With regard to solid matter, the power of readily circulating implies a readiness of combustibility, but it must be remembered that there is no hard line between combustible and incombustible. This is a matter of temperature, and many things which are incombustible here are said to be blazing in the sun. The combustion of organic matter may take place slowly, or with moderate rapidity, or with explosive violence. When we burn coal, which is a vegetable product, we find that the carbon and hydrogen escape as carbonic acid and water, accompanied by nitrogen, sulphuric acid, and volatile hydrocarbons. The residue consists mainly of silica and alumina, which are removed from the furnace in the form of clinker and ash. The water ultimately returns to the earth in the form of rain or dew; the carbonic acid is ultimately absorbed by green plants, and, by stimulating the growth of these, helps to furnish us with more combustible material; while the residue is almost a waste product. Thus, in this example we find that the carbon and watery vapour readily 'circulate,' while the residue can only do so after a long interval of time, and is practically lost. The volatile hydrocarbons and sulphuric acid, being poisonous to herbage, are a source of practical loss rather than gain. Let us take next the case of an animal, which is really a living furnace, browsing in a field; as it browses we may often see the breath, which is the smoke of this furnace, laden with carbonic acid and water, escaping from its mouth and nostrils, and it is probable that the green leaves of the herbage absorb this carbonic acid almost as soon as it escapes, and, appropriating the carbon, return oxygen to the animal to help its respiration and combustion. The animal as it eats continues to grow and increase in bulk and value, whereas the artificial furnace in which the coal is burnt tends steadily to wear out and decrease in value. As it browses and grows, the droppings of the animal nourish the herbage which here and there, by patches of more vigorous growth and deeper green, afford sure evidence of the value of these waste products. In this arrangement there is no waste, for both the animal and the herbage, by a process of mutual exchange and the circulation of organic matter, increase in value. Not only is there no waste, but, strange as it may seem, there is a positive gain, with no loss whatever. The furnace and the fuel are both increased! This increase can only be apparent, and not real, for it is well known that although we may alter the form of matter, we can add nothing to and subtract nothing from the sum total of the world.