Jan. 1, 1902.] THE TROPICAL AGRICUl-TURlST. 
445 
at Cawnpore will weigh (corn and straw) aboat 4,500 lb. 
per a':;re in the dry state, and such a crop will re- 
quire something like 1 ,8O0,OJ0 lb. or roughly 803 tons 
of water. Or again a cholum crop, including stems, 
leaves and grain, may weigh from 4,0 )0 to 7,0001b, 
in the dry state, and such crops will require 2.000,000 to 
3,500,000 lb. of water, or say 1,000 to 1,500 tons. 
These are of course very large amounts, but when 
we state them iu terms of rainfall, they are not so 
very surprising. The former is equivalent to 9 inches 
and the latter to about 10 inches to 15 inches of 
rvinfall. One would be inclined to ask why even 
more than this rainfall is necessary in order to grow 
heavy crops. If the rainfall were well distributed 
over the seasons, it would indeed go much further 
towards satisfying our Indian crops than it does, 
but unfortunately it is apt to fall in heavy showers, 
and a part runs off the land, whilst there is always 
some lost by drainage, the latter being indeed a 
necessary condition for all healthy soils. It follows 
however that, in order to grow heavy crops, even 
larger amounts of water are required than the few 
inches 1 have spoken of. 
It is necessary therefore that the soil shall hold, 
for periods of weeks and months, very considerable 
quantities of water, aud this it does in the manner 
I have indicated, namely, as a very thin film on the 
surface of the soil particles. 
A further part of Whitney's work consists in the 
determination of that proportion of moisture, or to 
state it popularly, that degree of dampness of any 
goil, which is most suitable to the crops grown. 
1 have already told you that these investigations are 
merely in their infancy, and I cannot do more at 
present than to say that there is every probability 
of reaping a rich harvest of information. 
In some soils, crops will grow best with very much 
less water than iu others. The following examples 
will better show the importance of the work. 
One soil was found to be too wet with 10 per 
cent- of wa^er to be too dry if the proportion fell 
below 5 per cent, and that the crop fared best with 
from 5 to 8 per cent. In another case, the soil was 
too wet if the moisture rose to 25, and it was too 
dry with 15 per cent. ; in yet a third case 23 per 
cent, of ^ater was too little. 
What this line of research may ultimately indicate, 
it is of course impossible to say, but one thing seems 
probable, namely, that although we cannot alter the 
rainfall, it may tell us in what manner we should 
irrigate our land, so as to produce the best crop 
with the least waste of water, and without damage to 
the laud. 
Sewage. 
The last subject which I shall refer to is the so- 
called biological treatment of sewage. It may appear 
at first sight that this matter belongs rather to the 
domain of the sanitary engineer than to the agri- 
cultural chemist. As a matter of fact, the disposal 
of sewage should engage the attention of both. To 
the one the question is — " How am I to get rid 
of this material at any price, and with the least 
possible amount of nuisance to the inhabitants 1 " 
Whilst to the other, the deposition of such matters 
in the soil, means an increased manure supply. 
That the land is the proper destination for the 
town " night-soil " has been admitted by all who have 
had to answer the question, but when the practical 
details of the working arrangements were taken in 
hand, the difficulties proved great. In many parts of 
Europe, America, and in fact wherever the sanitation 
of towns has been seriously taken in hand, the excre- 
mentious matters have been conveyed down closed 
sewers by water and emptied, with, or without pre- 
viously " purification," into the nearest river, and 
sanitary authorities have been very well satisfied if 
they conld do this successfully. 
The regular return to the land of those matters 
which had been withdrawn from it, although so de- 
sirable, has been considered as generally impractic- 
{blble, aud bas had to be classed among the many 
other interesting problems which remained unsolved. 
To sanitarians and to agriculturists alike, there- 
fore, the discovery of a means of disposing of sewage 
without causing any nuisance whatever, void of 
technical difficultiej, with a minimum of cost, and 
including as an integral part of itself, an effluent 
which no one would object to put on their flower 
garden, much less on their fields, has come as a 
matter of unusual interest. It is now, of course, 
a very old s'ory, that animal and vegetable refuse 
matters of all kinds disappeared when allowed to 
remiin in either water or the soil, and that this 
resolution is effected by lowly organisms which are 
commonly spoken of as microbes," better known 
to the scientific world as moulds, yeasts, fungi, and 
bacteria, organisms smtiU of stature, but performing 
nevertheless an all-important work. For many years 
attempts were made to utilise this army (for the 
number of such organisms is very large), for the 
disposal of town refuse. It was argued that because 
these matters do disappear if left in either soil or 
water, then why not put them on the soil or into 
the river ? And for a long time this procedure has 
been followed, but with the result, that in the majority 
of cases the river was rendered foul, or the land 
nnapproa,;hable. There was clearly something very 
wrong in our method of utilising the services of the 
wily microbe. 
For the last 10 years, however, efforts have been con- 
centrated by a number of investigators on the value 
of what is termed the " Biological Filter.'' The 
general meaning of the term "Filter" is a porous 
medium, through which a liquid will pass, but which 
will arrest solid matt»,vs, and prevent them flowing 
on with the liquid. Thus we use blotting paper in 
the chemical laboratory to separate our precipitates. 
Drinking water (with the idea that only solid sub- 
stances did us harm) has been filtered through such 
things as sand or charcoal, or more recently by the 
aid of the '^asteur cylinder of finely porous earthen- 
ware. The " biological '' filter is, however, in reality 
very different, at least in principle. It consists, it 
is true, of such material as broken cbarcoil or 
stones, but there is no attempt to make it impre- 
vious to the passage of fine solid matter. It had 
indeed become recognised that at least one of the 
reasons why the soil of the sewage farm did not 
purify its daily allowance of sewage, was because it 
was too tiue, and contained too small an allowance 
of atmospheric oxygen, resulting in harm to the army 
of microbes which it was hoped would consume the 
sewage. The idea of the biological filter was then 
to offer to the sewage as larger a surface as possible, 
with, at the same time, plenty of air. Liquid 
sewage run into such filters (which had been pre- 
viously aerated) and allowed to remain in them for 
a few hours, was found to have lost one-half or 
more of its organic matter. 
The Massachusetts Board of Health, in the U. S. A. 
and Mr. Dibdin, and Mr. Soott-Moncrief in England] 
must be named among those who cariied out the 
first experiments in this direction. At first, only a 
comparatively weak sewage was run into the bio- 
logical filter, but the process worked so exceedingly 
well, that very shortly these filters were supplied 
with the raw and unadulterated sewage, in the hope 
that this might be equally palatable and digestible 
to our friend the microbe. And indeed it was 
found to be so. The solids of the crude sewage 
disappeared, that is, they were dissolved and broken 
down, and this, as also the matter already dissolved 
in the sewage, actually disappeared to the extent 
of about 85 per cent. It thus became evident that 
the microbe could be persuaded to perform hia 
duties most efficiently, if the sewage were only 
given to him in a proper manner. 
Before telling you anything more about the form 
of the biological filter, or its analogue the Septio 
Tank, I must digress to say a word about micro- 
organisms generally, 
la the first place, as yoa know, they consist q( 
