328 ON THE CULTIVATION OF INDIGENOUS OPIUM. 
from the calcareous carbonate, the quantity of this must be estimated 
by the loss another weight of earth sustains by dissolving it in a given 
quantity of weak hydrochloric acid ; subtracting then the latter weight 
from that which the calcined residue expresses, the remainder is the 
weight of organic matter. The residue of the calcination is treated by 
hydrochloric acid in a little test tube ; all the oxides are dissolved, 
with the exception of the silica, which is collected by a filter, and which, 
after having been well washed in hot distilled water, should be calcined 
before its true weight can be taken. 
The hydrochloric separation is precipitated by a solution of bicar- 
bonate of potash. The bi-oxide of iron, the alumina, and the lime are 
separated, whilst the magnesia remains in the filtered solution and may 
be extracted from it by boiling. 
The precipitate formed by the bi-carbonate of potash is collected by 
decanting or filtering; whilst damp it is put into a solution of caustic 
potash and boiled to collect the alumina, whish is afterwards separated 
from this alkaline solution by hydrochlorate of ammonia. 
The insoluble portion of the precipitate in the potash only contains 
the bi-oxide of iron and the carbonate of lime ; these are to be dissolved 
again in hydrochloric acid, and in adding afterwards ammonia the 
bi-oxide of iron is isolated from the lime which floats upon the liquor, 
and in its turn is precipitated by a solution of carbonate of potash. 
Each principle thus separated should be strongly calcined and then 
weighed, that the properties may be known as they existed in the 
specimen of earth submitted to analysis. 
The name of humus has been given to the residue formed by the 
decomposition, more or less advanced, of organic substances exposed to 
contact with the air. This black residue, in consequence of its earthy 
appearance, is known also by the name of vegetable or animal earth, ac- 
cording as it proceeds from vegetable or animal substances. It supplies 
agriculture with an excellent manure, and appears to act on the process of 
vegetation, not only by the soluble saline principles which it contains, but 
by the property it possesses (as observed by de Saussure and Humboldt), 
of absorbing by its carbon a certain quantity of oxygen from the air 
and producing carbonic acid gas, which decomposed by the plants, 
becomes to them one of their principal elements. 
The enterprising researches of M. Theodore de Saussure have proved 
that vegetable earth contains a very small quantity of extractive 
matter soluble in water and alcohol ; but that it is almost entirely 
formed of a brownish-black matter, soluble alkaline solutions, and 
having the characteristics of ulmine ; and that in equal weights of each, 
it contains more of carbon and nitrogen, and less of hydrogen and oxygen, 
than the vegetables which have furnished it. 
Though the composition of earths comes very near in general to those 
which we have described, they vary according to the nature of the 
organic substances which produce them. 
