December 22, 1892] 



NATURE 



175 



On washing and riddling this clayey matter, well waterworn and 

 smooth gravel was separated from it up to about the size of a 

 bean, and on many of these minute pebbles, with the aid of a 

 microscope of low power, beautifully-developed striae are to be 

 seen, sometimes on more than one face. This latter fact appears 

 to have been unnoticed before. The conclusion I have come to 

 is that Ramsay had reasonable grounds for his belief in this 

 being a Permian glacial deposit, and think that if he had given 

 more details in his otherwise able paper, geologists would 

 probably have followed him more freely. 



T. Mellard Reade. 

 Park Corner, Blundellsands, December 5. 



The Earth's Age. 



As Dr. A. R. Wallace's " Island Life" may be regarded as 

 one of the best authorities on its subject, it appears desirable 

 that any errors in it should be pointed out, lest any of its numer- 

 ous readers should be misled. 



In Chapter X. (2nd edition, 1892) is an estimate of the earth's 

 age based on the following data : — Land area of globe, 

 57,000,000 square miles, coast line, 100,000 miles, width of 

 shore deposits, 30 miles, hence area of shore-deposits, 

 3,000,000 square miles, hence rate of deposition 19 times as 

 fast as that of mean rate of denudation, which latter is taken to 

 be I foot in 3000 years. 



Thickness of stratified rocks 177,200 feet, hence time required 

 for deposit 28,000,000 years. This last result is taken to be 

 approximately the earth's age. 



It appears to me that Dr. Wallace's data warrant no such 

 conclusion, for, in the 28,000,000 years in question, all that 

 would have been deposited would be a thickness of 177, 200 feet 

 of rock, over an area of only 3,000,000 square miles, whereas, 

 what has to be accounted for is an area of 57,000,000 square 

 miles (neglecting igneous rocks and sedimentary deposits be- \ 

 neath existing seas) of the same thickness. Therefore, so far 

 from Dr. Wallace's data leading to 28,000,000 years as the 

 earth's age, they actually lead to a result 19 times as great, viz. 

 532,000,000 years. 



Sir A. Geikie's estimate is (Nature, vol. xlvi., p. 322), 100 

 to 680 million years. Personally, I think, the method of taking 

 maximum thicknesses of deposits unsatisfactory, for it assumes 

 that every formation was deposited, with its maximum thick- 

 ness, over the whole land area of the globe. The absurdity of 

 this supposition is obvious. The only defence of it is that it is 

 held to make an ample allowance (of unknown amount) for re- 

 peated denudation. It would, perhaps, be better to ascertain 

 the actual thickness of a great series of successive formations, say 

 in the Colorado Caiion and other regions, and from such data 

 to estimate the total average thickness. This estimate, of course, 

 wouKi allow nothing for repeated denudation, but would enable 

 one to form an idea of the earth's minimum age. 



Bernard Hobson. 



Owens College, Manchester, December 5. 



I AM glad that Mr. Hobson has formulated his difficulty as to 

 the measurement of geological time by the comparative rates of 

 denudation and deposition, because it shows that I cannot have 

 explained my views as clearly as I thought I had done ; yet on 

 again reading over pp. 217-223 of "Island Life," I can hardly 

 understand how he has missed the essential point of the argu- 

 ment. Fortunately, there is no dispute as to the data, only as 

 10 the conclusions to be logically drawn from them. 



