One would conclude from the data

presented that the heat of wetting ceases in the finer silt,
but in some soils may be found in the coarser material also.
The reason that these coarse materials of the soil tends
to give the heat of wetting may be due to the presence
of colloidal coatings of gels around the particles, the
unremoved organic matter, or both.
This data tends to confirm Bouyoucos' (6) contentions,
as stated previously, that the finer silt should be included
in colloidal material as it tends to give the heat of wetting
property similar to that of clay and the colloid separates,

IS

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15

TABLE

2 -

HEAT

OB WETTIHG

OB SOIL SEPARATES EXPRESSED
PER GRAM OB MATERIAL

AS CALORIES

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but in a different degree*
Table 3 presents the heat of wetting expressed as
calories per gram of the mineral separates obtained.

One

will notice that all the ground mineral separates give a
definite heat of wetting in the fine material or for
particles smaller than 0.005 mm. in diameter.

The pure

zircon mineral gave a heat of wetting of over 0.3 calorie
per gram in the 0.005 to 0.01 mm. separate.

The biotite

gave 0.35 calorie for the 0.02 to 0.035 mm. size but the
heat of wetting of the remaining larger particles are within
the experimental error and should not be considered
significant.

The hornblende shows a large heat of wetting

even in the particles larger than 0.05 mm. in diameter.
This heat of wetting, 1.26 calories per gram, cannot be
disregarded as hornblende is a constituent of many soils
and must play an important role in the heat of wetting of
soils as a whole.

Cushman (7) suggests that a colloidal

film is formed on the surface of some minerals due to the
formation of insoluble hydrolytic products of a colloidal
nature.

This may be the explanation for the hornblende and

biotite minerals giving a higher heat of wetting than the
other minerals shown in the table.

From the results recorded

it shows that different minerals exhibit the property in
different degrees.

This bears out the theory of Harkins and

Ewing (10) that the quantity of the heat of wetting of a
colloid is indicative of the nature of the material.

The

compression force is considered to be due to the attraction

lA

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force between the molecules of the liquid and the molecules
of the absorbent.

The heat evolved by wetting is due

largely to the heat of the compression of absorbed liquid*
Bouyoucos (5) has shown that the force with which the soils
absorb and probably compress the water that produces the
heat of wetting is very high*
In comparing tables 2 and 3, it is observed that the
soil separates are larger in diameter than the mineral
separates where the heat of wetting property ceased to be
obtained.

This would be expected as the soil particles

are probably more porous and haue an increased degree of
activation, due to weathering processes.

It is probable

that the organic matter and gels had not been entirely
removed or that some particles are coated with hydrolytic
products of minerals similar to hornblende.
As was stated previously, all the organic matter is
not removed from some soils by treatment with hydrogen
peroxide.

Since the organic matter particles have a lower

density tftan

the soil particles, we would expect to find

most of the small size particles of organic matter in the
fine separates.

On the other hand, the large size organic

matter particles would be reduced in size as the hydrogen
peroxide would oxidize the easily oxidized part of the
particle and leave the more resistant.

In this manner, we

would expect to find the most organic matter in the colloid
and clay separates.
In order to ascertain if the separates after being

treated with hydrogen peroxide still contained organic
matter, their nitrogen content was determined.

Table 4

shows the per cent of nitrogen remaining in the soil
separates as determined by the Gunning-Hibbard method for
total nitrogen (16). All the nitrate nitrogen had previously'
been removed by repeated washings as determined by the
colorimetric method.

Table 4 shows that the fine materials

contain the largest amount of total nitrogen. In every case,
except the Missouri soil, the colloid separate contained
more nitrogen than the clay separate.

Me Cool et al (11)

showed that the fine soil material gives a high heat of
wetting and has a higher nitrogen content than the coarse
material.

In the Missouri soil the clay had the highest

nitrogen content.

The 0.005 to 0.01 mm. in diameter separates

also gives a fairly high percentage of nitrogen. It is very
uncertain whether the organic matter present as determined
by the total nitrogen determination causes the difference
between the heat of wetting of a given size soil separate
and the corresponding mineral separate, providing the
number of each size particles were equal in both oases.
Since the soils si©conglomerations of different minerals,
it was thought that possibly hornblende, biotite and similar
minerals in the soil would coat the other particles as well
as themselves with a hydrolytic colloidal coating, and thus,
increase their heat of wetting property.

Several samples

of different soil separates and mineral separates were treated
for ten days with hornblende particles less than 0.005 mm.

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in

diameter in suspension.

