304 
places in these islands native ; but only in very 
small quantities. Phosphate of lime is generally 
conveyed to the land in the composition of other 
manure; and it is probably necessary to com 
crops and other white crops. 
Bone-ashes ground to powder will probably be 
found useful on arable lands containing much ve- 
getable matter, and may perhaps enable soft peats 
to produce wheat ; but the powdered bone in an 
un calcined state is much to be preferred in all 
cases when it can be procured. 
The saline compounds of magnesia will require 
very little discussion as to their uses as manures. 
The most important relations of this subject to 
agriculture have been considered in the former 
part of this Lecture, when the application of the 
magnesian limestone was examined. In combin- 
ation with sulphuric acid magnesia forms a soluble 
salt. This substance, it is stated by some enquirers, 
has been found of use as a manure ; but it is not 
found in nature in sufficient abundance, nor is it 
capable of being made artificially sufficiently cheap 
to be of useful application in the common course 
of husbandry. 
Wood ashes consist principally of the vegetable 
alkali united to carbonic acid ; and as this alkali 
is found in almost all plants, it is not difficult to 
conceive that it may form an essential part of their 
organs. The general tendency of the alkalies is to 
give solubility to vegetable matters ; and in this 
way they may render carbonaceous and other sub- 
stances capable of being taken up by the tubes in 
