The history of diseases so originating scarcely supports this view. Typhus, which at times seems to originate in a miasm, at times seems to assume a contagious character. The same may be said of yellow fever. But however this may be, the distinction applies to such diseases as intermittent and remittent fevers, which originating in a miasm, itself springing from the putrescence of animal or vegetable bodies, gives rise to disease which does not reproduce the miasm. Now, between diseases so produced and those arising from contagion properly so called, there is this remarkable distinction: the blood once altered is no longer susceptible of the same contagion, whereas against miasms there is no such security.[51] [51] Blood has a _mordant_ given to it which dyes it red; when this is in excess, the blood becomes black, or very dark. This was the colour of the blood in cholera. Its crasis seemed to be broken down, and I have it on sure anatomical testimony, that in dissecting those who had died of cholera, the larger veins, when once opened, continued to pour out blood for many days. In every contagious disease, and perhaps even in those simply arising from miasms, there is an odour which fills the chambers of the sick, and is recognisable at once. Ammonia is very generally present, as it is wherever animal decompositions are going on, that is, putrefaction. The foul airs emanating from stagnant and neglected ditches is composed, as has been long known to chemists, of carbonic acid and sulphuretted hydrogen gases, and these are viewed by some as amongst the most dangerous of miasms. These gases may be destroyed by acid vapours now in common use.[52] From chemistry we also derive another valuable lesson in respect of substances directly destroying human life. The materials ready to undergo putrefaction, and thus to generate miasms, may all be present, and yet no miasms are given out, and man escapes; this security depends upon the absence of that third principle requisite to bring the others into activity.