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On the volcanoes on the Moon

editor’s introduction Kant published On the Volcanoes on the Moon in the March 1785 issue of the Berlinische Monatsschrift, which was edited by F. Gedike and J. E. Biester. The occasion for Kant’s essay was Aepinus’s claim that Herschel’s ‘discovery’ of volcanic activity on the Moon supported his view that volcanic activity could be invoked to explain the irregularities on its surface. Kant wants to reject this explanation in favour of the explanation of the formation of the Moon he had proffered earlier, in his Universal Natural History and Theory of the Heavens (Chapter 4, this volume). That is, Kant wants to maintain that the Moon, like the Earth and the other planets in the solar system, was formed from chaotic, gaseous material that gradually lost heat on the surface and solidified, albeit with irregular crevices. Therefore, the uneven geographical features of the Moon that could be perceived from Earth were due not to volcanic eruptions, but rather to other kinds of eruptions that occurred as the gaseous materials that constitute the mass of the Earth cooled and gave off heat. The primary novelty of Kant’s explanation here, compared to what he offered thirty years earlier, is his adoption of Crawford’s theory of heat.

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Right at the beginning of the Gentleman’s Magazine for 1784,1 there is a letter by the Russian Counsellor, Herr Aepinus,2 to Herr Pallas3 concerning an item of news that Herr Magellan4 reported to the Imperial Academy of Sciences in St Petersburg concerning a volcano on the Moon discovered by Herr Herschel5 on 4 May 1783. This news interested Herr Aepinus, as he says, all the more because in his opinion it confirms his hypothesis concerning the volcanic origin of the irregularities on the Moon’s surface, which he arrived at in 1778 and published in Berlin in 1781,∗ and in which, he is pleased to acknowledge, three natural philosophersd independently reached the same conclusion: he himself, Aepinus in St Petersburg, Professor Beccaria6 in Turin, and As attention in all countries had Professor Lichtenberg7 in Gottingen. ¨ been directed towards volcanic craters by Sir [William] Hamilton,8 the hypothesis may [according to Aepinus] be likened to an over-ripe fruit, which could not but fall into the hands of the first person who happened to touch the tree. Finally, in order not to create ill-feeling amongst his contemporaries by claiming the honour of being the first to make this hypothesis, he [Aepinus] adduces as its originator the celebrated Robert Hooke,9 in Chapter 20 of whose Micrographia (published 1665) he found the same ideas. Sic redit ad Dominume — However, Herr Herschel’s discovery does have considerable merit as a confirmation of the ambiguous observations made by Beccaria’s nephew and by Don Ulloa,10 and it leads us to [recognize] similarities between the Moon (and probably other heavenly bodies) and our Earth, which would otherwise have counted as no more than bold conjectures. But in my opinion it [i.e., Herschel’s discovery] does not confirm the hypothesis of Herr Aepinus. Despite the similarities between the circular marks on the Moon and [terrestrial] volcanic craters, there remains [on the one hand] such a great difference between the two, and, on the other hand, there is such close similarity to other terrestrial circular mountain ranges or ridges that are not volcanic, that another hypothesis about the formation of heavenly bodiesf is more likely to be confirmed by it [Herschel’s discovery], even though it is only partly analogous. It is true that the circular elevations on the Moon, similar to craters, make it likely that they originated as a result of eruptions. But on our Earth we find two kinds of circular elevations, of which one kind is always so small that they would not be visible from the Moon by any telescope; and their constituent materials show them to have originated in volcanic ∗

“On the Unevenness of the Moon[’s Surface]”; in the second volume of the Proceedings of the Gesellschaft Naturforschender Freunde.11

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eruptions. Others, however, cover whole countries or provinces, many hundreds of square miles in area, [each] with a circular ridge of land with mountain ranges of various heights. Only these could be seen from the Moon, and indeed would appear to be of the same size as those circular marks we see on the Moon, as long as their uniform covering (by forests or other vegetation) does not prevent their being distinguished from so great a distance. Thus these [circular mountain ranges] also would lead us to think that they had originated through eruptions, but on the evidence of their constituent materials they cannot have been volcanic [in origin]. – The crater of Vesuvius has a circumference at its summit of 5,642 Paris feet (according to della Torre12 ), which is about 500 Rhineland roods, and a diameter of nearly 160 roods; but such a crater certainly could not be discerned as such by any telescope on the Moon.∗ On the other hand, the crater-like mark of Tycho on the Moon is almost thirty German miles in diameter and is comparable [in size] to the Kingdom of Bohemia, while the nearby mark, Clavius,13 is similar in size to the Margraviate of Moravia. Now on the Earth these countries are enclosed by mountains, which also have the appearance of craters, from which mountain ranges radiate just as they do from Tycho. But if our crater-shaped basins enclosed by [mountain] ridges (all of which represent catchment areas for the river waters and cover the entire land-mass) were not to offer a similar appearance to [observers on] the Moon – as indeed can be supposed for only a few of them – then this could only be attributed to the accidental circumstance that the Moon’s atmosphere (the existence of which is proved by Herschel’s discovery, because fire does burn there) cannot extend nearly as far as ours (which is shown by the negligible refraction at this satellite’s edge). Thus the mountain ranges of the Moon extend beyond the limit of vegetation there; while on Earth the mountain ranges are for the most part covered with vegetation, and these of course would not be distinguished so readily from the area of the enclosed basin. Thus we have on Earth two kinds of crater-like land-forms: one, of volcanic origin [of the order of] 160 roods in diameter and thus about 20,000 square roods in area; others that are definitely not volcanic and are about 1000 square miles, that is 200,000 times greater in area. With which one of these do we wish to compare the circular elevations on ∗

