January 4, 19 12] 



NATURE 



by the one or more other minerals which it contains. We 

 have therefore scarcely started upon our investigation before 

 the need of an organised system is demonstrated : first 

 comes the chemist, who prepares and analyses the pure 

 mineral for investigation ; then the physicist, vvho provides 

 and measures the conditions to which it is subjected ; then 

 the mineralogist, who establishes its optical properties in 

 relation to the corresponding natural minerals. 



Having prepared such a mineral, of high purity and of 

 known crystalline character, we can ascertain its behaviour 

 at the temperatures which must have obtained during the 

 various stages of earth formation. We can study the 

 various crystal forms through which it passes on heating 

 and the temperature ranges within which these forms are 

 stable ; we can also melt it and measure the melting or 

 solidifying temperature. Another mineral, prepared with 

 the same care and studied in the same way, may after- 

 ward be added to the first, and the relation of these two 

 determined. If they combine, heat is absorbed or released ; 

 and this quantity of heat can be measured, together with 

 the e.xact temperature at which the absorption or release 

 takes place. If the mixture results in the formation of 

 one or more mineral compounds, we shall learn the con- 

 ditions of formation, the temperature region within which 

 the new forms are stable, and the changes which each 

 undergoes with changes of pressure and temperature, as 

 before. If the new forms show signs of instability, we 

 can drop them into cold water or mercury so quickly that 

 there will be no opportunity to return to initial stable 

 forms, and thus obtain, for study with the microscope at 

 our leisure, every individual phase of the process through 

 which the group of minerals has passed. 



Without complicating the illustration further, it is 

 obvious that we have it in our power to reproduce in detail 

 the actual process of rock formation within the earth, and 

 to substitute measurement where the geologist has been 

 obliged to use inference ; to tabulate the whole history 

 of the formation of a mineral or group of minerals under 

 every variety of condition which we may suppose it to 

 have passed through in the earth, provided only we can 

 reproduce that condition in the laboratory. 



During the past quarter of a century there has arisen 

 in the middle ground between physics and chemistry a new 

 science of physical chemistry, in the development of which 

 generalisations of great value in the study of minerals 

 have been established. So long ago as 1861 the dis- 

 tinguished German chemist, Bunsen, pointed out that the 

 rocks must be considered to be solutions, and must be 

 studied as such ; but inasmuch as comparatively little was 

 known about solutions in those days, and the recks at 

 best appeared to be verj' complicated, no active steps in 

 that direction were taken during Bunsen 's life. But in 

 recent years solutions have been widely studied, under 

 rather limited conditions of temperature and pressure, to 

 be sure, but it has resulted in establishing relations — like 

 the phase rule — of such effective and far-reaching character 

 that now, after half a century afterward, we are entering 

 with great vigour upon the prosecution of Bunsen 's sugges- 

 tion. It is now possible to establish definite limits of 

 solubility of one mineral in another, and definite conditions 

 of equilibrium, even in rather complicated groups of 

 minerals, which enables us not only to interpret the rela- 

 (ions developed by such a thermal study as that outlined 

 .'tlxjve, but also to assure ourselves that only a definitely 

 limited number of compounds of two minerals can exist, 

 that they must bear a constant and characteristic relation 

 to each other under given conditions of temperature and 

 pressure, and that changes of temperature and pressure 

 will affect this relation in a definite and determinable way. 

 Physical chemistry not only takes into account the chemical 

 composition of mineral compounds, but their physical 

 properties as well, throughout the entire temperature region 

 in which they have a stable existence, and therefore 

 furnishes us at once with the possibility of a new and 

 adequately comprehensive classification of all the minerals 

 ;ii\d rocks in the earth. The value of an adequate system 

 of classification appeals chiefly to those whose duties bring 

 them into intimate relations with the subject-matter of a 

 science ; but so much may appropriately be said, that a 

 consistent application of physical chemistry to the minerals 



NO. 2201, VOL. 88] 



may operate in the not far distant future to develop an 

 entirely new conception of the science of mineralogy. 



