I2 4 The Physical and Chemical [March, 
combination. And these compounds which nourish us are 
not eternal or indestructible ; they are very easily destroyed, 
and their elaboration is tedious. So far we cannot make 
even the simplest of them artificially. We cannot, so far, 
take the oxygen and hydrogen of water and the carbon of 
coal or chalk, or of the atmosphere, and combine them to- 
gether so as to form starch, or sugar, or oil. We have to 
depend on the agency of plants, and to wait while they 
effeCt the task, which, in regions outside the tropics, they 
only carry on for about half the year. Plants, then, can 
extract phosphorus, and lime and sulphur, &c., from the 
soils ; carbon, oxygen, hydrogen, and nitrogen from the air, 
the water, and from the remains of plants and animals 
which have ceased to live, and can elaborate the whole into 
compounds which we in turn can digest and assimilate, or, 
in other words, make part of ourselves. One task, how- 
ever, the plant is unable to do : it cannot, as the most care- 
ful experiments show, take the free nitrogen of the air and 
assimilate it. It has to depend upon certain compounds of 
nitrogen, such as ammonia and nitric acid, which exist in 
the air, the water, and the soil, but to a very limited extent. 
Now these compounds of nitrogen are being formed but 
slowly, and they are very liable to destruction, i.e., to a 
decomposition into free nitrogen. Whether the entire stock 
of combined nitrogen in the world is maintained at a con- 
stant level, or is decreasing, is a very doubtful question. It 
is certain that if we allow animal matter, blood, flesh, 
uiine, &c., to putrefy, a certain proportion of the nitrogen 
present escapes as free nitrogen. 
Whether man will ever succeed in making the nitrogen of 
the atmosphere available as plant-food, — and hence ulti- 
mately as human food, by converting it into ammonia or 
nitric acid, time only can show. Many experimentalists 
have grappled with this grand problem, and ammonia, &c., 
have been attually produced, but so far the cost of the 
article obtained has exceeded its value. Hence it is, for the 
piesent, premature to take the free nitrogen of the atmo- 
sphere into account as a possible source of food. 
Now why do we mention the above faCts, which, to many 
of our readers at least, must be a thrice-told tale ? Simply 
to remind them how very much more of the elements of 
food aie required than what is, at any given moment, locked 
up in human bodies, lo take a very simple illustration : a 
trader needs not merely capital sufficient to stock his shop : 
he lequires lunds sufficient to maintain himself, to meet all 
curient expenses, and to buy in fresh goods until the pay- 
