tion affecting a comet may be such that the return journey never occurs. They may be such that the comet goes on indefinitely travelling away from our sun, until he is caught by some other star, and his orbit changes its shape, with the new sun as attracting centre. These are the "wandering comets as distinct from the "periodic comets," which have been shown to conform to Halley's scheme of their movement and recurrence.

And now some one will ask, perhaps impatiently, "What, after all, is a comet?" We have seen that many are continuously, and others casually, members of the solar system. What do they consist of? Spectrum-analysis shows that they consist chiefly of the chemical element carbon.1 Though they have weight, and are attracted by the sun, yet they seem to be for all their size and terrifying shape and glare incredibly light and airy things. Herschell declared that the tail of a big comet probably consisted of but two or three pounds of solid matter-diffused, rarefied, and luminous. And the head or nucleus certainly does not weigh many hundreds of tons. In the eighteenth century astronomers observed a comet pass right in among the moons of the planet Jupiter. You might expect the moons to be terribly knocked about by such an impact. 1 I am indebted to Mr. Rolston, of the Solar Physics Observatory, South Kensington, for some information on this matter.

Generally speaking, it appears that the spectra of these bodies indicate carbon-in some form-as the principal constituent.

As to the particular form of carbon, there is still a considerable doubt, so much that, in describing the spectrum of Morehouse's comet, Professor Frost says (Astrophysical Journal, xxix., p. 59, 1909) :—“We avoid the still unsettled question of the 'carbon' bands (of the so-called 'Swan' spectrum) which have been so often ascribed to a hydrocarbon, specifically acetylene, and we use for them the simple designation 'carbon.""

In addition to this "carbon" there is the cyanogen spectrum present in most cases.

Sodium and iron have been detected in the spectra of some few

They were not; they were not deflected in the smallest appreciable degree from their position and regular movement! One is naturally inclined to look upon the tail of a comet as something like the smoke of a railway engine trailing behind the advancing "head." As a matter of fact, it does not always trail behind, but is always turned away from the sun, so that when the comet is travelling away from the sun the tail is in front! It is now held that the tail is caused by the radiant energy (light and heat) of the sun, blowing, as it were, the lighter particles from the incandescent head, and causing them to spread out in a long track of variable shape. The photographs of the third comet of the year 1908 show that the tail can vary to an astonishing extent and with great rapidity -that is to say, in four or five hours. It is seen in those photographs as a scimitar-like curved blade, then with a second head or nucleus behind the leading one, then actually bent like the letter Z, and then divided into seven distinct diverging "plumes," and then it returns to its former simple shape-all in the course of a few days. Astronomers have now shown that there is a close connection between comets and the showers of "shooting stars" or meteors which frequently strike the earth's comets, e.g. Wells (1882, ii.), whilst Holmes (1892) showed only continuous spectrum.

An interesting suggestion is made by Newall, namely, that the spectrum is not indicative of the comet's composition, but of that of the medium through which the body passes. Thus the persistent identification of the cyanogen bands in cometary spectra is attributed, primarily, to the "heating up" of cyanogen existing, free, in circumsolar space.

Till 1907 most of the cometary spectrograms showed only the carbon" and cyanogen radiations, but in Daniel's comet of that year, and in Morehouse's of 1909, other lines were detected for which origins have not, as yet, been found.

Thus, some form of carbon+unknown+(occasionally) sodium and iron seems to sum up our present knowledge of cometary composition.

atmosphere. It is considered probable that comets eventually break down into streams of meteors, and that their "life" (if one may use that term) is, relatively to that of other heavenly bodies (which are all undergoing change and, in many cases, decay), not a very long one. But there are no facts at present known which enable us to tell whether a given comet is young or old, and it would have been a decided shock had it been found that Halley's comet, which has so happily spent every seventy-sixth year with us for so many centuries, had "burst up," or by "indisposition" had been unable to pay his usual visit as expected in 1910.

XXV

ABOUT CHOLERA

HAT is this terrible disease which every few

WHA

years travels from the banks of the Indian Ganges, where it is always present, and makes its way to one or more of the great cities of Europe, killing its thousands with horrifying rapidity? The word "cholera " is used by the great Greek physician of antiquity, Hippocrates, and by his followers down to the days of our own Sydenham, to describe a malady which occurs commonly in summer, is often of severe character, but rarely fatal, and is characterised by the exudation from the walls of the intestine of copious fluid, usually accompanied by vomiting and sometimes by "cramps." This malady is now distinguished by physicians as "simple cholera," or European cholera, the last name being misleading, since the disease occurs all over the world. It is caused by a special microbe, which multiplies in the intestines and produces a poison. Other microbes produce similar results. One which causes luminosity in foul salt water has been found to produce cholera-like results when cultivated in a state of purity and swallowed by man. Other poisons besides those produced by microbes set up a sort of "cholera" in animals and man. Drugs of both mineral and vegetable origin have this effect, as every one knows, and are used in small quantities to produce

purging. Microbes which are noted for other obvious effects which they produce by the poisons they form in man's intestines-such as the microbe of typhoid feveralso produce cholera-like purging.

But the name "cholera," or "the cholera," is now applied without any further qualification to what would be more correctly described as “Indian cholera," or "epidemic cholera." It is a disease which first became known to Europeans in India in 1817, less than a hundred years ago. It resembles “simple” cholera in its general features, but is usually much more violent in its attack, and often causes complete collapse in two or three hours. from its onset, and death in as many more. The main point about it is, however, that it is a quickly spreading “epidemic” disease; it invades a whole population, and travels from place to place along definite routes. Although the outbreak of cholera in India in 1817 was the first to attract the attention of Europeans, it was nothing new in India, and was recognised in distant ages by Hindu writers. Its usual name on the delta of the Ganges is "medno-neidan." Ninety per cent. of the population perished of cholera in some districts of India in 1817, and English troops were attacked by it with terrible results.

Cholera gradually made its way in subsequent years through Persia to Russia, and at last to Western Europe; but it was not until late in the year 1831 that Indian cholera arrived for the first time in England, and in the following year it caused something like a panic. There have been at least three subsequent outbursts of Indian cholera (before that of the year 1908) which have reached Europe, and two of these have reached England and caused profound alarm and anxiety. That in 1854 reached us just before the Crimean War, and caused such rapid and numerous deaths in London, especially in the West End (St. James's, Westminster), that the corpses