indubitably established that insufficient air and light give rise to disease. Dwellers in towns are, as a rule, more delicate than those in the country, for they get less air and light than the latter. Air and light, then, are absolutely indispensable to health, and every one should remember all that we have said on the matter, and act up to it to the best of his ability.
As has been already pointed out, air is the most indispensable to our subsistence, while water comes next in order. Man cannot live for more than a few minutes without air, but he can live for a few days without water. And in the absence of other food, he can subsist on water alone for many days. There is more than 70% of water in the composition of our food-stuffs, as in that of the human body.
Even though water is so indispensable, we take hardly any pains to keep it pure. Epidemics are as much the outcome of our indifference to the quality of the water we drink, as of the air we breathe. The drinking of dirty water very often produces also the disease of the stone.
Water may be impure in either of two ways,—by issuing from dirty places, or by being defiled by us.