BULLETIN OF THE BUSSEY INSTITUTION. 205 
with water, a percolate was obtained that gave no precipitate with am- 
monia, while the percolates from those portions of the earth which from 
their position in the tube had been less strongly heated and had not be- 
come gray gave abundant precipitates with ammonia. In this experi- 
ment in the glass tube, carbonic acid began to be given off from the earth 
before the tube became very hot, certainly at low redness. 
The results of these experiments are manifestly akin to those ob- 
tained by Reichardt and Blumtritt ;* who found, for example, that while 
no carbonic acid could be detected in the mixture of gases expelled by 
heating some substances, such as oxide of lead, gypsum, and some 
samples of whiting, large quantities of carbonic acid were found in the 
gases expelled from garden earth, from the oxides of iron and man- 
ganese, from clay and river-mud, and various other solid bodies. As 
much as 80% of carbonic acid was found in the gases expelled from 
one sample of ferric oxide, and more than 33% in those from a sam- 
ple of air-dried garden earth. But while the experiments of Reich- 
ardt and Blumtritt, upon ferric oxide and the like, point merely to the 
power of these substances to absorb carbonic acid from the air, at the 
ordinary temperature, my experiments with the calcined loams enforce 
the lesson, that roasted earths can retain much carbonic acid that has 
been formed within them by the oxidation of organic matters at the 
temperature of calcination. The fact of this retention of carbonic 
acid at high temperatures, and the easy solution of super-carbonate of 
lime from the roasted earths on the addition of water, suggests the 
inquiry whether some part of the efficacy of the processes of “ paring 
and burning,” and “clay burning,” which have at times been highly 
esteemed in agricultural practice, may not be due to the reactions of 
the supercarbonate. It is not impossible, even, that some samples of 
coal ashes may have a slight agricultural value on this account. 
In respect to the loams proper that have merely been kept dry and 
not calcined, I am still in doubt whether the greater part of the car- 
bonic acid found in them has been formed there through oxidation of | 
the organic matters of the loam, or whether a good part of it may not 
have been absorbed from the air, as the experiments of Reichardt and 
Blumtritt suggest. This point is left undecided by the results of the 
following experiments, which show, merely, that considerable time is 
required in order that a dried earth shall become charged with enough 
* “‘ Journal fiir praktische Chemie,” 1866, 98. pp. 418, 476. 
