G28 
PllOGIlESS OF CHEMISTRY. 
the chcnnlsti-y of fanning, and several ingenious and sug¬ 
gestive theories relating to practical agriculture. In the 
introduction to the last German edition of his ‘Agricultural 
Chemistry/ Liebig refers in terms of studied disrespect to 
the investigations of Mr. Lawes, and uhilc misquoting a 
paragraph in one of Mr. Lawes^ publications, endeavours to 
convey the impression that that gentleman was unacquainted 
with the correct use of the term ‘ mineral/ and liad misun¬ 
derstood Liebig'’s mineral theory, which he is generally 
considered to have disproved. I mention this circumstance 
with pain, and have no doubt that all wdio value Liebig’s truly 
important scientific labours will regret it as much as 1 do. 
Another practical question which science has latterly brought 
])rominently before the attention of the public is that of the 
utilisation of the'drainage of towns. It is estimated that the 
quanlit}^ of nourishment for plants wasted in London alone in 
this form is worth about a million sterling per annum; but this 
valuable material is contained in so large a quantity of M ater, 
that no plan has come into 'working for separating it out 
profitably for use. Some persons are of opinion that the 
sewage might with advantage be conveyed through pipes for 
use in the fields, especially on meadow land, to which it is 
most easily applicable. Baron Liebig has written a letter on 
the subject, which is foi'M'arded by Alderman JMeclii to the 
Journal of the Society of Arts, containing a proposal to 
mix tlie liquid '*.vith siiperpliosphate of lime before distri¬ 
buting it, by which he considers that the value of the con¬ 
stituent already contained in the liquid M'ill be practically 
increased. It is, hoM’ever, not likely that the opinion of a 
chemist '^vill decide the authorities to adopt an experimental 
scheme of the kind, as it is really rather an engineering and 
commercial than a chemical question. The practical test of 
value commercially is ho'^v much an article uill fetch, and 
the data of this kind before us does not lead to the antici¬ 
pation of a profit at all approaching to M’hat theory suggests 
from the sale of this refuse. At Croydon (a town of about 
18,000 inhabitants) it appears that the sewage is sold for 
something over a thousand per annum. Another refuse ma¬ 
terial which has already come to possess great value is coal tar. 
Not only is our chief supply of ammonia, the food of plants, 
derived from that source, but those brilliant and varied 
colours M'hich are now so much in use for dying silk also 
owe their origin indirectly to the same source. There is, 
jierhaps, no more striking instance of the benefits which 
ultimately arise, even to the manufacturing arts, from every 
complete investigation of chemical substances, than is afforded 