But its fiery eruption could be seen during the lunar night. In the aforementioned letter there is a note referring to the observation of Herr Beccaria’s nephew and of Don Ulloa to the effect that both volcanoes must have been of terrifying size because Herr Herschel was only just able to see his with a very much larger telescope, and [furthermore] he was the only one of all the observers to notice it. However, in the case of luminous matter it is not so much the size as the brightness of the fire that matters for it to be seen clearly; and it is known that the flames of volcanoes sometimes emit bright light while at other times the light is obscured by smoke.

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the Moon (of which none of the observed ones are less than a German mile in diameter and some more than thirty)? – I think, judging by this analogy, [that comparison can only be made] with the latter, which are not volcanic. For shape alone is not decisive; the vast difference in size must also be taken into consideration. But then Herr Herschel’s observation has confirmed the idea of volcanoes on the Moon, though only ones whose craters have not been seen, nor can be seen, by him or anybody else. However, it [i.e., Herschel’s observation] has not confirmed the opinion that the circular configurations seen on the surface of the Moon are volcanic craters. For in all probability they are not (if we are to judge from the analogy with similar large basins on the Earth). Thus we should merely say that, since the crater-like basins of the Moon are very like those on the Earth that are catchment areas for rivers but are not volcanic, then it would presumably follow that the Moon also contains volcanic craters similar to those on Earth. It is true that we cannot see these latter on the Moon, but luminous points have been observed in the lunar night as proof of fire, which can best be explained in terms of the cause suggested by this analogy.∗ Leaving aside this minor equivocation in the conclusions of the eminent gentlemen referred to above – to what cause can one ascribe the non-volcanic craters found everywhere on the Earth’s surface, that is, river basins? Naturally, eruptions must be the cause, but they cannot be volcanic, because the mountains making up their rims contain no material of that kind, but appear to have been formed by an aquaeous mixture.g I think that if one imagines the Earth as having been originally a chaos in aqueous solution, then the first eruptions, which must have arisen everywhere even out of the greatest depths, would have been atmospheric (in the proper sense of the word). For it may well be assumed that our atmospheric ocean (aerosphere), which is now above the surface of the Earth, was formerly co-mingled in a chaos, along with the Earth’s other matter; that, together with many other elastic vapours, it burst forth from the heated globe as it were in great bubbles; that in this ebullition (which no part of the Earth’s surface escaped) the matters of the primeval mountains were ejected in the form of craters; and in this way laid the foundation for the basins of those rivers with which the whole land-mass is interwoven, like the meshes of a net. Since they consisted of watersoftened matter, these [crater] rims gradually lost their solvent water,h which in running off washed out the gaps which presently distinguish ∗

Beccaria regarded the ridges that radiate from the circular lunar elevations as lava streams, but the enormous difference in size between these and those that flow from the volcanoes of our Earth contradicts this opinion and make it seem likely that they are mountain ranges which, like those on our Earth, radiate from a central stem.

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those mountainous and saw-toothed rims from the volcanic [types] which have unbroken ridges. These primordial mountains consist of granite, after other matter such as hornstone or primeval limestone, which did not crystallize or solidify so rapidly, had been separated therefrom. The ebullition at a given place gradually becoming less and less, and its level progressively lower, these latter [softer materials] were deposited on the granite as outwash material in step-like sequence, according to their relative density or solubility in water. Thus the first formative cause of the irregularities of the [Earth’s] surface was an atmospheric ebullition, which I should prefer to call chaotic in order to emphasize its origins. On top of these, one must [further] imagine that a pelagic alluvion14 gradually deposited layered materials, which for the most part already contained marine creatures. For where there was a large number of these craters of chaotic origin grouped together as it were, they formed extensive elevated regions above other areas where the ebullition had not been so violent. The former became the land with its mountains; the latter became the ocean beds. As the superfluous crystallizing [i.e., solvent] water from those basins eroded their rims, and as the water from one basin ran into another, but all running down to the low-lying part of the Earth’s surface which was just forming (that is, the sea), it formed the defiles for future rivers, which we are still amazed to see flowing between steep walls of rocks which they now can no longer affect, and sought the sea. This, then, was no doubt the skeletal form of the Earth’s surface, insofar as it consisted of granite, extending under all the layered rocks that have since been deposited on top of it by pelagic alluvions. But it is precisely for this reason that the form of the land had to become cratershaped, even in places where more recent layers completely cover the ancient underlying granite, since its foundation was shaped in this way. Thus one can draw [mountain] ridges on a map (on which no mountains have been indicated) if one draws a continuous line through the sources of the streams that run into a large river; and this line will always enclose a circle, which is the basin of the river. Just as the ocean bed presumably became deeper and deeper, and thereby collected the water that ran out of the aforementioned basins, so also the river beds were created along with the whole present structure of the land, which makes possible the confluence of the water from so many basins into a single channel. For nothing is more natural than that the bed on which a river now carries the water from a large land [mass] should have been washed out by the retreat of precisely that water to which it presently flows, namely the sea and its ancient alluvions. In accordance with such a principle, this washing away cannot be conceived, as Buffon15 would have it, as being due to marine currents at the sea floor under a universal ocean, because under water there is no downflow according to 423

