4 2 
ELEMENTS OF AGRICULTURE. 
These are plaster, marl, magnesia, iron, and manga® 
nese.* 
41. Plaster, or gypsum,f is of importance not only as a 
3cnsti tuent part of certain soils, but also on account of the 
effect that it produces upon certain plants, to the growth 
of which it is particularly favorable. It is diffused through¬ 
out nature to some extent, and in some localities it is quar» 
ried in large quantities. After being pulverized, it is used 
either in building, or as an agent of vegetation. There is 
this difference between carbonate and sulphate of lime: 
when exposed to heat, the first loses its acid ; whereas the 
second, as it never separates from its sulphuric acid, ever 
remains in the state of a salt. 
42. Marls are found in many soils, and sometimes in 
such quantities as to make them unfruitful. They are of 
different colors — white, gray, blue, etc. Their elements 
are generally clay and carbonate of lime, often mixed with 
silica, onshells that also contain carbonate of lime. Marl 
is said to be more or less rich as it contains more or less 
carbonate of lime. 
43. Marls, as we shall see hereafter, are very useful in 
the amelioration of lands. They are distinguished from 
other earths by the effervescence produced when they are 
brought in contact with acids. This property is common 
to all substances containing calcareous principles. 
44. Magnesia, which in a pure state is a white, insolu¬ 
ble substance, is generally found in the soil in the state of 
a carbonate — that is to say, united with carbonic acid. 
Some writers pretend that this body, if employed without 
being calcined, is injurious to vegetation; but this is not 
* This last, as well as some other earthy bodies, are met with so rarely, 
and in such inconsiderable quantities, that we will make no further allusion 
to them, 
t Frequently called gypsum : it is. composed of sulphuric acid, lime, an * 
ater 
