May 19, 1904] 



NA TORE 



55 



and I think the poundal, as units of energy " looks as if I 

 confuse force and energy, the context shows that I object 

 to non-metric units as unscientific, and therefore do not care 

 which unit bears the name poundal. The statement that I 

 want to have Claus instead of Rank for the British unit of 

 entropy is wrong. The claus is the unit of entropy in the 

 practical metric system where the joule is the unit of energy. 



The rank is a name proposed by Prof. Perry for jiH/O, 



and as this is not entropy in any real change, I cannot 

 adopt it as a unit of entropy. As to dx> ' will deal with 

 that elsewhere ; it is a side issue. The statement that I 

 talk of " the entropy of a quantity of heat " is wrong. 

 Prof. Perry holds that entropy is a factor of heat. I dis- 

 sent, and agree with Prof. Planck that entropy is not a 

 factor of energy. So far from talking of the entropy of 

 a quantity of heat, I have explained very fully how and 

 why entropy is in no sense a factor of heat. 



I would not write were a review in Nature not particularly 

 important, and I trust you will, in fairness to my publishers 

 and myself, allow this letter to appear. 



41 Palace Court, W., May 1. James Swinburne. 



My sole object in the controversy to which Mr. Swinburne 

 refers was to show that, like most of the other writers of 

 whom he complained, I have never either made or 

 championed the mistakes he speaks of at the beginning of 

 this letter. As to my notice of his book, I cannot admit 

 that I have misrepresented him except as to the claus. I 

 made a mistake in saying that his claus is what is some- 

 times called a rank. .\s he now says that the momentum 

 of which he spoke was a scalar momentum, I submit that 

 I was quite fair in: my comments. I cannot admit that his 

 Bx diagram is a side issue. John Perry. 



Origin of Plants Common to Europe and America. 

 That there is a resemblance between the floras of Canada 

 and northern Europe, and again between the floras of 

 Canada and of eastern Siberia and Japan, is well known. 

 Including the horsetails and ferns with the flowering plants, 

 probably about 575 species are identical in Canada and 

 Europe, and again about 330 in Canada and Japan or the 

 River Amur country. A large number of these are common 

 to the three continents. The hypothesis generally accepted 

 has been that, in some comparatively recent epochs, there 

 has been a connection between Europe and America which 

 facilitated the intermingling of the plant life of the two 

 continents. The late Prof. Asa Gray suggested the prob- 

 ability that the migration of European plants had taken 

 place across Asia to America. Lesquereux, from his studies 

 of the flora of the Dakota group, on the other hand, main- 

 tained that the North American flora is not now, nor has it 

 been in past geological ages, the result of migration, but 

 that it is indigenous. It has long been known that species 

 now extinct occurring in the Miocene of Europe had 

 appeared in America at an earlier period. Lester Ward 

 enumerates eleven species — all now extinct — as common to 

 the Laramie group in the United States and the Eocene of 

 Europe, and shows further that at least two living species 

 now found in both Japan and America date their origin in 

 America as far back as the Eocene. Twenty years ago my 

 own studies in the distribution of Canadian plants also con- 

 vinced me that whilst facilities had existed for migration 

 in both an easterly and a westerly direction, Canada was 

 the point of origin of many of the species now identical in 

 Europe and America. This conviction has been heightened 

 by further knowledge of the range in Canada of these 

 identical species and by further discoveries during recent 

 years of plants in the Pleistocene clays of Canada. Of 

 seventy fossil species in these Pleistocene clays at Toronto, 

 Ottawa and elsewhere, twenty occur at the present day in 

 both Europe and Canada, fourteen are similarly Asiatic 

 and Canadian, whilst eleven are common to the three con- 

 tinents. This if it does not necessarily indicate that in 

 Pleistocene times the intermingling of these floras had 

 already been effected, at least shows that in this period 

 these identical species were present in Canada, and had 



NO. 1803, VOL- 70] 



here their place of origin if there is nothing to indicate 

 their presence at as early a period in Europe or Asia. In 

 its vast areas of exposed Laurentian and Huronian form- 

 ations, Canada has an old look about it, and must have 

 furnished a home through long past ages for the growth 

 and diffusion of northern temperate plant life, when other 

 sections of the globe have from time to time been under 

 water. 



The peculiarities of the present range over Canada of 

 many of these identical species also afford suggestions. 

 Whilst many of them are distributed somewhat generally 

 over the country, and many are high northern or Arctic, 

 quite a number do not range west of Lake Superior ; others 

 have not been found west of the Rocky Mountains, whilst 

 some are confined to British Columbia and Alaska. In 

 view of their occurrence also in either Asia or Europe, this 

 circumscribed range of so many species suggests their 

 antiquity, and that the elevation of that lofty barrier, the 

 Rocky Mountains, and the disturbance of the relations of 

 land and water in Manitoba and the North-West Terri- 

 tories in more recent times, has resulted in these plants 

 being confined to their present range where forest con- 

 ditions were more suitable, and has led to the treeless 

 prairies and plains being tenanted by new groups of species 

 specially suited to the new conditions there, when the land 

 rose to its existing level. A. T. Drummond. 



Toronto, April. 



Moisture in the Atmosphere of Mars. 



In vour issue of May 5 I see a note in the astronomical 

 column on Mr. Lowell's theory of the Martian canals. It 

 is perhaps not just to criticise it on so short a summary, but 

 there is a point on which I should like to ask a question. 

 If, as Mr. Lowell says, there is not sufficient ^moisture on 

 the planet to produce vegetation, how does the water return 

 to the poles ready for the next summer? The only way, it 

 seems to me, is by evaporation. His suggestion of artificial 

 waterways to carry the water from the polar caps implies the 

 existence of an atmosphere sufficiently dense to enable 

 intelligent beings to live. That being so, is it not just as 

 plausible that the evaporated water should condense in the 

 form of rain on the general body of the planet as well as at 

 the poles? although, of course, the excessive cold would 

 account for an increased fall at these extremities. 



Bournemouth, May 10. Arthur J. Hawkes. 



Radium and Milk. 



In the souring of milk the amount of lactic acid developed 

 may reach 080 per cent, in three or four days when the 

 milk solidifies. In view of Sir O. Lodge's suggestion 

 (Nature, October i, 1903), I have made experiments com- 

 paring the rate of acidification, in two to three days, with 

 and without the influence of radium rays from a 5 mgrm. 

 radium bromide tube. The differences in five cases did not 

 exceed the limit of experimental error, 001 per cent, of 

 lactic acid, and in a sixth case with the milk solidified the 

 difference only amounted to 005 per cent, of lactic acid. 

 It therefore appears to me that under normal conditions 

 radium rays have little or no effect on the functions of the 

 lactic acid bacillus. William Ackrovd. 



Halifax. 



THE BANTU RACES OF SOUTH AFRICA} 



NOTHING so good as this book dealing with the 

 Negro indigenes of southern Africa has yet ap- 

 peared. Mr. Dudley Kidd's work is therefore entitled 

 to take the first rank on this subject, at any rate as 

 far as the Bantu races of South Africa are concerned. 

 It is a national humiliation to us to reflect that 

 as a Government we have been connected with South 

 Africa for more than a century, that is to say, two- 

 thirds as long as our imperial connection with India 

 has lasted, and yet that by Government endeavour or 



1 "The E=senlial Kafir."_ By Dudley Kidd. Pp. xiii + 436. (London 

 A. and C. Black, 1904.) Price 18^. net. 



