303 
are injurious ; but it is probably in consequence of 
their supplying an excess of ferruginous matter to 
the sap. Oxide of iron in small quantities forms 
an useful part of soils ; and, as is evident from the 
details in the Third Lecture, it is found in the 
ashes of plants ; and, probably, is hurtful only in 
its acid combinations. 
I have just mentioned certain peats, the ashes of 
which afford gypsum ; but it must not be inferred 
from this that all peats agree with them. I have 
examined various peat-ashes from Scotland, Ire- 
land, Wales, and the northern and western parts 
of England, which contained no quanti ty that could 
be useful ; and these ashes abounded in siliceous, 
aluminous earths, and oxide of iron. 
Lord Charleville found in some peat-ashes from 
Ireland sulphate of potassa, i. e . the sulphuric acid 
combined with potassa. 
Vitriolic matter is usually formed in peats ; and 
if the soil or substratum is calcareous, the ultimate 
result is the production of gypsum. In general, 
when a recent peat-ash emits a strong smell re- 
sembling that of rotten eggs when acted upon by 
vinegar, it will furnish gypsum. 
Phosphate of lime is a combination of phosphoric 
acid and lime, one proportion of each. It is a com- 
pound insoluble in pure water, but soluble in water 
containing any acid matter. It forms the greatest 
part of calcined bones. It exists in most excre- 
mentitious substances, and is found both in the 
straw and grain of wheat, barley, oats, and rye, and 
likewise in beans, peas, and tares. It exists in some 
