Thus it happens that in his extensive and elaborate inquiries, Major Tulloch was continually met by difficulties which overthrew at once all existing medical theories, rendering it even probable that the supposed relation of cause and effect between marshes and miasms, and miasm and fever, was merely accidental. In what that third element consists, that immediately exciting power which urges on the decomposition to an extent it had not before attained, rendering that miasm mortal, or at least most dangerous, which heretofore the vital force was able to resist, has not yet been discovered. Is it electricity? is it ozone?[53] or does it depend on some unknown principle in the elements of the atmosphere, for the detection of which we have no instrument? Does security in such cases depend on the presence in the atmosphere of some such principle as ozone? Whatever be the cause, the fact is certain; epidemics follow cycles of increase and decrease; like comets, they come and disappear at long intervals. Our business in the mean time lies with what is constantly present, in a more or less aggravated form--the malaria continually reproduced, always efficient in certain regions of the earth; in the overcoming of which, as I have endeavoured to show, well-directed human industry is far from unavailing.[54] [53] Ozone is said to oxidize the poison. It destroys sulphuretted hydrogen and all oxydable miasms, and is the most powerful disinfecting agent, but is itself unfit for respiration: it causes suffocation. Air in its normal state contains one ten-thousandth part of ozone; when raised to one two-thousandth part it is sufficient to kill small animals.