Causes of Fertility or Barrenness of Soils. 
191 
their place, formin£j a double silicate of alumina and ammonia — 
a compound alinost insoluble in water, but slif^htly soluble in 
rain-water, which contains carbonic acid. By this process the 
volatile and soluble ammonia, not required for the immediate use 
of the plant, instead of being- carried away in the water of drain- 
age, or evaporating into the atmosphere, is stored up in the soil 
in such a manner as to allow of its being given up slowly when 
required. Moreover he found that salts of potash, in the same 
way, can displace any of the other bases named, but not ammo- 
nia. Magnesia follows next, then lime, and lastly soda, which 
being the least valuable because the most abundant, is displaced 
by any of the others. To use the Pi^ofessor's own words, for the 
preservation of ammonia four compounds are made responsible, 
for potash three, for magnesia two, and for lime only one. We 
cannot fail to admire the beautiful economy of nature, which thus 
takes the greatest care of the most valuable substances. It was 
mentioned that the ammonia double silicate was only slightly 
soluble, and hence, it might be feared, would be too securely 
locked up to become available for the wants of plants ; but Pro- 
fessor Way has discovered that lime, when brought into connec- 
tion with the ammonia double silicate, causes it to be set free. 
Common salt also appears to possess the same property. May 
not this explain the well-known beneficial effect of small doses 
of salt and lime on clay soils for the wheat crop ? It would be 
quite out of place here to enter into any detail of the course of 
experiments by which these results have been reached ; they will 
be found most lucidly described in former volumes of the Journal. 
We have mentioned them because, if confirmed, they must exer- 
cise a great influence on the future progress of agricultural 
chemistry, and because they bear out what has been stated before, 
that it is to the peculiar state of combination, and not to the 
existence only of the various mineral substances in a soil, that its 
fertility or barrenness is due. Henceforward the point in an 
analysis will be, the proportion and description of the double 
silicates ; and the advice as to manure, rotation of crops, &c., 
will depend upon this. For instance, suppose a soil deficient in 
double silicates, then addition, if practicable, would be the first 
point ; if this could not be done, we should at least be aware of 
our weakness, and apply our manure in such a manner that the 
roots of our crop could absorb it before it passed away into the 
subsoil ; we should not lay on large dressings as food for a 
succession of crops, but should rather aim at giving a small dose 
to each. Should our land, on the contrary, be rich in double 
silicates, we might fearlessly bury our manure, confident that the 
soil would preserve it, and annually or frequently topdress with 
small doses of lime and salt. Perhaps we ought not to place 
