June 1, 1891.] 



KNOWLEDGE. 



lU.j 



the invasion of any particular area of the earth's surface by 

 the volcanic forces is heralded by subterranean shocks 

 causing earthquakes. A little later on, fissures are formed, 

 as indicated by the rise of saline and thermal springs, and 

 the issuing of carbonic acid and other gases at the surface 

 of the earth. As the subterranean activity becomes more 

 marked, the temperature of the springs and emitted gases 

 increases, and at last a visible rent is formed, exposing 

 highly heated and incandescent rocky matter below. From 

 the fissure thus formed, the gas and vapours imprisoned in 

 the incandescent rocks escape with such violence as to dis- 

 perse the latter in the form of scoriie and volcanic ash, or 

 to cause them to pour out in streams or lava flows. The 

 action generally becomes concentrated at one or several 

 points along the line of action — that is, the line of fissure 

 and dislocation. In this way, a chain of Volcanos is 

 formed, which may become the seats of volcanic action for 

 a long time. When the volcanic energies are no longer 

 able to raise up the fluid materials so that they shall flow 

 out of the cones which have been built up, nor to rend 

 their sides and form parasitic cones, fissures with small 

 cones may be formed in the plains around the great 

 central Volcanos. Later on, as the heated rocky matter 

 below cools down, the fissures become sealed up by con- 

 solidating lava, and the Volcanos fall into a condition of 

 quiescence, after which they begin to suffer from the 

 effects of exposure to the atmospheric agencies of decay, 

 and thus become more or less worn away or " denuded." 

 But still the existence of heated rocky matter at no great 

 depth below is indicated by outbursts of gases and vapours, 

 the formation of geysers, mud-volcanoes and ordinary 

 thermal springs ; gradually, however, even these manifesta- 

 tions become more feeble, and thus all v-isible signs of 

 volcanic energy die away in the district. Such a cycle of 

 changes may require millions of years, but by the study of 

 Volcanos in every stage of their growth and decline, it is 

 possible to reconstruct this outline of their life history. 



That Volcanos are built up along lines of fissure in the 

 earth's crust does not admit of any doubt. The present 

 distribution of Volcanos over the earth is a striking proof 

 of this, and, moreover, we have further evidence derived 

 from the study of old volcanic areas, which have been, as 

 it were, dissected and so brought to light by long-continued 

 erosion or denudation. Let us look a little more closely at 

 the present distribution of Volcanos on the earth's surface, 

 for it reveals some interesting facts which nnist be borne 

 in mind in forming any conclusions with regard to the 

 possible cause of volcanic action. One rule wo have 

 already observed, viz., that Volcanos arc mostly distributed 

 along lines. Secondly, they seem to follow or coincide 

 with great mountain chains, such as the Andes, Eocky 

 Mountains, or the ranges of C!entral America. Thirdly, 

 there is some kind of connection between Volcanos and 

 tiie coast lines of continents. Fourthly, they are always 

 near some liody of water (j.c, when in the active stage). 

 Fifthly, they are situated in regions of the earth which are 

 undergoing slow H])huaval, and are absent from regions 

 where subsidence is taking place. 



In framing any conclusions with regard to the problem 

 under consideration, we must renu-niber that volcanic action 

 depends mainly on two things — (1) a high temperature 

 below the region of activity, (ii) the presence of steam at 

 a high pressure. 



Superheated steam evidently plays a very important 

 part, and the force which raises masses of molten lava 

 to the surface may be that due to the expansive power of 

 steam. Volcanic eruptions, then, are essentially gigantic 

 explosions, such as are faintly imitated in the bursting of 

 steam boilers. This is good as far as it goes, but we 



cannot take it as an explanation of volcanic action ; for we 

 require to know the source of the steam, and of the lava, 

 as well as the reason for the high temperature necessary 

 for the production of both. Where does the heat come 

 from ? and what is the source of so much steam ? Sir 

 Humphry Davy, discoverer of the metals of the alkalies 

 and alkaline earths at the commencement of the present 

 century, showed that the metals potassium and sodium 

 when touched by water develop a great deal of heat ; in 

 fact they burn on water, decomposing it and uniting with 

 the oxygen. This led him to throw out the idea that if 

 pure metals exist far down in the earth's interior, the 

 access of water and air might give rise by oxidation to a 

 large amount of heat, sufficient in fact to produce volcanic 

 phenomena. But later on he confessed that this chemical 

 theory of Volcanos was unsatisfactory. If it were true, 

 enormous quantities of hydrogen gas would necessarily be 

 emitted during volcanic action, but this is not the case. 



