126° The Transactions of the South African Philosophical Society 
with sterilized gelatine, and kept the gelatine plates in the usual way 
for two days. 
On plates Nos. 1, 2, and 4, not a single colony of microbes appeared ; 
on plate 3 their number was considerable, viz., 27,000 per cem. of sand, 
and on plates 5 and 6 much larger, viz., 118,000 and 60,000 respectively. 
These results coincide well with those of the chemical analysis, and 
prove that the nitrification of these materials is a process quite similar 
to that in other countries. 
Having ascertained by local observations and subsequent experiments 
that the nitric acid of these nitrates and nitrate soils is derived from 
the accumulated refuse and remains of animals, specially of the rock 
rabbits, it will be necessary to show whence the other ingredient of the 
saltpetre, viz., the potash, is derived, and how the pure substance could 
have accumulated in such considerable quantities as one finds it. 
The potash is probably derived from the rock as well as from the 
feces. A sample of rock which I analyzed had the following composi- 
tion : 
Silica - - 35:3 Magnesia - Ses 
Ferric oxide - 55°66 Potash - - ee ioe. 
Alumina - Ss geste Soda - - =) 6 
Lime hy Ls) Moisture - = chs 
Of special importance for the process of nitrification is the presence 
of lime in the rock, for Professor 'Tolomei’s experiments have shown that 
the nitrifying microbes grow only in the presence of a basic substance | 
like lime or magnesia. He has also demonstrated that porous material 
favours the nitrification considerably. 
Both these conditions are well fulfilled in our case. The finely 
laminated shales become impregnated with the compounds of ammonia 
contained in the drainage from the animal remains, and in the fumes 
formed during their putrefaction. The fine fissures between the layers 
act like the pores in the soil, and the ammonia is well exposed to the 
action of the nitrifying agents. 
The result is the formation of the nitrates of potash and lime, of 
which the latter is very hygroscopic, and attracts moisture from the 
atmosphere whenever the air becomes damp. One day I could hardly 
notice any saltpetre along a cliff which I had seen glistening with white 
efflorescence the day before, although it had not rained as yet. The 
moisture of the atmosphere had been sufficient to dissolve the film of 
nitrates on the surface of the rocks. This solution of nitrates spreads 
‘and descends along the shales, and the saltpetre crystallizes again when 
the moisture evaporates. Being thus repeatedly dissolved and 
recrystallized, it becomes purer and purer, and being a very soluble 
