ulphuret
of calcium. It is well known that all sulphurets are prejudicial to
vegetable life, and hence, when fresh gas lime is used, its effects are
often injurious rather than beneficial. But if it be exposed for some
time to the air, oxygen is absorbed, the sulphur is converted into
sulphuric acid, gypsum is produced to the extent of some per cent, and
the lime then becomes innocuous. When composted with dry soil, the
admission of air into the interior of the lime is facilitated, and this
change takes place with greater rapidity. The waste lime from
bleach-works, tanneries, and other manufactories, is occasionally used
by farmers; but unless obtained at a nominal price, it cannot compete
with good quick lime, owing to the large amount of water it contains,
and the consequent increase in the cost of carriage.

_Sulphate of Lime or Gypsum._--Gypsum has been extensively used as a
manure, and is found to exert a very remarkable influence upon clover,
and leguminous crops generally. It is employed in quantities varying
from two cwt. per acre up to a very large quantity, and almost
invariably with good results, in some instances even with the production
of double crops. Much speculation has taken place as to the cause of
this action which is so specific in its character, and from Sir Humphrey
Davy down to the present time, many chemists and agriculturists have
considered the matter. Sir Humphrey Davy attributed its action to its
supplying sulphur to those plants which, according to him, contain an
unusually large quantity of that element. That opinion has been since
entertained by others, but it can scarcely be considered as well
founded, for the more accurate experiments recently made do not point to
any conspicuous differences between the quantities of sulphur contained
in these and other plants. It is, moreover, to gypsum alone that these
effects are due, and if it were merely as a source of sulphur that it
was employed, there are other salts which could be equally, perhaps more
advantageously, used; such, for instance, as sulphate of soda. Others
have attributed its action to its power of fixing ammonia, but this
explanation is certainly untenable, for the soil itself possesses this
property very powerfully, and it is inconceivable that the addition of a
few hundred weights of gypsum should have any effect in promoting this
action. The experiments which have been made with gypsum leave no doubt
as to its effect, more es