344 



NATURE 



[June 



191. 



times the amount of heat now given out in one 

 year. Kelvin, after making- this kind of calcula- 

 tion, said : " It seems, therefore, on the whole, 

 most probable that the sun has not illuminated 

 the earth for 100 million years, and almost certain 

 that he has not done so for 500 million years." 



The* possibility that the existence of radium 

 in the sun might increase the calculated age has 

 been carefully considered, and it is found that it 

 will not do so; it has no practical effect on the 

 result if the proportion of radium to other sub- 

 stances is taken to be the same as it is on the 

 earth. 



It may have been the possibility of a much 

 less radiation from the sun in the past 

 that caused Kelvin to be so generous. But, 

 making every allowance of this kind, it 

 is difficult to imagine a greater age than 100 million 

 years ; indeed, it is difficult to imagine so great 

 an age. It seems absolutely necessary to find 

 more energy than mere gravitational energy, and 

 we are very loth to assume that the matter which 

 now forms the sun had once much greater atomic 

 energy than it possesses now. It is curious that 

 the mathematics of a spherical mass of gas, pub- 

 lished in Nature, July 13, 1899, pp. 250 and 252, 

 should lead to a speculation of this very kind ; 

 that is, that the mass of gas could not 

 exist unless there was originally some more atomic 

 energy than we find it to possess in the laboratory. 

 Lord Kelvin thought that this conclusion merely 

 meant that such a body would collapse until its 

 stuff ceased to behave as a perfect gas. In these 

 days when the facts of radio-activity are unsettling 

 our beliefs, and it is necessary to get the sun's heat 

 argument into agreement with the others, there is 

 a temptation to let our thoughts linger on the other 

 speculation, although, indeed, it must be quite 

 absurd. And yet we know that the second law of 

 thermodynamics is being evaded somewhere in the 

 universe. 



Our thanks are due to Mr. Holmes for this very 

 welcome and interesting little book. J. I'. 



POPULAR BOTANY AND GARDENING. 



(1) Trees and Hoiv They Grow. By G. Clarke 

 Xuttall. With 15 Autochromes by H. Essen- 

 high Corke. Pp. xi + 184 + plates. (London: 

 Cassell and Co., Ltd., 1913.) Price 6s. net. 



(2) Wild Flowers as They Grow. Photographed 

 in Colour Direct from Nature by H. Essenhigh 

 Corke. With Descriptive Text by G. Clarke 

 Xuttall. Fifth Series. Pp. viii+200. (London: 

 Cassell and Co., Ltd., 1913.) Price 55. net. 



(3) Garden Flowers as They Grow. Photographed 

 in Colour Direct from Nature by H. Essenhigh- 

 Corke. With Descriptive Text by H. H. 



NO. 2:75. VOL. 91] 



Thomas. Pp. iii+197. (London: Cassell and 

 Co., Ltd., 1913-) Price 55. net. 



(4) Garden Work: A Practical Manual of School 

 Gardening. By William Good. Pp. xvi + 399 



+ plates. (London: Blackie and Son, Ltd.. 

 1913.) Price 35. 6cl. net. 



(5) Dahlias. By George Gordon. Pp. xi+115-f 

 8 coloured plates. (London and Edinburgh : 

 T. C. and E. C. Jack.) Price is. 6d. net. 

 (Present-Day Gardening.) 



IT is somewhat difficult to know just what to 

 say about the majority of the numerous 

 popular works on botany and gardening that are 

 turned out in such rapid succession in these days 

 — in some cases the writers of such books rival 

 even the most popular of popular novelists in their 

 industry, turning out half-a-dozen or more size- 

 able books a year. If one is to judge them critic- 

 ally, one is bound to say that these books are, 

 on the whole, rather poor; if inclined to cynicism, 

 one would certainly say that most of them are 

 totally unnecessary ; but, after all, one cannot but 

 rejoice at the increasing interest in plant-life and 

 gardening of which this flood of good, bad, and 

 indifferent books may be regarded as the out- 

 come and reflection. One may at least admit that 

 compilers of books of this kind are making fairly 

 good use of improved methods of colour and other 

 illustration processes ; that the letterpress, though 

 too often hasty and slipshod, is freer from actual 

 inaccuracies than one might have expected ; and 

 that these books are likely to arouse the reader's 

 interest. So much to the good ; only, since the 

 writing of such books appears to be fatally easy, 

 let us hope that some few readers may resist the 

 temptation to write books themselves. 



(1) This is a readable and interesting account 

 of a number of common trees, with fifteen coloured 

 plates by Mr. Corke, in addition to which the 

 author contributes a large number of remarkably 

 good photographs, including stages in the germin- 

 ation of the seeds and the unfolding of the buds 

 in the majority of the trees dealt with. So much 

 has been done in the letterpress to make the 

 story of these trees attractive that it seems a pity 

 the author did not give, either as introduction or 

 appendix, a general account of the growth of trees 

 and the many interesting biological features (leaf- 

 mosaics, for instance) which they present, and 

 perhaps some account of the ecology of wood- 

 lands as developed in Britain — to mention only one 

 or two of the aspects of tree-life not touched upon. 



(2) Messrs. Nuttall and Corke are steadily work- 

 ing their way through the British flora; in this 

 volume the former describes, and the latter depicts, 

 a fifth batch of twenty-five native wild flowers. 

 The coloured -nl?t-?s are unusually good, even for 



