Maj 9, 1878] 



NATURE 



43 



The extent of volcanic action is partly lost on the earth 

 by the effects of air and water ; but when they looked at 

 the old rocks, they found there had been volcanic action 

 almost everywhere. In our limestone rocks, for instance, 

 there are the basaltic veins, which in some parts go by 

 the name of toadstone, which are certainly the result of 

 volcanic heat enough to produce fluidity. Almost every- 

 where they found that there were volcanic streams inter- 

 mixing with all the rocks ; and even although the surface 

 of the earth had been free from volcanoes in a given 

 district for a time, yet there had always been volca- 

 nic action very near, enough to force in veins of 

 Java from time to time. It seems, therefore, that we 

 are entitled to say that we have always been near a 

 gxQdiX. deal of heat — probably we have been much 

 nearer it than at the present time, but still we are 

 near enough to experience a great deal even in these 

 countries. Repeated experiments have been made on the 

 increase of temperature as you go down in mines, and 

 the conclusion has been come to that the temperature 

 rises one degree Fahrenheit, sometimes in sixty and 

 sometimes in 100 feet. There is a mine in Cornwall in 

 which he had walked in a stream of water at the bottom 

 actually scalding to the legs ! and everybody knows what 

 quantities of water there are in the hot springs. And then 

 there is the great display of the volcanoes, which come 

 from a great deal of heat somewhere ; and in places where 

 volcanoes are extinct we can trace a sort of basaltic con- 

 tinent, so to speak, up to the very mouths of the craters 

 from which the lava has come. So that there has been 

 in all former ages undoubtedly much more heat than at 

 present. There was another matter on which he would 

 desire to speak, but with no great boldness, and that was 

 the change in magnetism. The subject of terrestrial 

 magnetism is one of the most obscure in the world ; 

 nevertheless, looking at the direction in which it always 

 is towards the colder parts, and tracing its general phe- 

 nomena, it may be effected by thermo-electricity, and 

 tliat may be produced by the constant wear going on in 

 the interior of the earth, where the fluid lavas are con- 

 solidating themselves. Within a few years the voyage of 

 the Challenger has been made, and he had little hesita- 

 tion in saying it was one of the most important in the 

 scientific history of the world. In crossing the great seas 

 they sounded to great depths, and measured in a satisfac- 

 tory way the temperature of the water down to the depth of 

 five miles. They always came to cold at the bottom ; and 

 there are great controversies whether the cold can come in 

 deep sea streams from the frozen regions of the north. 

 He thought that had some influence ; but he thought the 

 bottom of the water and the ground at those great depths 

 is cold — he did not think that part of the earth partakes 

 of the same heat as other parts ; that he only expressed 

 as his opinion, in which, of course, he might be met 

 by the disbelief of a great many persons. That was the 

 state of things as we know it regarding the temperature 

 of the earth — that there is evidence everywhere that there 

 has been enormous heat almost all over the earth. Some 

 parts of the crust of the earth under the deepest seas are 

 still perforated by volcanic islands. In some places the 

 heat comes very near the surface. That he looked upon 

 as an important fact, leading them to a theory of what the 

 state of the earth really is. 



On entering upon a matter which was undoubtedly one 

 of the boldest speculations in modern science, which was 

 the formation of the earth — he could not say its crea- 

 tion, but the way it got into its present shape — he had to 

 premise that the theory on which he had to speak, which 

 is known as the nebular hypothesis, is the conception of 

 a very bold and vigorous intellect indeed. Laplace it 

 was who remarked that all the planets and satellites 

 revolved in the same direction round the sun, and all of 

 them turned on their axis in the same direction : and it 

 was difficult to deny that there must be some general 



cause for this. It naturally occurred to Laplace that if 

 we can find something which is contracting its dimen- 

 sions, and which has a little rotation to begin with, then 

 with every contraction of dimensions that rotation would 

 become more rapid, till it might go to any degree, depend- 

 ing upon the condensation of its various parts and its 

 density before. Then can we come to look at any matter 

 which is being thus condensed, and which might so form 

 systems such as ours, with sun, planets, and satellites ? 

 There are a series of bodies in the sky which did not 

 attract much attention in former days, mainly because 

 telescopes were not so large, but which are now catalogued 

 by thousands. These are the nebulae. The name denotes 

 their cloudy appearance. They are small bodies among 

 the stars, sometimes appearing to have stars in them, or 

 to be connected with stars, and sometimes not. They 

 have the strangest and most capricious shapes imagin- 

 able. If this nebula is contracting its parts together so 

 as to form a world, that rotation in the course of conden- 

 sation will become so rapid that it may form suns and 

 planets and earths around it ; and on this supposition 

 there is no difficulty in making a complete solar system 

 cut of such a mass as that of the nebula in Orion. 

 Observations made lately by the largest telescopes — 

 those of Lassell and Lord Rosse, both of which are re- 

 markable telescopes of the largest class — have brought to 

 light a number of nebulae possessing a spiral appearance ; 

 and they seem to have some bearing on the supposition 

 that the nebulas are contracting and getting into a rotatory 

 state. But these changes go on so slowly that they had 

 not been able to answer with certainty for any of the 

 changes of which he now spoke. The whole thing is 

 theoretical, and yet, as it seemed to him, in the highest 

 degree probable. Supposing this to be the case, these 

 nebulae would rotate, and in their compression would get 

 very hot. There is no doubt that condensation would 

 produce enormous heat, and it seems we have there suffi- 

 cient explanation of the great heat we find below the 

 surface of the earth and in other places. We suppose 

 that the stars generally have been formed from the 

 condensation of nebulae; and there is a circumstance 

 which was worthy mentioning. A series of observations 

 founded upon optical experiments has come to light 

 within late years which has done more to reveal the 

 secrets of nature than anything before — this was by 

 means of the spectroscope. By voltaic action sparks 

 may be produced which derive their character — sparks 

 like those of an electrical machine— in a great measure 

 from the metals from which they spring. A spark springs 

 from metal to metal, and the character of the metals 

 gives different characters to the sparks. We have one 

 set of these spectra produced by iron, another by nickel, 

 others even by hydrogen gas, and so on, and these are 

 observed and catalogued with great care. When we 

 come to observe the light in the stars in the same 

 manner, we find there are no two stars alike ; some of 

 them have the same spectra as that given from iron, and 

 others have spectra from a number of different things ; 

 and we are actually able, by legitimate reasoning from 

 this, to say from what the stars are made— what metals 

 and other things they are made of, and, as a generat 

 thing, there are no two stars alike. So that in this 

 nebular hypothesis we are not bound to say that the 

 nebulae are all of the same materials, and we conceive 

 that by comparing the bodies which we know in the 

 solar system with those of the stars, we may arrive at 

 an idea of the variety of materials of which the planets 

 are composed. We cannot find anything different in 

 comparing the light of the planets, because they all 

 derive their light from the sun, and they do not present 

 any difference of appearance in the spectrum. But we 

 can draw conclusions from their relative density. As he 

 had said to them, the average density of the earth is 

 probably five and a half tirties that of water. They knew 



