212 
Journal of Agricultural Research 
Vol. VIII, No. 6 
bromid, aluminium chlorid, etc., yielded far greater depression of the 
freezing point than would be theoretically expected. Jones attempted to 
explain these abnormal results by assuming that these hydrates take up 
water, forming complex compounds with it, and thus remove it from 
the field of action, so far as the freezing-point lowering is concerned. 
(4) The chemical composition of the soil itself would probably seem to 
presuppose that some of the water in the soil must be chemically com¬ 
bined, if some of our prevalent chemical knowledge is correct. The 
present status of our knowledge of the chemical composition of soils 
indicates that the soil contains many colloidal hydrates, such as alumi¬ 
nium, silica, iron, and magnesium, in simple or in complex combina¬ 
tions or zeolites. If these compounds exist in the hydrate form, then 
the water of hydration, according to our present conception, is probably 
chemically combined. 1 The amount of water that many of these hydrates 
take up and combine with it, probably chemically, is really very great, 
as will be'seen from the following formulae: A 1 2 0 S * 38 H 2 0 , Si 0 2 • i. 35 H 2 0 , 
Fe20 3 *4.25H 2 0, Na 2 S 0 4 • ioH 2 0 , Na^COg* ioH 2 0 , CaCl 2 - 6 H 2 0 , etc. The 
force with which this water of hydration is held by the different 
hydrates varies considerably. Such hydrates as Na 2 S 0 4 *ioH 2 0 lose 
most of their water upon being exposed to the ordinary atmosphere, 
while others, such as CaCl 2 * 6 H 2 0 , will lose it only at elevated tempera¬ 
ture. Furthermore, all the water in any hydrate is not held with the 
same force but with a different degree of force. This is shown by the 
fact that the same hydrate possesses different degrees of aqueous pres¬ 
sure at the various degrees of hydration or formula weights of water. 
Thus, copper sulphate in the form of CuS0 4 *5H 2 0 exhibits an aqueous 
pressure of 47 mm., while in the form of CuS 0 2 -H 2 0 it shows an aqueous 
pressure of only 4.5 mm., at a temperature of 50° C. That all the water 
in a hydrate is not held by the same force is further confirmed by experi¬ 
mental data, which show that the greatest portion of water of hydra¬ 
tion of most hydrates is lost below the temperature of ioo°, while the 
remainder is given off above this temperature. 
(5) That some of the unfree water may exist in the soil as chemically 
combined is further indicated by the researches of Muntz and Gandechon 
(8) on the heat generated by dry soils upon being wetted. These investi¬ 
gators found that a large amount of heat was generated when the soils 
were wetted with water, but very little if any when they were brought in 
contact with benzene and toluene. They reasoned that, if the heat 
1 The vapor tension of the salt hydrates, such as the CaCl2-6H20, NaaSO^ 10H2O, etc., behaves quite 
different from that of the colloidal hydrates, such as the Al203*38H 2 0, SiCVi^sHaO, etc. In the former 
hydrates the vapor tension decreases by sudden steps as water is being withdrawn, indicating that definite 
hydrates are being formed. In the latter hydrates, however, the vapor tension decreases continuously 
without any sudden break in the curve, indicating probably that no definite hydrates are being formed. 
As a result of this difference in the vapor tension of the two classes of hydrates some investigators are led 
to believe that the water in the colloidal hydrates exists as physically adsorbed water and not as loosely 
chemically combined water. The opposite view is held by other investigators. Apparently the subject has 
not been definitely solved one way or the other. The problem is exceedingly difficult of definite solution. 
