directly measure the depth of each sents. We see that the radiation of little cloud. There are, however, many the sun, both as to light and as to heat, scores of thousands of these cloudlets is almost entirely dispensed from thes to be seen, and as they all seem much clouds of the photosphere. We seen of the same shape we may feel pretty that these clouds are contained in a confident that we are not looking at shell which lies in the outer parts of rod-shaped objects turned endways the sun. We see that the interior of towards us. There cannot be a doubt the sun is comparatively dark, that even that we may safely estimate the depth through the pores, and through the of each granule as a dimension resem- spot openings, it transmits but little bling on the average the length and radiant light and but little radiant heat. the breadth of the objects which are There is, however, no reason to think measured by us. We thus see that the that the internal parts of the sun are solar cloudlets are generally presented less hot than these photospheric clouds as more or less irregularly globular to which we owe so much. Indeed, objects a thousand or more miles in the contrary is, and must be, the case. diameter. We know that a heated body like the The photosphere, which is the term sun must be hotter in the inside than which astronomers apply to the shell it is on the outside. We know that or stratum that contains the luminous clouds, is thus seen to consist of floating cloudlets of dimensions comparatively small, when the diameter of the photosphere is considered. We do not say that there is only a single layer of these objects, but it is plain that the total thickness of the stratum must be of insignificant dimensions relatively to the radius of the sun. Dr. Stoney, in his paper to which I am referring, speaks of the photospheric shell as a "film" encompassing the interior of the sun. I do not see that we have any means of determining the thickness of the shell accurately, but to do so is not essential for us. It may be that the photosphere bears the same ratio to the bulk of the sun that the delicate skin of a peach bears to the luscious interior. It may be that the rind of an orange bears a proportion to the fruit inside, which represents the relation of the cloud-bearing stratum to the internal parts of the sun. In this latter case we may imagine that each of the photospheric clouds would be small in comparison with the thickness of the entire shell. In this case, too, the clouds must of course be much more sparsely distributed throughout the extent of the shell, for otherwise they would not permit us to obtain any glimpses whatever of the interior. from the outside inwards the temperature on the whole gradually increases, and thus we learn that the dark inner regions of the sun, of which we obtain glimpses through the openings in the spots, must be actually hotter than the dazzling clouds of the photosphere. Here, then, we seem almost in the presence of a paradox. It is not from the hottest part of the sun that the heat or the light is chiefly dispensed. If the photosphere were removed it seems that a portion of the sun far hotter than the photosphere would then be fully exposed. It is, however, almost certain that in such a case the light and heat we get from the sun would drop to the tenth part, perhaps to the hundredth part, or even to some smaller fraction of that copious radiation which we now enjoy. It is quite plain that the inhabitants of this earth are indebted, not merely to the fact that there are vast supplies of heat stored in the sun, but to the circumstance that the photosphere is of such a particular constitution that it is enabled to transmit to us some of that wealth of energy, which without the assistance thus rendered the sun would apparently experience much difficulty in getting rid of. And thus we are brought face to face with the great problem as to what parAnd now we are able to state the ticular element it is whose presence remarkable problem which the sun pre-confers on the photospheric clouds that remarkable property which is of such water. To an observer in remote space vital importance to us. This is the who should view the earth as a whole, problem which Dr. Stoney has attacked it is the atmospheric clouds which and of which he has offered the solu- would present the most conspicuous tion which seems to answer all the features. They would be the objects requirements. It is my object to set that would command most special atforth some account of this remarkable tention, just as from our point of view extension to our knowledge, which, it is the clouds, perhaps of water also, even though it was made many years which are the chief features on Jupiago, appears to be still unknown to ter, and just as the photospheric clouds many to whom such matters are of are the chief features in the sun. Of interest. There is the more justifica- course I do not for a moment suggest tion for treating of the subject at the that the same elements which form our present time, because recent researches | clouds are also the constituents of the have tended in a remarkable manner solar clouds. The simplest considerato confirm the doctrines first enunci- tion of the facts of the case would inated by Dr. Stoney. deed preclude such a view. But the The analogy of the luminous cloud, illustration is of use, inasmuch as it in the sun, to the watery clouds in our suggests that as a single material sufown atmosphere, may be first referred fices to produce the characteristic terto. The vapor of water is well known restrial clouds, so a single material may to be diffused in more or less abun-suffice to produce the characteristic dance throughout the whole depth of solar clouds. the air. Under certain conditions of Let us then see if we have the necestemperature and of the quantity of sary data for ascertaining what this water present, this vapor may be con- solar material must be. We are first densed into clouds, and may thence be confronted with the fundamental quesprecipitated in rain. Under other cir- tion as to whether it is likely to be cumstances the clouds are again dis- composed of elements found on the solved into vapor according to the earth. There was a time, no doubt, ever-varying conditions of the air. when it might have been urged that The transition of water from the form in all probability the solar elements