March 24, 1892] 



NATURE 



487 



the differences above noted ' are of specific value." - I may add 

 that I have recently seen the specimens of Tudor limestone ex- 

 hibited in the Peter Redpath Museum, and my estimate of their 

 value coincides exactly with that of Sir J. \V. Dawson in 1888. 

 As Sir J. W. Dawson most kindly promises his assistance to 

 other workers, perhaps he would submit to some of them any 

 specimens from Tudor which he regards as more conclusive than 

 his original type. 



It would seem rather unnecessary for anyone to trouble to 

 infer from my paper that Sir J. W. Dawson has "regarded the 

 Madoc and Tudor specimens as ' Lower I.aurenlian,' " when 

 that is so directly stated by Sir J. W. Dawson in his description 

 of his figure ; viz. "Specimen oi Lozoon canadense embedded in 

 a dark-coloured homogeneous limestone occurring in the Lower 

 I aurentian series at Tudor, Canada West" {Quart. Journ. Geol. 

 Soc, vol. xxiii. p. 265). J. W. GREGORY. 



British Museum (Natural History), S.W. 



The Theory of Solutions. 



In his last letter (Naturk, March 3, p. 415) Prof. Oslwald 

 repeats his opinion that a theory is " a complex of laws, grouped 

 around and derived from a main law," and infers from my letter 

 that what I term a theory he would term an hypothesis. 



If this were the whole point at issue, I could meet it in no 

 better way perhaps than by referring Prof. Ostwald to his own 

 works. For example, in his " Outlines of General Chemistry," 

 are to be found not only numerous instances of the use of the 

 word theory in its ordinary and accepted sense {e.g., p. 58) 

 but also cases in which it is employed as synonymous with 

 hypothesis {e.g., p. 187). 



With regard to the definition of s.lutions as mixtures, Prof. 

 Ostwald maintains that even if hydrates are formed in a solution, 

 the solution is finally a mixture of the hydrates and the remain- 

 ing solvent. The real question involved is unaffected by this 

 explanation. There is no doubt whatever that to the majority 

 of readers the definition, without any qualifying clause, that 

 solutions are mixtures leads to one conclusion and no other — 

 namely, that between solvent and dissolved substance there is no 

 interaction of a chemical nature. Prof. Ostwald has in his 

 letters stated that in some cases he considers such interactions 

 occur ; he has also stated that between chemical and physical 

 processes he knows of no distinction. The definition is at 

 variance with both these views, and it seems but fair to conclude 

 that such discordant statements tend in no way to obviate that 

 misconception which Prof. Ostwald so often deplores. 



In defence of the application of van der Waals's equation to 

 solutions, a process questioned by me in my letter. Prof. 

 0^twald states that van der Waals himself has taken up this 

 very question. The method by which van der Waals approaches 

 the subject, curiously enough, furnished thfc main grounds for my 

 objections. The most superficial comparison of the complex 

 formula which van der Waals deduces for a mixture of two sub- 

 stances, with such an application of his simple gas equation to a 

 solution as that given in Prof. Ostwald's book, is ample justifica- 

 tion for my strictures. But apart even from such evidence as to 

 the inadequacy of the application, the form which it is finally 

 made to assume is in itself a proof of its incompleteness. By 

 judicious simplification the application is made to lake the shape 

 of a linear equation in which " pressure forces due to the inter- 

 actions of molecules are absent." That is to say, the cohesion 

 of solvent and dissolved substance, and the mutual reactions of 

 both, are alike ignored. Further comment on such a method of 

 accounting for the phenomena of solutions appears to me to be 

 superfluous. J. W. Rodger. 



London, March 7. 



The Limpet's Strength. 



The limpet experiments of youresteerned correspondent, Mr. 

 Percy .\ubin, as reported in Nature of March 17 (p. 464) 

 would have been still more intf-resting and instructive had he 

 weighed the animals deprived of their shdlls. 



On April 10, 1890, I publi-shed my experiments showing that 

 the shell-less limpet pulls 1984 times in the air its own weight, 

 and about double when immersed in water. 



' i.e. between the specimen from Tudor and those from other localities. 

 - "Specimens of Eozoon canaiiense," Mem. Peter Redpath Mus., 1888, 



NO. II 69. VOL. 45] 



Fasting fleas on an average pull 1493 times their own dead 

 weight. 



Other experimenters give the pulling power of the shell- 

 deprived Venus verrucosa of the Mediterranean, a cockle-like 

 creature, at 2071 times the weight of its own body. 



The force required to open an oyster appears to be I3I9'5 

 times the weight of the shell-less oyster. 



J. Lawrence-Hamilton, M.R.C.S. 



30 Sussex Square, Brighton, March 19. 



Technical Education for Novelists, 

 j Amidst the many schemes for technical education, could you 

 j not put in a plea for the "author of the popular novel"? 

 Perhaps the need will best appear from these illustrations taken 

 I from the first 100 pages of a recently published and loudly 

 I heralded work. 



(1) Scene — Kinder Scout, Derbyshire. Date — "after the 

 ! snows and rains of early April," 1864. Time — after Z p.m. "It 

 ' was a clear, frosty night, promising 2, full moon." 



(2) Same place, Easter Eve, 1864. "The wooded sides of 

 the great moor were fading into dimness, and to the east a 

 young xnooxi was rising." W. 



I March 12. 



THE ORIGIN OF THE YEAR. 

 L 



, T T would seem that in the dawn of civilization it was 



! ■*^ not at all a matter of course that the sun should be 

 taken as the measurer of time, as it is now with us ; and in 

 this connection it is worth while to note how very various 

 the treatment of this subject was among the early peoples. 

 Thus, for instance, it was different in Egypt from what it 

 was in Chalda;a and Babylonia, and later among the Jews. 

 In the Egyptian inscriptions we find references to the 

 moon, but they prove that she occupied quite a subordin- 

 ate position to the sun ; while in Chaldaea it would seem 

 that the moon was the chief thing worshipped, and it 

 was thus naturally the chief means used for measuring 

 time, and, so far as months were concerned, this, of course, 

 was quite right. In Chaldaea, too, where much desert 

 travel had to be undertaken at night, the movement of 

 the moon would be naturally watched with great care. 

 An interesting point connected with this is that, 



; among these ancient peoples, the celestial bodies which 

 gave them the unit period of time by which they reckoned 

 were practically looked upon in the same category. 

 Thus, for instance, in Egypt the sun being used, the 

 unit of time was a year ; but in Chaldaea the unit of 

 time was a month, for the reason that the standard of 

 time was the moon. Hence, when periods of time were in 

 question it was quite easy for one nation to conceive that 

 the period of time used in another was a year when 

 really it was a month, and vice versa. It has been sug- 

 gested that the years of Methuselah and other persons 

 who are stated to have lived a considerable number of 

 years were not solar years but lunar years — that is, pro- 

 perly, lunar months. This is reasonable, since if we 

 divide the numbers by 12 we find that they come out 

 very much the same length as lives are in the present day. 

 There seems little doubt that the country in which the 

 sun was first definitely accepted as the most accurate 



, measurer of time was Egypt. 



"The Egyptians," says Ranke in the first chapter of 

 his " Universal History," which is devoted to Egypt, 

 " have determined the motion of the sun as seen on earth, 

 and according to this the year was divided, in comparison 

 with Babylon in a scientific and practically useful way, so 

 that Julius Caesar adopted the calendar from the Egyp- 

 tians and introduced it into the Roman Empire ; the 

 other nations followed suit, and since then it has been in 

 general use for seventeen centuries. The calendar may 

 be considered the noblest relic of the most ancient times 

 which has influenced the world." 