As the number and scope of such exact measurements 

 increase, we gradually build up what mav be called a 

 geologic thermometer. Just as the location of fossils offer> 

 a basis for estimating geologic time, it often happens that 

 a mineral takes on a variety of different crystal habits, 

 according as it happened to form at one tenriperature or 

 another. Quartz, for example, which is one of the 

 commonest of natural minerals and one of the most 

 familiar, undergoes two changes in its crystal form which 

 leave an ineffaceable record. One occurs at 575° and the 

 other at 800°. An optical examination of even a minute 

 quartz fragment from the mountain side wiU reveal to the 

 skilful petrologist whether the crystal formed at a tempera- 

 ture below 575°, between 575° and Soo°, or above 800°. 

 And if we could have at our disposal a great body of such 

 exact measurements of the temperate region within which 

 particular crystals originate and remain stable, we could 

 apply that directly to terrestrial formations in which this 

 mineral occurs, and read therein the temperature which 

 must have obtained during their formation. All this will 

 not be done in the first year, and perhaps not in the first 

 decade ; but the ultimate effectiveness of this method of 

 procedure in establishing the relations between the minerals 

 and the valuable ores is now as certain of success as the 

 operations of any of the sciences which have now come to 

 be characterised as exact, as opposed to descriptive. 



There is one important difference between the great 

 laboratory of nature and its feeble human counterpart. 

 Nature operated with large masses, mixed with a generous 

 hand, and there was always plenty of time for the growth 

 of great individual crystals, at which we marvel whenever 

 we encounter them, and which we have sometimes come 

 to regard highly as precious stones. To carry these pro- 

 cesses into the laboratory is necessarily fraught with certain 

 limitations. The quantities must remain small, and the 

 time and available financial resources will always be 

 limited. So long as we are able to ascertain the optical 

 character of a crystal with equal exactness, whether the 

 crystal is of the size of the proverbial mustard-seed or a 

 walnut, the scientific laboratory cannot properly afford the 

 time necessary to produce the large crystals which nature 

 offers so abundantly, furthermore, the crystals of nature 

 often owe their brilliant colouring to slight admixtures of 

 impurity, which to the scientific laboratory spell failure, 

 and are avoided with the utmost care. Most of the mineral 

 crystals, when reproduced in the laboratory, are quit 

 colourless. And so, although the question is often raisi 

 whether we are not really engaged in the artificial pro- 

 duction of gems, and although the seductive character of 

 such an investigation would no doubt appeal to many, it 

 must be admitted that the geological laboratory is not, and 

 probably will never become, the serious competitor of 

 nature in those directions in which nature has produced 

 her most brilliant effects. 



In what has preceded I have laid emphasis upon the 

 value of experimental measurements in the systematic 

 development of a more exact science of the earth. It is n 

 fair question, and one which is very often raised, wheth. 

 all this investigation has a utilitarian side — whether th' 

 knowledge obtained in this way, and with such difficulty, 

 will help to solve any of the problems arising in the 

 exploitation of our niiiKi.il r.-sourccs or assist in our 

 industrial development. Ii i- neither wise nor expcdien 

 in entering upon a new field of research to expatiate Ion;, 

 upon its practical utility. Its principles must first be 

 established, after which there is no lack of ingenuity m 

 finding profitable application of them. 



The development of thermoelectric apparatus for ili 

 accurate measurement of high temperatures was begun 

 and has been perfected in the interest of geophysical r— 

 search, and it has already found such extended application 

 among the technical industries as to demand the manu- 

 facture and calibration of thousands of such high-tempera- 

 ture thermometers ■ •• The tempering and 

 impregnation of steel .nger dependent upon the 

 more or less trained • workman, but are done at 

 measured temperatun- r known conditions which 

 guaranl.r llu; unifnnnHN .., ,li.' proHu.t nnd admit of 



