It is interesting to note the tendency of organic matter, when mixed with water, to give rise to explosive and combustible products. Explosions in cesspools and sewers have occurred many times. When wet hay is stored in stack it catches fire. When we stir the mud at the bottom of a pond or river, bubbles of combustible marsh-gas rise to the surface. The coal measures are due to the storing under water of semi-aquatic plants which have been preserved by being silted up, and we know that coal is full of olefiant gas, marsh-gas, sulphuretted hydrogen and carbon monoxide, which are all combustible, and that the carbonaceous residue, charged with volatile and combustible hydrocarbons, forms the chief fuel of the civilised world. Peat is formed in ways analogous to that of coal, and the so-called mineral oils are certainly the products of organic matter which has been silted up. These subterranean stores of combustibles, all of organic origin, are, as we know, prodigious in quantity. Nobody can predict the time which it will take to exhaust the coal measures of the world, and we know for a fact that the sacred fires of Baku on the Caspian, fed by subterranean reservoirs of naphtha, have been burning for centuries. When we see the end of a tin of 'preserved meat' bulged, we know that the gas-forming organisms have been at work within, and when the bed of the lower reaches of the Mississippi rises as a small mud mountain, spluttering with carburetted hydrogen, we know that analogous forces have been in operation. It seems, indeed, to be a law of Nature that the ultimate destiny of organic matter is to 'circulate,' and that if it does not do so quietly, as in the ordinary processes of nutrition in plants and animals, it merely bides its time, and ultimately attains its end with more or less destructive violence. Nitre (nitrate of potash or nitrate of soda) is an organic product, and sulphur is an essential constituent of all or nearly all organisms. Of the three ingredients of gunpowder, two (charcoal and saltpetre) are, it is certain, of exclusively organic origin, and the third, sulphur, may be so also. All the common combustibles with which we are familiar are certainly of organic origin, and one is almost forced to the conclusion that in this world life must have preceded combustion. If we are to explain what _has been_ by what _is_, such a conclusion is irresistible. Are we quite sure that volcanoes, which are seldom far from the sea, are not fed by old deposits of organic matter which has collected in the primeval ocean, and, like the more recent coal measures, have been silted up?