332 



•ir Idintifv. 



f the «nrth> 



if only we 

 >n» throiq^ 



From thU point tl 

 !, and cnn be 

 <l^e of the mi 



il tiv stage!* oi incjr development. 

 It mutt have been a very turbulent s<-a, the ntoltcn 



irface of our earth upon which the rockv . r,,^i i„... .„ i,. 



rtn. The first pntchcs of crust were p: 



■ r .tnd (ivor again by rsrnping gasr>s 



ri<<, of which our waning volcanic activity ix but a 

 •>cho. If the earth wait first gaseous and th«> outT 



gradually condensed to a liquid, its om 



ist must have been whirled and turn 



,,pi, . v..,i in a few thousand years (whiti. ■- .. >.•. 



I the formation of an earth), to mix its 



its pretty thoroughly. It has accordingly 



• just how it came to separate into in- 



if such widely different appearance and 



( haiict'jr. * )t course, the number of its ingredients wa*i 



large. We have already discovered eighty or more 



different elementary substances in the earth, and there is 



an almost endless nimiber of more or less stable compounds 



of these. The fnM'zing of an earth is therefore different 



from the freezing of pure water, but the freezing of salt 



water offers a clue to the explanation of the way in which 



the earth solidified as we find it. When salt water freezes 



the salt is practically all left behind. The ice contains 



much less salt, and the remaining water relatively more 



salt than before freezing began. .Applying this familiar 



observation to the supposed molten surface of the earth 



as it begins to solidify, we have a suggestion of order and 



re.ison in its separation into so many kinds of rocks. 



Now it happens that in the recent development of chem- 

 istry much attention has been given to the study of solu- 

 tions of various kinds, and a great body of information 

 has been gathered and classified, of which our observation 

 upon thp 1roo7\r\fi of salt water is a simple type. Still 

 niMi,. ti(. ntl\ ujiiite lately, in fact) it has occurred to many 

 siud-iits of th' (nrth that here lies not onlv the clue, but 

 ptrhaps the k. \ , to their great problem. If the individual 

 components whldi .-ire intimately mixed in solution separate 

 wholly or partially in some regular w.ny upon freezing — and 

 n-ririv iill the solutions which have been studied appear 

 to show such segregation — we have a quantitative system 

 which will probably prove adequate to solve the problem of 

 rcK-k formation, provided only that the experimental difficul- 

 ties attending the study of molten rock and the complica- 

 tions imposed by the presence of so many component 

 minerals do not prove prohibitive. This is a verv simple 

 statement of the point of view which has led to the experi- 

 mental study of rock formation in the laboratory as a 

 natural sequence to statistical study in the field. 



(ieophysics, therefore, does not come as a new science, 

 nor as a restricted subdivision of geology, like phvsiographv 

 or stratigraphy, but rather to introduce into the studv of 

 the earth an element of exactness, of quantitative relation. 

 It may include physics or chemistry, biologv or crvstallo- 

 graphv or physical chemistry, or all of these at need. 

 The distinctive feature of geoph\'T!ic.s is not its scope, which 

 niav well be left to the future, but its quantitative 

 rharacti-r. TIi.- geophysical laboratory of the Carnegie 

 Institution at Washington has entered uoon some of the 

 investigations suggested by this long prelimin.nrv studv of 

 the earth — the physical properties and conditions of forma- 

 tion of the rocks and minerals. The department of terres- 

 trial magnetism of the same institution has undertaken 

 another — the earth's magnetism ; the German Geophysical 

 Laboratory at Gcitlingen a third — the earthquakes ; and 

 these will no doubt be followed by others 



The first effect of calling exact science into consulta- 

 i'<"n upon geolocjic problems is to introduce a somewhat 

 i) = fferent viewpoint. It has been our habit to studv the 

 minerals and the rocks as we find them to-dav, after 

 lyany of the c.iuses which have had a share in their evolu- 

 fon have ceased to be active — after the fire has gone out. 

 If we attempt to reconstruct in our minds the operations 

 which enter into the formation of an igneous rock or of 

 a body of ore. we must infer them from present appear- 

 .Tnces and environment. The experimental geophysicist, 

 on t\v oiher hand, confronting the same problem, savs to 

 himself : Can we not construct a miniature volcano in the 

 'iboratory? Can we not build a furnace in which an 



NO. 220I, VOL. 