of vapor to the form of clouds is ef- were so far different from any bodies fected by a process of condensation, known to terrestrial chemists that the when the vapor, finding itself at a solar clouds must be constituted of cooler temperature than is compatible something altogether beyond our cogwith its retention of the gaseous form, nizance. But this view cannot be passes into the liquid state. Each molecule of the vapor thus allies itself with other neighboring molecules, and so a little bead of water is formed and the myriads of beads thus arising constitute a cloud. It may be, indeed it probably is the fact, that the nucleus of each little droplet, around which the molecules of watery vapor cohere, is provided by some minute particle of floating matter, such as one of the motes with which the atmosphere, even when purest, is still largely tenanted. But the essential point for us to notice in our present inquiry is, that the ordinary atmospheric clouds acquire their special character from the presence of a single substance, namely, sustained in the present state of science. Nothing is more remarkable in the recent advance of knowledge than the clear demonstration of the fundamental unity between the elements present in the celestial bodies and those elements of which the earth is composed. It is, no doubt, true that we have found grounds for believing that there may be one or two elements in the sun which we do not find here. We have, indeed, assigned to these dimly discerned elements the hypothetical names of coronium and helium. But even if such bodies exist at all, they are certainly wanting in the essential qualities that must be attributed to any element which purports to be the active component of the photospheric | stituent of the glowing clouds must clouds. There cannot be a reasonable possess. It must plainly be of the doubt that the sun is mainly composed most refractory or infusible nature, for of elements both well known and abundant on the earth. It is clearly among those known bodies that it is our duty to search for the characteristic photospheric material. it will be noted that the photosphere only derives its capacity to radiate white light-that is to say light of every hue-from the fact that the incandescent particles from which the light is dispensed must be at least liquid, if not actually solid. No truly gaseous body could emit light possessing the properties of photospheric radiation. We thus see that whatever be the photospheric material, it must be something which is able to remain a liquid, if not indeed a solid, while at a temperature still so high that the other constituents of the solar atmosphere are retained in the form of perfect. vapor. As the terrestrial clouds consist of water, they are derived not from a simple element, but from a composite body formed of the gases, oxygen and hydrogen. The multitude of composite bodies is, of course, innumerable, and the task of searching for the solar constituent would therefore seem to be an endless one, unless we were in some way enabled to restrict the field of inquiry. This is just what the vast temperature of the sun permits us to do. It is well known that at a heat resem- As an example of a material of which bling that at which the photosphere is the solar clouds are certainly not commaintained, chemical compounds can-posed, we may take the case of iron. not in general exist. Ordinary chem- This element requires, of course, ical compounds exposed to temperatures powerful furnace to transform it into a of such elevation are instantly resolved liquid, and at a still greater temperainto their elementary components. It ture it is known to pass from the liquid is thus manifest that in the endeavor to form to the gaseous form. In this gasfind the photospheric material we have eous state it is extremely abundant in not to scan the illimitable field of chem- the sun, as is evident from the fact ical compounds, we have only to con- that many hundreds of lines in the sider the several elementary bodies solar spectrum are thus accounted for. themselves. But it seems certain that the photo a of something, that is, which will have passed into the liquid state from the gascous state at a temperature which suffices to retain iron in a gaseous condition. This at once cuts iron off from the inquiry as well as all those elements which are more fusible than iron. Indeed, we may at once take the further step and say that no metal at all can fulfil the conditions that will be necessary. The field is thus restricted to the metalloids. Thus, at once, the research is nar-spheric material must be composed of rowed to a choice among some sixty-something much more infusible than four different materials, this being iron about the number of the different elementary bodies. Most of them have already been actually detected in the sun, and it is very likely that the others do really exist there also in some part or other of the sun's mighty volume. The mere presence of an element in the suu is, however, a very different thing from the presence of an element in the photospheric clouds. There are many materials in the earth, but only one forms the clouds of our atmosphere, so there may be many materials in the sun, but only one of them may be required to give character to the photospheric clouds. The list of possible substances receives a further reduction by the application of the important principle which Dr. Stoney was, I believe, the first to develop in his paper already referred We shall be guided in the selection to. In our endeavor to understand of the right material by the fundamen- problems of so complex a nature as tal properties which the effective con- those which the sun offers, where the conditions as to temperature and as to are animated by an average velocity pressure are so totally different from appropriate to that particular gas if it anything which we can observe around had been isolated and maintained at us on the earth or from the experi- the temperature of the mixture. Thus, ments which we can try in our labora- at the present moment the molecules tories, there is a particular advantage of oxygen and nitrogen in the air to be gained from any guidance which which we breathe being at the same the laws of dynamics can afford. These temperature, are nevertheless by no laws here come to our aid in a very means moving with the same average significant