3IO 



SCIENCE. 



[Vol. XXI. No. 540 



These general estimates are indefinite, and the minima, mean, 

 and maxima are alike unworthy of final acceptance; but they 

 stand for a real problem and not a merely ideal one, and repre- 

 sent actual conditions of the known earth; and, so far as the 

 science of geology is concerned, the maximum estimate is quite 

 as probable as the minimum, while the mean is much more proba- 

 ble than either. 



As commonly made, the physical and astronomical estimates of 

 the age of the earth are based on the assumption that the planet 

 is(l) homogeneous, and (3) simple in structure. Thus the cooling 

 of the earth would appear to be assumed analogous to that of a 

 heated spheroid immersed in an ocean, and cooling at a rate de- 

 termined by relative temperatures of spheroid and water, i.e., at 

 a progressively decreasing rate. Now the actual planet is (1) 

 heterogeneous and (2) complex in structure; and it may be ques- 

 tioned whether sufficient allowance is made for these facts in the 

 non-geologic estimates. 



By reason of terrestrial heterogeneity, the temperature of the 

 earth's surface is not directly dependent on the relative tempera- 

 tures of the terrestrial interior and surrounding space, but is 

 chiefly determined by a complex and wonderfully efficient me- 

 chanism for collecting and conserving solar heat, in which the 

 atmosphere and the liquid envelope play important roles. Most 

 geologists and physicists are of opinion that glacial periods might 

 be explained by geographic changes, and hesitate to adopt such a 

 theory only because of the dearth of positive evidence, or the ex- 

 istence of negative evidence, of such changes; and it is commonly 

 recognized that, other conditions of sun and earth remaining un- 

 chauged, the earth might be materiall.y chilled or warmed if the 

 land and sea were disposed in zonal lu- meridional belts in such 

 manner as to cut off or facilitate aqueous and aerial circulation. 

 There is, indeed, reason for supposing that if the earth with its 

 present mean interior temperature were divested of its heat-con- 

 serving mantles of air and water it would become a frozen planet. 

 Thus, whatever may have been the case in the pre-geologic 

 stages of planetary development, the present temperature of the 

 external earth, and so its rate of cooling, depends on the sun 

 rather than on the proper heat of the planet; and if (as is proba- 

 ble) the aggregate quantity of air and water enveloping the 

 planet Is diminishing, the efficiency of the terrestrial mechanism 

 for conserving solar energy must have been even greater during 

 the earlier ages of geologic development than now. 



Again, the earth is complex in chemlc constitution, and, more- 

 over, it is probable, if not certain, that this complexity is corre- 

 lated with temperature. If the course of terrestrial development, 

 as commonly recognized, could be reversed for a time, the con- 

 stitution of the earth-crust would be materially modified ; as the 

 temperature rose through a few degrees, the oxidation and fer- 

 mentation of certain substances would doubtless be accelerated; 