NATURE 

 r 



[January 4, 1012 





uilrO' 

 f(*r thft 

 ' ter of 



wat<r 



iglMoiu rock can be formed under such oondi 

 ran observe its minutfi^t chang*? He prnjx 

 ducff t«>mp<-raiur<*-fneai>uring devices and .i;i|).u 

 I if pressure, to investiyaf th- 



.; atmosphere and tli' qu.mtif, 



may b«* •" — ■" ' '■ •'■• 



of everv 



i^uards it 



In thcM; various ways he will undertake to a»«.rriain tlie 



'•xwt mngnittide of all the causes, both physical and 



■ 1) have been at work in his miniature rork- 



her with the physical characteristics of the 



i'' """"■' • 



A very practical question now arises : Can he do all this 

 successfully at the temperatures where the minerals forni? 

 We must press this question and insist upon a satisfactory 

 answer, for it is by no means obviotjs th.<»t th** relations 

 which the physicist and chemist 1: i at the 



temperatures of everyday life — en density, 



solubility, viscosity, dissociation- — um <i..iinm. to hold 

 when substances are ciu-ried up to a white heat. Th«? 

 substances, too, are different from those with which the 

 chemist and physicist have been generally familiar. 

 Instead of simple metals, aqueous .solutions, and readily 

 soluble active salts, we encounter silicates and refractory 

 oxides, inert in behaviour and capable of existing together 

 in mi.xtures of great complexity. We must therefore ex- 

 tend the range of our physics and our chemistry to a 

 scope in some degree commensurate with the wide range 

 of conditions whicJi the earth in its development has passed 

 through. Let us follow for a little the actual progress of 

 such an attempt. 



The first step is to provide the necessary temperatuc 

 Obviously, the common fire-clay crucible and the smelter s 

 furnace, with its brick lining, will not serve us here, for 

 all these are themselves mineral aggregates. The charge, 

 furnace lining, and crucible would go down together in a 

 fall as disastrous as Humpty Dumpty's. But experiment 

 has taught us that platinum crucibles, magnesia furnace 

 tubes enclosing an electrically heated helix of platinum 

 wire, and electric temperature-measuring devices, provide 

 a furnace in which nearly all the important minerals 

 can be successfully studied, which is not enough to melt 

 zinc, silver, gold, copper, nickel, or iron readily, and 

 where any temperature up to ifSoo** C. can be maintained" 

 perfectly constant, if need be, for several weeks. All 

 these temperatures can be measured with no uncertainty 

 greater than 5°. This equipment preserves the chemicaf 

 purity of the mineral studied, and enables the temperature 

 to be controlled and measured at every step of the experi- 

 mental work. Or an iridium furnace tube and an iridium 

 crucible can be substituted for platinum, the magnesia 

 supports can still be used, and we have it in our power to 

 go on to 2000° C, which is quite sufficient for all the 

 more important minerals which we know. 



The physicist has therefore found a suitable meltin;, 

 pot, and means of ascertaining what goes on within th»- 

 pot ; but he at once encounters another difficulty. Natxire 

 has provided us with relatively few minerals of high 

 chemical purity. If a natural mineral is chosen for experi- 

 ment, however typical it may be, several per cent, of other 

 minerals may be expected to be present with it, the effect 

 of which is at present quite unknown. Now the first 

 axiom of the investigator in a new field who desires to 

 undertake measurements which shall have a real value is 

 that the number of unknown quantities in his equations- 

 must not be greater than he can eliminate by his experi- 

 mental processes ; in other words, he must begin with con- 

 ditions so simple that the relation between a particular 

 effect and its cause can be absolutely establish*Kl without 

 leaving undetermined f.-ictors. Having solved the simple 

 case, it is a straightforward matter to utilise this informa- 

 tion to help solve a more complicated one. If we would 

 therefore reduce the mineral relations to an exact science, 

 which is our obvious purpose, it is necessary" from the 

 outset to prepare minerals of the highest puritv" and m 

 establish their properties. Having obtained such a pur" 

 mineral type, it may be, and often is, in the powf^r of the 

 mineralogist and his microscope to determine, by direct 

 compari-ion with its natural prototype, the kind antf 

 amount of effect actually produced in thf> natural minpral 



I 



88] 



