[54] Hydrogen, or inflammable air, is the lightest known substance; its specific gravity is to that of air as 732 to 1000. The gases, into the composition of which it enters, rising from these ditches and banks of mud carry with them dried humus, and even animal matter in a state of putrefaction, which, being dry or moist, may act as strongly as variola itself, in respect of its injurious effects on man, who breathes it either as it rises from ditches, or is driven by currents of air circulating round watery places covered with humus. It is even (_onctueux_) so strong that it will sustain seeds and dust upon water, as I have witnessed at Amsterdam, Rotterdam, Verona, Bologna, Venice, and even in the canals of Lambeth and Deptford. By means of hydrogen we raise a balloon; can we not imagine it to be equal to the raising up of humus? It is generally supposed that sulphuretted hydrogen is amongst the dangerous miasms, but it cannot be so hurtful, for no boat can go into canals without disturbing it, and yet we see no evil results from this; but if the water-level lowers, and leaves vegetable or animal matter upon mud in a state of slow combustion, then it is that fevers commence--a fact, I think, I have proved by an appeal to the history of pestilences in ancient and modern times. CHAPTER X. ON THE SERVITUDE OF RIVERS. If the servitude of rivers be the noblest and most important victory which man has obtained over the licentiousness of nature,[55] then assuredly ancient civilizations bear away the palm in this respect from the modern, and Britain must be permitted to occupy perhaps the lowest place in the scale of those empires and nations who, by their industry and knowledge, overcame the difficulties which the right management of river courses presents to civilized man. [55] “Decline and Fall,” vol. iii. p. 391, Milman’s edition.