It is one of the most certain of facts relating to sanitation that comparative density of population affects disease, and especially the zymotic diseases, more than any other factor that can be ascertained. It is mainly a case of purity of the air, and consequent purification of the blood; and when we consider that breathing is the most vital and most continuous of all organic functions, that we must and do breathe every moment of our lives, that the air we breathe is taken into the lungs, one of the largest and most delicate organs of the body, and that the air so taken in acts directly upon the blood, and thus affects the whole organism, we see at once how vitally important it is that the air around us should be as free as possible from contamination, either by the breathing of other people, or by injurious gases or particles from decomposing organic matter, or by the germs of disease. Hence it happens that under our present terribly imperfect social arrangements the death-rate (other things being equal) is a function of the population per square mile, or perhaps more accurately of the proportion of town to rural populations. In the light of this consideration let us again compare these diagrams of Irish, Scottish, and English death-rates. In Ireland only 11 per cent. of the population live in the towns of 100,000 inhabitants and upwards. In Scotland 30 per cent., and in England and Wales 54 per cent.; and we find the mortality from zymotic diseases to be roughly proportional to these figures. We see here unmistakable cause and effect. Impure air, with all else that overcrowding implies on the one hand, higher death-rate on the other. This explains the constant difference between London and rural mortality, and it also explains what seems to have puzzled the Commissioners more than anything else—the intractability of some of the zymotics to ordinary sanitation, as in the case of measles especially, and in a less degree of whooping-cough—for in their case the continual growth of urban as opposed to rural populations has neutralised the effects of such improved conditions as we have been able to introduce.