Saturn's ring is composed of small asteroids (meteoroids, aërolites).

There was also a time, not yet more than a century or so removed from us, when many philosophers held that the Earth's atmosphere extended indefinitely around the globe, becoming more and more rarefied-the density decreasing in geometrical ratio, whilst the distance from the surface increased in arithmetical ratio, according to the celebrated Halley—until it blended with the rarefied atmospheres of other planets. But when it was found that no atmosphere of any kind could be detected around the Moon, this theory began to give way; and it was supposed, henceforward, that the Earth's atmosphere, whether it extended to 50 or to 500 miles, was limited; that it participated in the diurnal and annual movements of the globe, and was held to the Earth by gravitation, just as a pebble on a garden walk. But just as the pebble may, under the influence of the spade or rake of the gardener, have its own independent motions, so our atmosphere, under that of the Sun's rays, is in a constant state of change and activity.

CHAPTER XXI.

The Organic Matter of the Atmosphere-Effluvia, Miasma, Malaria, etc.

THE organic matter contained in the Earth's atmosphere has been touched upon in several previous sections of this little work, but a few more observations are requisite to complete our notions upon this important subject.

When a current of atmospheric air is passed, for a certain length of time, into pure sulphuric acid, this liquid, which is originally as white and transparent as water, becomes more and more coloured, and finally dark brown. The quantity of air thus passed, before the brown colour is obtained, supplies some indication of the absolute amount of organic matter present in the atmosphere of a given locality at the time the experiment is made.

This organic matter has been found to consist chiefly of microscopic cells, quite invisible, and not affecting to any great extent the transparency of the air. These cells are brought down in large quantities by snow and rain; they can be filtered from the air by passing it through cotton wool, as two German chemists, Schroeder

and Dusch, showed many years ago; and they are completely destroyed by fire, which is the most effective of all disinfectants.

These microscopic cells are constantly present in the atmosphere in all parts of the globe, to a greater or less extent, according to circumstances; they are organized beings which play an important part in the economy of Nature. Before they had been scrutinized by the scientific methods of modern times, their existence had long been suspected; and they have been successively alluded to as effluvia, malaria, miasma, bacteria, microbes, germs of disease, ferments, etc. They are extremely numerous, and varied, infinitesimally minute, and are the promoters of fermentation, decay, and diseases of all kinds, both in animals and plants; at the same time, they contribute to the fertility of the soil, and the purification of air and water, thereby promoting health and vitality.

One of the greatest achievements of the science of the nineteenth century is to have brought to light the nature of what the old naturalists and physicians used to speak of vaguely as miasma and virus. The successive experiments and observations of Schwann of Berlin in 1837 (who calcined his air), followed by those of Schultze, who passed the atmospheric air through sulphuric acid, and found it incapable of producing life in organic liquids which had been boiled, whilst air not so treated allowed the production of algae and infusoria of all kinds; of Schroeder and Dusch (in 1854 and 1859), who filtered air through cotton wool, and found

it, also, incapable of promoting the decomposition of sterilized beef-broth, the cotton wool having retained the germs; and finally, the observations of Dr. Davaine (1856), and M. Pasteur (1859), have brought this branch of knowledge to a great degree of perfection. At the present time hundreds of investigators are following up the researches of these clever observers in all parts of the civilized world.

When water is boiled for some time, these microscopic organisms which, like the air, it always contains, are destroyed. The water is then "sterilized," as it is termed, and no life can be detected in it. But by exposing it for a certain time to the atmosphere, and to sunlight, green matter is seen to form in it: microscopic plants, the germs of which have fallen from the air, develop in it rapidly, and are soon buoyed up to the surface of the liquid by the bubbles of oxygen gas which they secrete.

The ease with which the germs of these extremely minute organisms are dispersed through the atmosphere became very apparent in one of my recent experiments; a few bubbles of carbonic acid gas having been passed into a long tube nearly full of nitrogen, through water in which this green matter had developed, I noticed in a few weeks some excessively minute specks at the very top of the tube, a foot and a half from the surface of the water. In time, these tiny specks turned green and increased in size; they were found to be algo, identical with those in the water below. Their germs had been carried up through the nitrogen gas, and had

developed on the moist sides of the tube, near its upper extremity.

All these minute mono-cellular organisms, which are quite invisible in the air, have always played a most important part in Nature, from the earliest geological ages of our globe to the present time. They were, as I have shown, the first producers of free oxygen gas in the atmosphere of the Earth, by which, alone, animal life became possible. They are the cause of biological phenomena of all kinds, by which the Earth retains its fertility. They multiply with the most astonishing rapidity, which compensates for their minute size and short cycle of existence. They are the first principles of life, and of vital activity in health and disease.

These microbes, as they are now often called, have been classed into two categories-those which appear to be harmless, and those which give rise to disease in plants and animals. But this classification is very arbitrary. Certain kinds appear to be present in greater numbers in times of epidemics; and it is generally supposed that epidemics of measles, scarlatina, diphtheria, small-pox, cholera, influenza, typhoid fever, etc., are due to the prevalence, at certain times, and in certain localities, of the special microbes of these various maladies. However that may be, it is quite clear that the whole population of a district is not attacked at once, and that, in the most virulent of epidemics, a great number, we may say by far the greatest number of people, are never affected at all, though they must all have been penetrated with the microbes in the same manner as