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the slope of the bottom, which after all is the most essential point in this instance.∗ The volcanic eruptions appear to have been the most recent ones, that is after the Earth’s surface had already solidified. They did not form the land with its hydraulically regular structure for the run-off of rivers, but merely formed individual mountains, which are only insignificant details compared with the edifice of the whole of the dry land and its mountain ranges. Now, the use to which the ideas of the aforementioned eminent gentlemen may be put, and which Herschel’s discovery confirms, albeit only indirectly, is of significance in relation to cosmogony: namely [it suggests] that the celestial bodies were all formed originally in a similar manner. Initially, all were in a liquid state; this is proved by their spherical shape, which is, where observable, [seen to be] flattened in accordance with their axial rotation and the gravitational weight of their surface. But there can be no fluidity without heat. “Where did this primordial heat come from?” To attribute it, as does Buffon, to the heat of the Sun, of which all the planetary spheres are merely ejected fragments, is only a temporary expedient; for “where did the heat of the Sun come from?” If it be assumed (and for other reasons this seems very likely) that the original matteri of all celestial bodies, in the whole vast space in which they now move, was initially distributed in gaseous form, and was formed initially in accordance with the laws of chemical attraction, and subsequently chiefly [according to the laws] of gravitational attraction, then Crawford’s discoveries16 suggest how the formation of celestial bodies [is linked with] the production of the requisite enormous degrees of heat. For if the element of heat is distributed uniformly in space, but attaches itself to various substances only in proportion to their several attractions; if, as he shows, materialsj distributed in gaseous form contain far more elemental heat, and indeed require it for their distribution as vapours, than they can hold once they become solids, that is to say, when they coalesce to form celestial spheres, then these spheres must contain matter of heat in excess of their natural balance with the matter of heat in the space they occupy, that is, their relative heat will have increased in comparison with outer space. (Thus when vitriolic acid in gaseous form comes into contact with ice it immediately loses its vaporous state and thereby the heat is increased to such an extent that the ice melts at once.) We do ∗

The flow of rivers seems to me to be the real key to the theory of the Earth, for this requires: first, that the land be divided through its shifts into pools, as it were; second, that the floor on which these pools conveyed the water from one to another, in order ultimately to drain it into a channel, was formed by that water which gradually receded from the higher basins to the lowest one, namely the sea.

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not know how great the increase might be, but the degree of original rarefaction and subsequent condensation and the short amount of time involved appear to be relevant. But since the latter [i.e., the time] depends on the degree of attraction that brought the scattered matterk together, and this in turn depends on the quantity of matterl in the bodym being formed, it follows that the degree to which this body is heated must also be in proportion. In this way, we can also have insight into why the central body (being the greatest mass in any cosmic system) has the greatest amount of heat, and is the sun in every case. Similarly, we can speculate with some confidence that the outer planets, partly because they are for the most part larger, and partly because they are constituted of more rarefied matter than the inner ones, contain more internal heat, which they also appear to need (as they only receive from the Sun just about enough light to see by). Furthermore, the creation of the mountains on the observable surfaces of the celestial bodies, that is [the surfaces] of the Earth, the Moon, and Venus, by atmospheric eruptions of their primaevally heated, chaotic liquid masses, appears to us to be a fairly general law. Finally, the volcanic eruptions of the Earth, the Moon, and even the Sun (the craters of which Wilson17 saw [or rather detected] in sunspots by cleverly comparing their appearances one with another, as Huygens18 did with the rings of Saturn) could be explained by and derived from the same universal principle. Now if anyone wished to turn my criticism of Buffon against me, and ask where the first motion of the atoms in space came from, then I would reply that I have not offered to give an explanation of the very first change in nature, which is indeed impossible. Nevertheless, in the case of a natural phenomenonn such as the heat of the Sun, which has similarities to appearances whose cause we can at least surmise in accordance with known laws, I think it unacceptable to come to a halt and in desperation invoke an immediate divine decree as an explanation. This latter must admittedly form the conclusion of our investigation when we talk of nature as a whole; but in every epoch of nature, since no one of them can be shown by direct observation to be absolutely the first, we are not relieved of the obligation to search among the causes of thingso as far as is possible for us, and follow the causal chain in accordance with known laws as far as it extends.

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