After this time, the fine

hornblende particles were separated by sedimentation and
the heat of wetting determined.

No increase in heat of

wetting property could be detected after this treatment.
Since the fine material of the soils and minerals
were present, it was decided to determine the effect of
the heat of wetting property.

Bonyoucos (4) has shown

that at 800° 0. the physical properties of soils, such
as plasticity, heat of wetting, and unfree water, had
completely disappeared.

The separates at hand were heated

at different temperatures up to 830° G. in an electric
muffle furnace.

Table 5 shows the gradual decrease in the

heat of wetting property after exposure to higher temperature.
All of the materials except limonite decreased gradually
until at 830° C. only the kaolin, limonite and silica gave
any heat of wetting.

The limonite at 110° G. gave a heat

of wetting of 1.12 calorie per gram, at 308° G. it gave
3.48 calories per gram and then gradually decreased until
at 830° C. it gave 0.49 calorie per gram'.

This is interesting

as there was an increase and then a decrease with the addition
of heat.

We know that there are many forms of the oxide of

iron, the only difference in chemical composition being
the water of hydration.

In heating the limonite up to

305° G. some water may have been driven off or the material
ohfemically changed.

It is doubtful if by dehydrating it,

we had decreased the size of the particles materially.

Therefore, the increase in heat of wetting per gram of
material must have been due to a physical change of the
surface caused by a chemical change through the ignition
process.

This rearrangement of molecules oh the surface

or change of surface due to the loss of water gave the
material a greater attraction force for the water and
thus increased the heat of wetting.
The moisture contained in each sample used here
should be the same at the time the heat of wetting of the
heated material was determined as it was originally.
Each material was heated in the muffle furnace and then
allowed to cool at room temperature before being placed
in the heat of wetting tube and heated to 110° C . in the
electric oven.

It was thought that by this procedure the

moisture content would be the same in each case as it was
originally providing that no other changes had taken place
during the heating to 830° 0.
But why did, the materials decrease in their heat of
wetting property as is shown by the other minerals and
soil separates in table 5?
Since one observes that the sharp edges of a jagged
piece of glass will fuse long before the main piece of glass,
it was thought that possibly the small particles had fused
together.

The substances were then placed in distilled

water and separated by the application of Stokes’ law to
determine if the particles had increased in size.

Table

6 shows the different size particles after heating to
830° G.

In every case there was 89$ or more of particles

TABLE

5 - CALORIES PER GRAM OP SOIL COLLOIDS
f0.005 to smaller) AT DIFFERENT
OP IGNITION

AND THREE MINERALS
TEMPERATURES

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22.

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6 - PEE GMT OP DIFFERENT SI2E PARTICLES
MATERIALS TO 850° CENTIGRADE

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over 0.02 mm. in diameter left in each sample.

This

would indicate that the particles had fused together
and as the particles become larger the heat of wetting
decreased.

The particles present after heating, as shown

by separation, indicated that different minerals and soil
particles have different fusion points.

The silica has a

high fusion temperature. Therefore, it lost its heat of
wetting property gradually as the size increased, but at
830° G. the heat was not sufficient to form all particles
that give no heat of wetting.

In the case of soils, their

composition is heterogeneous and contain some minerals that
will fuse at a low temperature and will thus coalesce to the
minerals that have not fused.

By doing so, the size of

the particles will increase proportional to the number of
low or high fusion minerals present in the sample.
The Florida colloid and Kaolin mineral after heating
and separating into their different size particles were
reground by hand to try to restore their original heat of
wetting.

The Kaolin was increased to 0.82 calorie per

gram while the Florida sample was increased from 0 calorie
per gram to 1.53 calorie per gram.

This is very small

indeed when one remembers that the heat of wetting of the
Florida material was 15.4 calories per gram before heating.
Still one must consider that those particles were very
small and a larger amount of energy must be expended to
break down a particle 0.02 mm. in diameter to 0.005 mm.
and smaller.
There is no doubt, of course, that the soil material

which gives rise to the phenomenon of heat of wetting is
the colloidal matter and their degree of activation
developed by the weathering processes.

Therefore, one

would expect to restore the original heat of wetting of
the Florida soil on grinding by hand.

25

SUMMARY '
In this paper it was proposed to show the size of
particles from different soils at which the heat of
wetting property ceased to he manifested.

In order to

secure suitable samples of different soil separates from
soils for heat of wetting determinations, they were
treated with dilute hydrochloric acid to remove the
carbonates present and with hydrogen peroxide to remove
the organic matter.