It will readily be perceived that all explanations of 

 volcanic action resolve themselves finally into the question 

 of the condition of the earth's interior, with regard to 

 which we can at present only speculate ; hence the absence 

 of any complete and consistent explanation of the volcanic 

 problem. 



Certain facts undoubtedly tend to establish the idea, 

 once firmly maintained, that the whole of the earth's 

 interior is in a highly heated state, but they do not prove 

 it. The well-known increase of temperature as we descend 

 mines, which is about 1'^ F. for every 50 or GO feet, is 

 not sufficient proof, for the nitc af incnu.sc does not seem 

 to be maintained as we descend to the greatest depths, 

 and it is possible that the centre may be cold." Still, 

 astronomers tell us that the earth has been for ages a 

 cooling globe, so that it would seem natural to suppose 

 that there are still vast stores of heat within ; but they 

 may be more or less local. 



It has even been argued, at one time, that the whole 

 interior of the earth is in a molten condition, with only a 

 thin crust of solid matter forming a kind of shell or outer 



* It is conocivablo, though it is not probal)lt', thiit tlic central 

 portions might not l)e wanner tlian the regions which liave been 

 already explored; but it is impossible that, after the lapse of ages, 

 they should remain cooler than the exterior layers. The sun was 

 formerly supposed by Bode, Sir .lolin Iferseliel, and other distin- 

 guished astronomers to be a cool body surrounded by two 

 atmospheres, the inner one a partially opaque lieal-ahsorbini 

 atmosphere, an<l the outer phofuxphere :i l»rilliant lu'at-ixi\ ing 

 atn\osphere. If it had not been for th<" great di'sirc whii'h n\ankind 

 has always had to suppo.se that other boilies are inhabited by beings 

 similar to ourselves, such an iih-ii wouhl prolialily ne\er have been 

 entertained. Sir Isaac Newton considered and rejected the theory 

 lliat tin' sun might-lnivc a cool boily and yet be sumiun(hHl by a hot 

 atmospliere. Hut his reasv)ns weri* ordy giviMi in a U'tter, and wer«^ 

 probably not gencnilly known till tlie midille of this century. Tliey 

 are so elearlv cx|in'ssc"d that Ibev could not fail to have c.mviuced any 

 intelligent thinker. Tlic l.-tteroi' Sir Isaac Newton 1 refer to was pub- 

 lished by Sir David Brewster in Ills Life of Newton, Vol. 11.. p. -t-V.. Sir 

 Isaac says, " Thougli llie inward jiart of the sun were an earthy sross 

 substance, yet if tlic Hipiidsliiniug substance .Mr. l'"him.steeil su])poses to 

 swim npon it be I lien hot. it will beat the matter within it as certainly 

 as nielteil lead woidil heal an iron buUet inunersed in it. Nor is it 

 material whether the lii|uid matter on the sini be of any considerable 

 thickness. An iron buUet woidd licat as fast in a quart as in an 

 ocean of melted lead. Ibis dinVrence only I'xcepted, that the bullet 

 would cool a small (|uantily of lead more than a givat one. If, then, 

 the liquid matter swimming on the sun be but so thick as not to bo 

 ci)ole<l by the central (as it must be), it will certainly heat the central 

 parts, for it imjxirts heat to the contiguous parts as fast as if it were 

 thicker, by which means the central )«irts must become as hot a.s if 

 the hot fluid matter surrouiuling it equalled the whole vortex. The 

 whole body of the suti, therefore, must be red hot, and consequently 

 void of magnetism, uidess we suiqiose its uiagnetisra of another kind 

 from anv we have." A. C. R.^NY.MiB. 