manner. They provide us, velocities. It is certain that, on the in fact, with another criterion by which whole, the molecules of nitrogen are we can sift out from the remaining list moving with a greater average speed of possible bodies that one element of than is pursued by the molecules of which it now seems certain the photo- oxygen. At the freezing temperature spheric clouds must be mainly com- we may think of the average speed of posed. the molecules of oxygen as about fifteen hundred yards per second, and of nitrogen as being a little greater. Here we must resort to that molecular doctrine of the constitution of gases which is now universally accepted. It The molecular constitution that we teaches us that a gas is composed of have attributed to elements which are myriads of molecules darting about, gaseous at ordinary temperature, may each molecule travelling quite freely also be ascribed to the gases into which except during these enormous but brief other elements are transformed when intervals when, by a collision or en- the necessary supplies of heat have counter with some other molecule, it is been forthcoming. The vapor of iron, deflected from its path. It is an essen- no less than the vapor of water, is tial part of this doctrine that the average velocity with which the molecules are animated depends upon the temperThe higher the temperature the greater the speed with which on an average each molecule hurries along. ature. composed of molecules which are dart- It For our present purpose it is essential to consider the case of two different gases at the same temperature. The velocities of the molecules are not, on the whole, equal in the two gases. It can be shown that the molecules of the gas constituted by an element with a low atomic weight will have, on the average, a higher velocity than is possessed by the molecules of a gas which, though at the same temperature, has a higher atomic weight. Thus, for instance, one gas might have molecules which were moving on the average about as swiftly as a rifle-bullet, while another gas at the same temperature bat of less atomic weight would have an average speed correspondingly beow that of the rifle-bullet. It is espe- roam about with the freedom enjoyed ally important to notice that in the se where different gases are mixed gether, the several molecules of each by the molecules composing any ordinary atmosphere under like conditions of pressure. The essential property of carbon, so | commingled. At the same distance far as our immediate object is con- from the centre of the sun we may ascerned, consists in the fact that a mole-sume that the temperatures are equal. cule of this element, be it either in the This being so, the several molecules of solid, the liquid, or the gaseous state, the different elements in the mixture has but a small mass when compared will be moving with varied velocities, with the molecules of most other ele- corresponding on the average to their mentary bodies. We have reason to various molecular weights. Specially believe that each molecule of the same noticeable among them will be the element always has the same mass, and molecules of carbon. They are in great that different elements have in general abundance, and they are distinguished different molecular masses. It hap- from the great majority of the subpens that the mass of a molecule of stances with which they are associated carbon is much less, not only thau that by the high speed at which they are of a molecule of iron, but even than generally darting along. Their motions that of a molecule of oxygen or nitro- are of course pursued in every direcgen. I do not, of course, assert that tion, myriads of molecules are flying the molecule of carbon is lighter than downwards, myriads are flying horizonthe molecule of any other element. tally, myriads are flying upwards. It That is not the case. The molecule of is these last which are at present imhydrogen, for example, is much lighter portant. than that of carbon. Indeed, the ele- As a molecule is flying upwards it ment just named has the lightest mole-experiences not alone all the chance cule of any.terrestrial substance. Out encounters with the other molecules, of the sixty-four elements there are, but it is also directly subjected to a however, only five with a less atomic reduction of its velocity in consequence weight than carbon. of the gravitation of the sun. That gravitation is vehement in proportion to the great mass of the sun. Thus the attraction of the sun on the molecules must be about twenty-five times the attraction which the earth exerts on bodies near its surface. Those molecules which move comparatively slowly must in their occasional vertical flights respond to the solar attractions more promptly than the molecules better endowed with velocity. It thus appears that when the molecules of carbon happen to be darting upwards and outwards from the sun, their comparatively high velocities will enable them to attain generally greater altitude in the solar atmosphere than is permitted to the molecules of a less lively character. Thus we see that the molecules of carbon will on the whole tend to soar aloft to greater altitudes than are attained by the majority of solar materials. But what it concerns us specially to notice is that among elements which may be deemed refractory, that is to say, among elements which retain the solid or the liquid state until a temperature has been attained high enough to drive most other substances into vapor, the molecular mass of carbon is exceptionally small. We may, in fact, assert that so far as elements which are likely to exist in abundance on the sun are concerned, the case of carbon is unique. This element combines an excessively high refractory nature with an excessively low molecular mass. It will not be hard to deduce from these facts Dr. Stoney's very remarkable conclusion that carbon is the effective constituent of the clouds in the photosphere. It is, indeed, very interesting to trace out the ingenious line of reasoning by which this conclusion is established. I shall here give an outline of the argument. Picture the condition of affairs in the upper part of the solar atmosphere, where the vapors of many elements are This consideration excludes a number of elements from possible participation in the clouds of the photosphere. Their molecular velocities are not suffi 武隆 |