The soils thus treated were separated

by applying Stokes' law of sedimentation.
All the soil separates give a heat of wetting in the
finer silt separate while a few possess this property in
the coarse material, 0.02 to 0.35 mm. in diameter.

It

appears that the finer silt should be included in the
colloidal content of the soil as they possess the property
of heat of wetting.
Pure minerals finely ground and separated did not give
such a high size limit as the soil at the place where the
heat of wetting ceased.

Hornblende and biotite gave a

high heat of wetting even in the large silt particles.
The amount of nitrogen in the soil separates varies.
Larger amounts of nitrogen and, therefore, organic matter
were found in the colloidal material of the soil.
o r g a n i c - ' matter

This

may account for part of the heat of wetting

above that of the same size material separates.
Soil colloidal materials and finely ground minerals
were ignited to 850° G. to determine why the heat of wetting

was destroyed.

After heating to 830° 0. and separating

into the different size separates, it was found that in
every oase, the percentage of 0.02 mm. in diameter
particles was over eighty-nine.
One soil colloid and one mineral after ignition
were reground and the heat of wetting was partly restored.

#kr trfhe Scat af lettihg of % i l Colloids" *

t» Mm8<

dcmr* Agr. Res* Yol* 2t* Ho* 9 Pp.927-937,
S*

Anderson*

Fry#

?«3*» Otta* P.X*.

and
by
Colloidal and Hon
Soil
Constituents". TI*8*D.A* Bui# 1122, 20 p#
"

Bouyoueo s ,G-.^*

\

M i

•-

1

of Igja.ition a$ Furious
feraperatures upon Gertai* Physical
Properties of Boils** Soil Dei # Yol* 17

Ho. Zm PP* 135-139. 1923*
"Heat of Hotting of Soils Dried at
Different Temperatures and the Fore®
at VfMch Soils Absorb Water? Soil* Sol#
Sfc* 1* H
§'
4k'

■Mk® Hjdroaoter as a Sew and Sapid Method
fox* DewersJiaijqg the Colloidal Content
Of Soil*# Soil Dei* Yol* 23. So* 4
pp*

3X9*331*

I92Y*

"She Hydrometer Method for Studying Sofia*,
Soil Sol# Yol* 25* So* 5 pp* 365-369#

6#

7*

Oushmn, A*S*

8*

Davis* S«B*

**

%

"Th© effect ef later on Rock Powders"#
U*3#D*A* -Cur# of Cheinistry« Bui# 92# 1905«
of Olay and its
to Mode of Origin"*
frass*

American Ins* Min* Yol* 57* pp 451.488i
9*

Oil©* P*£#, Middleton* W*H»* Robinson, W*0*t Fry* W.H., and
Anderson* M*S* - "Estimation of Colloidal
Material in Soil by Absorption"* U.S.D.A.
Dal.* 1193* 42 pp* 1924*

10*

Harkijoa, W#D** and Ewing, D*T*, - "A h igh Pressure due to
Absorption and the Density and Tolume
Relation of Charcoal". lorn8* Amer# Chem.
Soe# Yol. 43* PP. 1787—1802* 1921.

11*.
^

McCool* M.-M*# Weidemann, A.G*, Schlnbatis, G.R. - "Further
*
Studies on Michigan Soil Profiles with
Special References to Dispersed Materials"♦
Soil. Sci. Yol. 23. Ho. 5 pp. 391-397.1927.

z$
s

2Z*

Pate, W.W.

—

"The Influence of the Amount and Nature

of th© iteplacable Bases upon the Heat
of Wotting of Soil Colloids? Soil. Sei.
Vol. BO pp. 329-837, 1925.
13#

Regn&uXt# Joiy, Bartholi and xionchon. — Cited from
Mnebarger, €#£» "The Heat Bnvolved
vdien liquids are brought in Contact with
reorders”* Phys. Res. 13s48-54* 1901.

14#

Robinson, W*0*

18.

S.5ollema -* Sited from Cameron# P.K* and Beix# J.M.

*• ttBet©rmination of Organic Matter in
Soils by Means of hydrogen xeroxide" #
jour. Agr* Res. ¥ol. 84* No. 4 pp. 339-356
19S7*
"fh© Mineral Constituents of the Soil
Solution"* U,S.I>*A. Bur. of Soils.
Bui. No. 30.

16.

Methods of Analysis, A*0*0. 1924.

Total Nitrogen

BetermiJmtiona - pp. 27.

17,

International Society of Soil Science. P. 18 - 1927*
Held at Rothamated Harpenden in 1926*