496 



NATURE 



[September 24, 1903 



molecular size and constitution of the heavy carbo- 

 hydrates, like starch and glycogen, and the family of 

 <lextrins intermediate between these and the sugars, 

 we have at present little more than guesses to go upon. 

 To give, as the present author does, long lists of re- 

 actions with iodine and other reagents, and on the 

 strength of differences in these to describe as separate 

 substances amylose, amylosan, amylodextrin, and 

 other forms of dextrin, and to add to the list amylo- 

 porphyrin and amylorubin, does not really advance 

 matters much. Biitschli apologises at the start for his 

 lack of chemical knowledge, and in the end admits 

 that several of his preparations are mere mixtures ; 

 we therefore fear that, from the chemists, his work 

 will meet with but scant courtesy. He has neverthe- 

 less succeeded in producing a very readable little 

 brochure, and if his main contention is accepted, his 

 labours will not have been useless. 



Lessons on Country Life. By H. B. M. Buchanan 

 and R. R. C. Gregory. Pp. xi + 330. (London: 

 ' Macmillan and Co., Ltd., 1903.) Price 3s. 6d. 



One of the authors of the above book, Mr. H. B. M. 

 Buchanan, produced a little time ago two small 

 "Country Readers," most excellent books for the 

 children of a rural elementary school, in which our 

 common domestic animals were discussed from a full 

 knowledge in an easy, pleasant style. We are sorry 

 we cannot give the same praise to the " Lessons " 

 before us ; the educational value of the former book 

 has disappeared, and the authors have allowed a 

 craving for completeness to swamp -their judgment, 

 so that the result is a miniature and scrappy encyclo- 

 paedia instead of a book. 



Country life is a vast subject, so vast that no child 

 •can learn during his school life even a fraction of 

 the information it may be desirable he should possess 

 in his after life ; the teacher, then, must abandon the 

 attempt to impart information, but devote his energies 

 to instilling into his pupils the right way of looking 

 at things, the method which they can employ them- 

 selves when going about the world. The method 

 ■consists in a trainmg in observation and experiment. 

 Here instead we have first a sout of abbreviated text- 

 book on live stock, hints on breeding and feeding, 

 twelve breeds of cattle described at lengths varying 

 from a page down to two lines, horses, sheep and pigs 

 to correspond, analyses of milk, rules for making 

 butter and cheddar cheese; with such a programme 

 what chance is there of observation or experiment for 

 school children ? 



The latter portion of the book deals with common 

 birds and mammals in a much better spirit ; strike out 

 the unnecessary Latin names for orders, families and 

 species, and it forms a fair reading book. The last 

 section, on insects, is again spoiled by a wholly un- 

 necessary passion for classification ; classification is 

 only grammar, and the parts of rvnTa are just as 

 good in this way as " Coleoptera, Euplexoptera, 

 Orthoptera, Thysanoptera, &c. " We know by sad ex- 

 perience how easy the schoolmaster finds it to write 

 these things on the blackboard and make his class copy 

 them with due attention to neatness and spelling; 

 •observation and experiment require both labour and 

 thought. We grieve to speak unkindly of Mr. 

 Buchanan, who has done such excellent work before ; 

 there are good things in the book, e.g. the section on 

 poultry and the illustrations, but, like the curate's egg, 

 it is only good " In parts." If the new teaching on 

 country life is to succeed in our schools, it will be in 

 virtue of the spirit, and not of the information which 

 the teacher imparts to his pupil, and we consider that 

 ^^hls book fatally misses the spirit. 



NO. 1769, VOL. 68] 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



Radio-activity and the Age of the Sun. 



In the Appendix E of Thomson and Tait's " Natural 

 Philosophy," Lord Kelvin has computed the energy lost in 

 the concentration of the sun from a condition of infinite 

 dispersion, and argues thence that it seems " on the whole 

 probable that the sun has not illuminated the earth for 

 100,000,000 years, and almost certain that he has not done 

 so for 500,000,000 years. As for the future, we may say, 

 with equal certainty, that inhabitants of the earth cannot 

 continue to enjoy the light and heat essential to their life 

 for many million years longer, unless sources now unknown 

 to us are prepared in the great storehouse of creation." 



The object of the present note is to point out that we 

 have recently learnt the existence of another source of 

 energy, and that the amount of energy available is so great 

 as to render it impossible to say how long the sun's heat 

 has already existed, or how long it will last in the future. 



Ihe lost energy of concentration of the sun, supposed to 

 be a homogeneous sphere of mass M and radius a, is 

 ifiM^/a, where n is the constant of gravitation. On in- 

 troducing numerical values for the symbols in this formula 

 I find the lost energy to be 2-7x10' M calories, where M 

 is expressed in grammes. If we adopt Langley's value of 

 the solar constant this heat suffices to give a supply for 12 

 million years. Lord Kelvin used Pouillet's value for that 

 constant, but if he had been able to use Langley's his 100 

 million would have been reduced to 60 million. The dis- 

 crepancy between my result of 12 million and his of 60 

 million is explained by a conjectural augmentation of the 

 lost energy to allow fgr the concentration of the solar mass 

 towards its central parts. I should have thought the 

 augmentation somewhat too liberal, but for the present 

 argument it is immaterial whether it is so or not. 



Now Prof. Rutherford has recently shown that a gramme 

 of radium is capable of giving forth 10' calories. If, then, 

 the sun were made of such a radio-active material it would 

 be capable of emitting lo" M calories without reference to 

 gravitation. This energy is nearly forty times as much as 

 the gravitational lost energy of the homogeneous sun, and 

 eight times as much as Lord Kelvin's conjecturally con- 

 centrated sun. 



Knowing, as we now do, that an atom of matter is 

 capable of containing an enormous store of energy in 

 itself, I think we have no right to assume that the sun 

 is incapable of liberating atomic energy to a degree at 

 least comparable with that which it would do if made of 

 radium. Accordingly, I see no reason for doubting the 

 possibility of augmenting the estimate of solar heat as 

 derived from the theory of gravitation by some such factor 

 as ten or twenty. 



In an address to Section A of the British Association in 

 1886 I discussed the various estimates which have been 

 made of geological time, and I said, " Although specula- 

 tions as to the future course of science are usually of little 

 avail, yet it seems as likely that meteorology and geology 

 will pass the word of command to cosmical physics as the 

 converse." I think the recent extraordinary discoveries 

 show that this forecast was reasonable. 



It is probable that the bearing of radio-activity on the 

 cosmical time-scale has occurred to others, but I do not 

 happen to have seen any such statement. 



Cambridge, September 20. G. H. Darwin. 



The Principle of Radium. 



Would some of your readers inform me whether the case 

 of the radium phenomena is quite unique? When a small 

 magnet in my drawer has been ready to act on a compass 

 at any time during the last twenty years, and has not 



