On subjects such as the conditions of existence on planetary bodies altogether inaccessible to direct observation it is imperative that we should distinguish between (1) known and established facts, (2) inferences of high probability, based on established facts, but still made subject to various assumptions, and (3) speculations as to facts which may possibly be the result of highly hypothetical conditions. Most of those who deal with this subject are unable to keep these three categories distinct, and stumble in the half-light of imperfect knowledge. Mr. Maunder walks with a sure step in the light of clear and definite knowledge, and we are therefore fortunate in having him for our guide.

The following paper was then read by E. WALTER MAUNDER, Esq., F.R.A.S.:

THE CONDITIONS OF HABITABILITY OF A PLANET; with Special Reference to the Planet Mars.

TH

HE first thought which men had concerning the heavenly bodies was an obvious one: they were lights. There was a greater light to rule the day, a lesser light to rule the night, and there were the stars also.

But with the acceptance of the Copernican theory, this world on which we live, while losing its pride of place as the centre of the universe, from another point of view received a promotion, in that itself it became a heavenly body of the same order as some of those that shine down upon us. And, as the earth is an inhabited world, the question naturally arises "May not these bright lights of heaven also be, like it, inhabited worlds?" There is a strong and natural desire to obtain an affirmative answer to the question; all men would greatly delight to be able to recognize the presence of races similar to our own upon other worlds in the depths of space.

What do we mean by an "inhabited" world? We know quite well what we mean by an inhabited" island. When an explorer in his voyage lights upon a land hitherto unknown, no richness of vegetation, no fullness and complexity of animal life will warrant him in describing it as inhabited. He can only give it that title if he should find men there. Similarly, if we speak of a planet as being habitable, we mean that it is suitable

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for the presence of beings that we could recognize as being essentially of the same order as ourselves, possessing an intelligent spirit lodged in an organic body. Animals without intelligence could not be dignified by the title of " inhabitant,' nor could disembodied intelligences, such as men have fabled to live in rocks, or streams, or trees-fairies, nymphs and elves and the like-be accurately described by the same term. We may readily imagine that in outward form the inhabitants of another world might differ very greatly from ourselves, but, like us, they must be possessed of intelligence and self-consciousness, and these qualities must be lodged in and expressed by a living, material body. Our inquiry is a physical one; it is the necessities of the living body that must guide us in it; a world unsuited for living organisms is not, in our sense, a habitable world.

What constitutes a living organism? It is almost impossible to give a comprehensive and satisfactory definition, yet we all know some of the chief characteristics of an organism. In the first place it is a machine. Like man-made machines it is a storehouse of energy, but it differs from artificial machines in that, of itself and by itself, it is continually drawing non-living matter into itself, converting it into an integral part of the organism, and so endowing it with the qualities of life, and it derives from this non-living matter fresh energy for the carrying on of the work of the machine. The living organism, therefore, is continually changing its substance, while it remains as a whole essentially the same. As Professor S. J. Allen has remarked: "The most prominent and perhaps the fundamental phenomenon of life is what may be described as the energy traffic, or the function of trading in energy. The chief physical function of living matter seems to consist in absorbing energy, storing it in a higher potential state, and afterwards partially expending it in the kinetic or active form."

Here is the wonder and mystery of life, the power of the living organism to assimilate dead matter, to give it life, and bring it into the law and unity of the organism itself. But it cannot do this indiscriminately; it is not able thus to convert every dead material; it is restricted, narrowly restricted, in its action.

First of all, living organisms are not built up out of every element; four elements must always be present and be predominant; the four being hydrogen, oxygen, nitrogen, and carbon. The compounds which these four elements form with each other in living organisms are most complex and varied,

and they also admit to combination, but in smaller proportions, a number of the other elements, of which we may take sulphur as an example.

This fact disposes at once of the vague plea which is sometimes raised, "Is it not possible that there may be life upon other worlds under physical conditions totally different from those which prevail here?" We cannot think it, for the evidence of the spectroscope has shown us that the same elements that are familiar to us here are present, not only in our sun, but in the most distant stars. And more than that, the elements have the same properties there as here. For the evidence of the spectrum of a body is evidence of its essential structure, far more searching than any chemical analysis could possibly give; it reveals to us the qualities of its ultimate molecules.

The same elements therefore exist throughout space, and exist with the same qualities. Nor are we able to call into imagined existence other elements of which we know nothing with properties quite unrelated to those of the known elements. For the Periodic Law has shown us that the elements do not exist as isolated phenomena, to which we could in imagination add indefinitely in any direction, but that they are strictly related to each other in all their properties. If, therefore, organic life on another world could be built up of elements other than the four which form its chief basis here, we should have the same phenomenon occurring within our own experience. We may therefore dismiss, as a wholly chimerical hypothesis, the suggestion that the conditions of life as we find them here may be abrogated elsewhere.

