We have been long accustomed to hear that our chief sanitary necessity in this world is pure water. This would be quite true if we were fish. But it is obvious that the purity of the air we breathe is of far greater importance than the purity of the water we drink, seeing that we must take a draught of air about twenty times a minute, while many of us do not take a draught of raw water from week's end to week's end. If the huge death-rate of the Strand were due to impure water, we may be sure that there would be no lack of discussion thereupon; but as it is due to overcrowding, and the filthiness of the air consequent upon overcrowding, we hear nothing about it. To make any serious attempt to check overcrowding would interfere with trade, and therefore it is considered silly and futile to discuss such a matter. Any man who supposes that considerations of hygiene will be allowed to interfere with trade, is, I think, deceived. At all events, I have no such belief. My sole object in discussing these matters is to warn country places against blindly following the lead of London in sanitary matters. CHAPTER V THE CIRCULATION OF ORGANIC MATTER[3] It is quite impossible to define 'organic matter,' or to indicate the line, if there be any, between organic and inorganic. Organic matter is the material of which living things are made. When a chemist analyses anything which is the product of life, whether vegetable or animal, he often speaks of his incombustible residue or ash as 'inorganic matter,' but this is clearly an arbitrary use of the term, for this incombustible residue has formed an indispensable part of one living thing, and may in due time be incorporated with other living things as something which they cannot do without. It may well be that everything of which we have knowledge (even including the igneous rocks) has at one time or another formed part of a living organism, and it is certain that a large proportion of the commoner chemical elements may form a part, more or less indispensable, of the bodies and framework of plants or animals. Oxygen, hydrogen, nitrogen, carbon, chlorine, sulphur, phosphorus, iron, sodium, potassium and calcium seem to be indispensable to almost every living thing. Many more of the elements are constantly found in some organisms, while others, such as lead, mercury, silver, &c., may be temporarily incorporated with living bodies. We shall deal mainly with those elements which are pre-eminently mobile, which are constantly changing and exchanging, combining and separating, and which are readily combustible. For practical purposes one might, indeed, use the terms 'organic' and 'combustible' to signify the same thing.