It is within the experience of every one that most dead animal and many dead vegetable matters rapidly decompose if kept in a warm, moist condition; and that while decomposing they emit ill-smelling and unwholesome vapors. One of the problems of large cities is how to efficiently and economically remove the "garbage," which, if allowed to accumulate and decompose, would poison the community.

This decomposition, named putrefaction, is due to the activity of extremely minute living things which, wafted everywhere in the air, sooner or later settle on animal or vegetable remains and destroy them. The living things which cause putrefaction are known as Bacteria. Other kinds of Bacteria are the causes of most infectious diseases. Many have the faculty of lying dormant in a dry condition for months or years. In this state they form light dust, capable of transference by the softest breeze; but as soon as they settle on a suitable material they become active, and multiply with great rapidity.

The Bacteria of putrefaction are among those which can live in the dry state for some time; and they always form part of the dust floating in the air; so it is easy to understand why fresh meat or fish left exposed to the air soon putrefies. As the Bacteria multiply much more rapidly in warm than in cold weather it is harder to keep meat in summer than in winter.

Give some examples of putrefaction. To what is it due?