[37] For the professional reader I may here throw out a hint--referring to the doctrine of epidemic disease stated in the Fifth Annual Report, that this apparent healthiness of districts after certain epidemic invasions probably bears relation to a temporary exhaustion of their zymotic atmosphere under the action of a specific ferment, and is in some respects analogous to that immunity from an infected fever which belongs to an individual who has recently suffered its attack. See also page 235.--J. S., 1854. Nevertheless, that the sanitary condition of the City has undergone considerable improvement within the last two years is a fact which no one can gainsay; and that a considerable share of the mitigation in mortality arises from this improvement cannot reasonably be questioned. If even a third of the mitigation in question, if a reduction of ten _per cent._ on the preventable mortality of the City, may be inferred from the materials which I lay before you, it is indeed matter for the utmost congratulation; and a continuance of the same reduction year by year, perpetuated (as doubtlessly it may be) by a continuance of the same exertions, would soon raise the City of London above all fear of comparison, on the ground of healthiness, with urban or suburban populations. Thirdly, I would beg the attention of your Hon. Court to those very important _local differences_ of death-rate which may be deduced from a study of our death-register. I have already had the pleasure of citing to you the low rate of mortality which has prevailed during the last year in the north-west sub-district of the City of London Union. The rate of death in the north division of the West London Union was nearly double that proportion; and between these extreme terms of disparity there were many intermediate degrees. Similar inequalities of mortality were observable in last year’s record. In the healthiest sub-district of the City the year’s death-rate was about 22 in the thousand; while in the worst it stood above 41; and for the whole West London Union exceeded 38.[38] [38] I have here availed myself of the corrections given in the note of page 6. Mainly and essentially these local differences of mortality depend on the proportion in which _preventable deaths_ enter into the total; the differences, however partial, depending on the operation within certain districts, of removable deleterious influences which do not exist in certain other districts.