 with a few dozen degrees increase, life would be destro.ved and 

 the highly complex comjiounds manifesting that form of energy 

 would be broken up ; with a few hundred degrees rise, the coals 

 would be consumed and the carbonaceous shales and limestones 

 would be transformed, and these changes would be accompanied 

 by profuse development of energy in the form of heat; and with 

 a few thousand degrees increase in temperature, most of the com- 

 pounds of the earth-crust would be modified or destroyed and the 

 elements separated or re-combined in simpler forms. Considera- 

 tion of the effects which would necessarily follow reversing the 

 mechanism of planetary development indicates that the history of 

 planetary growth is one of chemic differentiation coupled with 

 molecular degradation, in which at least such molecular undula- 

 tions as those of light and heat have progressively decreased in 

 vigor. Moreover, this law appears to pervade the cosmos. It is 

 probable that, as long since suggested by Kirkwood, the tempera- 

 ture of the cosmic bodies varies directly, while their chemic com- 

 plexity (as determined by the spectroscope) varies inversely with 

 their volume; and the meteorites, which give some indication of 

 the constitution of other parts of the solar system, if not of still 

 more distant portions of the cosmos, are made up chiefly of ele- 

 ments common to the earth, yet are united m frequently distinct 

 and usually simpler compounds. Thus the phenomena of the 

 planet, of the cosmos in general, and of meteorites appear to ex- 



press a law of inverse relation between chemic constitution and 

 temperature, i e., a law of chemic differentiation accompanying 

 molecular degradation; and this law is in accord with the results 

 of some of the latest researches concerning the ultimate relations 

 of matter and energy. It follows that an aged planet like the 

 earth must have stored up within it a vast amount of latent 

 molecular energy; and incidentally that the law of cooling based 

 on bodies of simple constitution is inapplicable. So it may be 

 questioned whether the simple law of cooling, supposed to indi- 

 cate the age of the earth, is more trustworthy than would be a 

 formula for the volume-temperature relations of H^O, derived 

 from laboratory experiments on ice vvhen extended to a body of 

 the same substance passing through the gaseous, liquid, and solid 

 conditions; or whether the simple law of coaling deduced largely 

 from laboratory experiments conducted under circumscribed con- 

 ditions are much more applicable to the highly complex earth than 

 to the body of a hibernating animal. 



In short, the geologic estimates of the age of the earth are 

 based on direct observation under actual conditions so fully 

 known, that, although certain factors are variable, all may be 

 safely assumed to be known; while the factors involved in the 

 non-geologic estimates — surface and sub-surface temperatures, 

 thickness of the earth-crust, properties and conditions of rocks, 

 etc. — must be furnished by the geologist, so that, at the best, 

 such estimates represent nothing more than the grist ground from 

 a mathematical mill ; and, moreover, it usually happens that un- 

 known factors are introduced to give texture to the product, but 

 which, at the same time, so far adulterate the grist as seriously 

 to affect its value. The geologic estimates concerning the age of 

 the earth are based on real processes and actually observed condi- 

 tions in such manner as practically to eliminate inaccuracies 

 growing out of complex and unknown factois, and are thus 

 strictly pertinent to the case; while the non-geologic estimates 

 are based on ideal conditions immeasurably simpler than those 

 actually attending a planet, and thus, interesting and instructive 

 as they are in the abstract way, have very little to do with the 

 concrete case. 



It is significant that the discussion of geologic process by stu- 

 dents who are not geologists is commonly trammeled in two 

 diametrically opposite ways. The student of the " exact " sciences 

 is seldom willing to grant so high a degree of mobility in the ter- 

 restrial crust as is required by the geologist to explain current 

 co.ntinent movements, and is given to rejecting or ignoring the 

 evidence of such movements; while, on the other hand, he is the 

 first to reject as excessive the time-estimates of the geologist based 

 in part on, and in complete harmony with, these observed move- 

 ments. This mental habit, growing out of the methods and 

 postulates employed in certain lines of study, is constantly to be 

 borne in mind in weighing non-geologic opinion concerning the 

 rate of geologic process, just as the opposite tendency on the part 

 of geologic study is to be guarded against. 



THE STANDARD COLOR SCHEME. 



BT J. H. PILLSBURT, NORTHAMPTON, MASS. 



In Science for Feb. 26, 1893, I gave a brief account of a color 

 scheme, first proposed by myself in ltj80, and set forth in more 

 elaborate form in a paper read before a meeting of the Society of 

 American Naturalists held in Boston, Dec. 31, 1890. During the 

 present year, through the courtesy of the Department of Physics 

 of Wesleyan University, of whose laboratory and apparatus I was 

 allowed free use, the standards previously selected by the con- 

 sensus of a number of color experts have been located by wave- 

 lengths and, as far as possible, also by the prominent absorption 

 lines of the solar spectrum. Since there are vibrations of an in- 

 finite variety of wave-length, any number of standards might be 

 selected, but it is not, of course, desirable to select a larger num- 

 ber than the eye can readily distinguish. Six colors are clearly 

 recognized by every normal eye in the solar spectrum, and this 

 number has been chosen for the scheme of standard colors, as 

 being both convenient and practical. These colors are red, orange, 

 yellow, green, blue, and violet. For the area of the solar spec- 



