germs of algæ, seeds of grasses, and other plants, etc., to immense distances, and they germinate and flourish wherever the conditions are favourable. These conditions appear to be periodically favourable only, for any given point of the globe; and when they cease to be so, the plant, or animal, disappears. It is a subject well worthy of more investigation than it has hitherto received.

Waterspouts have been known to carry frogs for many miles from marshy districts, and deposit them in large numbers in places where they were never seen before.

In 1864 I analysed1 some fragments of stone that were carried through the air from the neighbourhood of Dudley to Birmingham, where they fell in the streets during a violent storm, and were supposed to be aërolites. My examination showed that they were really fragments of greenstone rock, known in the locality as "Rowley Rag" (the "rock of Rowley "which forms a hill near the village of Rowley in Staffordshire).

Many old accounts of "rain of Sulphur" apply only to a fall of the yellow pollen of the northern pine trees, which is scattered by the wind; but there are wellauthenticated cases of real sulphur falls, to one of which I have made special allusion on a preceding page. Volcanic ash of various colours-red, grey, and black-is not unfrequently brought down by the rain at great distances from its origin; and storm rains in

1 Brit. Assoc. Report, 1864.

the West India islands often yield a large amount of sea-salt. The same thing occurs in other localities.

The principal extraneous substances which, according to my personal experience, appear to exist at almost all times in the atmosphere of the Earth, may be rapidly enumerated as follows:

Water; carbonic acid; sulphur; chloride of sodium ; sulphate of soda; iron (meteoric); silica (fossil bacteria, or micrococci); desmids and foraminifera; spores of Algo; unicellular Algo (always present in rain and snow); carbonaceous matter of unknown nature; hairs (pili) of various plants; débris of human vestments (cotton and woollen fibres of various colours); essences of flowers (odour of the air after a summer shower); vapour of characin (cause of the odour of marshy soils and ditches); traces of iodine and hydrochloric acid (in seaair); traces of nitric acid (in the rain of thunderstorms); ammonia; dust of carbonate of lime; volcanic ash, grey, red, and black; organic matter of various kinds, that can be condensed in strong sulphuric acid, which it turns brown.

In the dense London fogs seleniuretted hydrogen has been mentioned, but without sufficient proof, as the cause of the choking sensation these fogs produce. Sulphurous acid is often present (from the combustion of pyritous coal, and in volcanic regions); it first reddens blue litmus paper, and then bleaches it. Hydrochloric acid, present in the air near chemical works, only reddens litmus, and turns the leaves of trees yellow. Sulphuretted hydrogen is often present in town

air, and organic compounds of sulphur, due to the fermentation of sewage and refuse of various kinds. Ammonia is present in the air of the open country to the extent of 3 parts for every 100 parts of carbonic acid; on some occasions it has been detected in much larger quantities in the air of Regent Street, London, where it turns to a blue colour a strip of moist red litmus paper carried on the hat. London air, also, often shows carburetted hydrogen, due to gas escapes; and, in the country, proto-carburetted hydrogen, or marsh gas, constantly rises in bubbles from stagnant pools which have much decaying vegetable matter in their mud, and finds its way into the air. This is also the gas which issues from the seams of coal in mines, and gives rise to disastrous explosions. Boussingault has detected it, more than once, in very minute quantities, in atmospheric air taken from various localities, far from towns.

The germs of various pathogenic bacteria can be collected in cotton wool, or on plates of glycerine, such as I have before mentioned; they can then be transferred to a sterilized gelatine mixture, in which they develop, and can afterwards be inoculated into the tissues of various animals to prove their toxic nature.1

Dr. Miguel has collected the urea ferment in the streets of Paris (and it may readily be met with in our metropolitan railway stations). Previously to this,

1 Pasteur used guncotton for these experiments, which, after it has collected the germs, etc., in the atmosphere, can be dissolved in a mixture of alcohol and ether, and so set them at liberty for microscopic examination.

Pasteur collected the grape vine ferment during vintage time in France. I was desirous of repeating this experiment of the celebrated savant, whose friendship I enjoyed during my four years' residence in Paris, and whilst a heavy gale was blowing for about three days in London, at the time of the vintage in France, I passed some hundreds of gallons of air into a solution of sugar with the hope of thus catching some grape ferment. The solution was afterwards placed in a stove, kept, for some days, at about 70° Fahr. to promote fermentation. No alcoholic fermentation was obtained (I am afraid my solution contained too much sugar); but the bottle having been kept hermetically sealed for several months, there developed in it an extraordinary brownish-white fungus in long filaments, a species of Byssus, whilst the liquid became of a golden-yellow colour, and slightly acid.

CHAPTER XII.

Air essential to Sound-Mountain Air-Dr. Viault's important Observation-Height of the Atmosphere-Determination of Altitude-Effects on the Barometer and ThermometerTemperature at the Limits of the Atmosphere-Hermite's recent Experiments with small Captive Balloons-Heights of Clouds-Determination of Water-vapour in the AirThe Rain-band of the Spectroscope.

ATMOSPHERIC air is a good conductor of sound, and a better conductor when moist than when dry.

If no atmosphere existed, sound would be absent from our world.

As we rise on the mountains, the air becoming less and less dense, sound diminishes remarkably; so that the crack of a rifle is quite a slight noise in the hands of a Chamois hunter in the Alps; and even thunder is far less terrible on the mountain heights than in the valleys.

Mountain air affects the breathing of persons unaccustomed to it: the air being less dense contains less oxygen in a given volume, whilst the capacity of the lungs, of course, remains the same. But Nature ensures a proper degree of hæmatosis by increasing the number of blood-corpuscles of those who reside for any length of