What are the conditions of habitability here on this world? They have never been more happily stated than by Ruskin ir his Modern Painters.

"When the earth had to be prepared for the habitation of man, a veil, as it were, of intermediate being was spread between him and its darkness; in which were joined, in a subdued measure, the stability and the insensibility of the earth and the passion and perishing of mankind.

"But the heavens also had to be prepared for his habitation. Between their burning light-their deep vacuity-and man, as between the earth's gloom of iron substance and man, a veil had to be spread of intermediate being-which should appease the unendurable glory to the level of human feebleness, and sign the changeless motion of the heavens with the semblance of human vicissitude. Between the earth and man arose the

leaf. Between the heaven and man came the cloud. His life being partly as the falling leaf and partly as the flying vapour."

The leaf and the cloud are the signs of a habitable world. The leaf, that is to say, plant life, vegetation, is necessary because animal life is not capable of building itself up from inorganic material. This step must have been previously taken by the plant. The cloud, that is to say water-vapour, is necessary because the plant in its turn cannot directly assimilate to itself the nitrogen from the atmosphere. The food for the plant is largely brought to it by water, and it assimilates it by the help of water. Life on a planet therefore turns upon the presence of water, the great neutral liquid and general solvent, the compound of the two most abundant elements, hydrogen and oxygen. There is no other compound of like properties and simplicity of constitution that could take its place, or that the elements could supply in such abundance. We cannot imagine a world wherein bisulphide of carbon or hydrochloric acid or any other such compound could discharge the functions which water fulfils here. It is, therefore, upon the question of the presence of water that the question of the habitability of a given world chiefly turns. In the physical sense man is "born of water," and any world fitted for his habitation must "stand out of the water and in the water."

Where shall we find such another world? There were two bodies whose surfaces men could study to some extent, even before the invention of the telescope-the sun and the moon. But we are able now to determine the temperature of the sun with some approach to precision, and we know that not only is it far too hot for the presence of vegetation, but it is so hot that oxygen and hydrogen would usually refuse to combine there. The components of the molecules of water would be driven asunder; water would be dissociated. And as with the sun so with all the stars, for they, in various measures and degrees, are all suns. The moon also is without the leaf and the cloud; its surface has been drawn, photographed and measured over every square mile, until the side visible to us has been more thoroughly surveyed than our earth, but it shows us only bare unchanging rock. A man placed there could draw no nutriment from the atmosphere around him, or the soil beneath; no vapour would ever soften the hardness of the heaven above, no leaf the hardness of the rock below.

But what of planets? There may be planets circling round the stars, or there may not be; we have no means of knowing,

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On subjects such as the conditions of existence on planetary bodies altogether inaccessible to direct observation it is imperative that we should distinguish between (1) known and established facts, (2) inferences of high probability, based on established facts, but still made subject to various assumptions, and (3) speculations as to facts which may possibly be the result of highly hypothetical conditions. Most of those who deal with this subject are unable to keep these three categories distinct, and stumble in the half-light of imperfect knowledge. Mr. Maunder walks with a sure step in the light of clear and definite knowledge, and we are therefore fortunate in having him for our guide.

The following paper was then read by E. WALTER MAUNDER, ESQ., F.R.A.S. :

THE CONDITIONS OF HABITABILITY OF A PLANET; with Special Reference to the Planet Mars.

HE first thought which men had concerning the heavenly

Tbodies was an obvious one, they were lights. There was a greater light to rule the day, a lesser light to rule the night, and there were the stars also.

But with the acceptance of the Copernican theory, this world on which we live, while losing its pride of place as the centre of the universe, from another point of view received a promotion, in that itself it became a heavenly body of the same order as some of those that shine down upon us. And, as the earth is an inhabited world, the question naturally arises "May not these bright lights of heaven also be, like it, inhabited worlds?" There is a strong and natural desire to obtain an affirmative answer to the question; all men would greatly delight to be able to recognize the presence of races similar to our own upon other worlds in the depths of space.

What do we mean by an "inhabited" world? We know quite well what we mean by an "inhabited" island. When an explorer in his voyage lights upon a land hitherto unknown, no richness of vegetation, no fullness and complexity of animal life will warrant him in describing it as inhabited. He can only give it that title if he should find men there. Similarly, if we speak of a planet as being habitable, we mean that it is suitable