179 
employed as manures produce effects in times proportioned to 
their degree of putrefaction ; those substances which are most 
putrid producing the most speedy effects, and of course soonest 
losing their efficacy. Having manured two pieces of the same 
kind of soil, the one with a mixture of dung and straw highly 
putrefied, the other with the same mixture newly made, and the 
straw almost fresh, he observed, that during the first year, the 
[)lants which grew on the land manured with the putrefied dung 
produced a much better crop than the other ; hut the second 
year (no new dung being added), the ground which had been 
manured with the unputrefied dung produced the best aop : the 
same thing took place the third year, after which both seemed 
to be ecjually exhausted. Here it is evident that the putrefied 
dung acted soonest and was soonest exhausted. It follows from 
this, that carbon only acts as a manure when in a particular 
state of combination ; and this state, whatever it may he, is evi- 
dently produced by putrefaction. Another experiment of the 
same chemist renders this truth still more evident. He allowed 
shavings of wood to remain for about ten months in a moist 
place till they began to putrefy, and then spread them over a 
piece of ground by way of manure. The first two years this 
piece of ground produced nothing more than others which had 
not been manured at all ; the third year it was better, the fourth 
year it was still better, the fifth year it reached its maximum of 
fertility; after which it declined constantly till the ninth, when 
it was (piite exhausted. Here the effect of the manure evidently 
depended upon its progress in putrefaction. 
When vegetables are allowed to putrefy in the open air, they 
are converted into a loose black substance, well know n under the 
name vegetable mould. On this mould plants grow with great 
vigour. It is the substance which renders newly-cultivated 
lands in America, &c. so fertile. When exposed to the air, in 
the course of cultivation, it is gradually w asted and destroyed, 
and the lands are thus impoverished. This vegetable mould, 
therefore, is obviously one of the grand sources of the food 
of plants. It deserves, therefore, an accurate examination. 
182 Acid Soils. Einhof examined a sort of mould to which he 
has given the name of acid vegetable mould. It occurs in 
low-lying meadows and marshes, and the plants which grow 
upon it in these situations are the different species of carex, 
loo adctaricu. 