Mr. Hobson says that I account for a deposit of 177,200 feet 

 (the supposed thickness of all the stratified rocks) over an area 

 of 3.000,000 square miles (the estimated area over which at 

 any one epoch stratified rocks are being deposited) in 

 28,000,000 years (the deduced estimate of known geological 

 time) ; and then adds: " Whereas, what has to be accounted 

 for is an area of 57,000,000 square miles of the same thickness " 

 (my italics). This seems to me a most amazing misconception; 

 lor it means that every single formation and every stratum or 

 member of each formation, was deposited to the same average 

 thickness over the whole land surface of the globe (area 

 57,000,000 square miles) ! And this implies that at every suc- 

 cessive period, from the Laurentian to the Pliocene, the con- 

 ditions of denudation and deposition were totally different from 

 what they are now, since at the present time it is demonstrable 

 that the area of deposition of continental debris is only a frac- 

 tion of the whole continental area. It implies further, that 



during each geological period the whole of the existing land 

 area must have been, either at once or in rapid succession, sunk 

 beneath the sea in order to allow of its being all covered with 

 each successive formation — an amount of repeated upheaval and 

 depression which hardly the most extreme convulsionist of the 

 old school would have po.stulated. I cannot make the matter 

 clearer, and trust that on further consideration Mr. Hobson will 

 admit that his objection is invalid. Alfred R. Wallace. 



The Colours of the Alkali Metals. 



In Nature (vol. xlvii. p. 55) is a communication by Mr. G. 

 S. Newth, entitled " Note on the Colours of the Alkali Metals." 



I write to call attention to my article on " The Colour and 

 Absorption-Spectra of Thin Metallic Films, and of Incandes- 

 cent Vapours of the Metals ; with some observations on Electrical 

 Vitality," published in the American Chemical Journal {yo\. xiv 

 p. 185) and reprinted in the Chemical Nexvs (vol. Ixvi. p. 163), 

 which gives the method employed by Mr. Newth, as well as 

 other methods for obtaining metals in thin films. 



In it attention is also called to the fact that the colour of the 

 film of the metal and the colour of the vapour are widely 

 different. 



Mr. Newth, however, succeeded in getting a film from sodium 

 on glass, while I did not, and his success was probably due to the 

 use of a higher vacuum than I employed. He also obtained a 

 rubidium film. 



In my paper I called attention to the similarity in colour of 

 the film by transmitted light and that of the incandescent vapour 

 which is very striking in many cases. In this respect the film of 

 rubidium as obtained by Mr. Newth follows the rule fairly well ; 

 but the film which he got from sodium is exceptional, as accord- 

 ing to the analogy furnished by other metals it should be yellow. 

 The presence of potassium, however, might cause the green 

 colour which he observed, by the combination of yellow and 

 purple. Wm. L. Dudley. 



Vanderbilt University School of Chemistry, 

 Nashville, Tenn. , December 2. 



Osmotic Pressure. 



In an article on osmotic pressure, in N.A.TURE {ante, p. 103), 

 Mr. Rodger very truly remarks that "at present the attitude of 

 the prominent upholders of the new theory [of solutions] is one 

 of indifference as to the exact mechanism of osmotic pressure. 

 The numerical agreement between the measurements on solu- 

 tions and those on ga-es is regarded as ample justification for 

 considering dissolved substances to be in a pseudo-gaseous con- 

 dition." Such an indifference is surely to be regretted from any 

 truly scientific point of view, especially as those explanations 

 which have been given of the mechanism of o.smotic pressure have 

 been based on the supposition that the dissolved substance is in 

 a veritable and not merely /j^«^^-gaseous condition. There are, 

 however, many reasons for supposing that while the dissolved 

 substance may for many purposes be regarded as analogous to 

 a gas, it must in reality be in a very different condition, and that 

 osmotic pressure is not due to the bombardment of the free 

 molecules of the dissolved substance against a diaphragm through 

 which they cannot pass. The impenetrability of the diaphragm 

 to certain molecules can scarcely be attributed to any other 

 cause than that the molecules are too large to pass through the 

 interstices of the former, and it is scarcely conceivable that the 

 molecules of water which do pass through can be much smaller 

 than the molecules of .simple salts, which do not pass through ; 

 still less that they can be smaller than the single atoms into 

 which these salts are said to be dissociated. 



A very simple experiment, which I mentioned some time ago 

 in the Ber. d. deutsch Chem. Gesell. (24, 3639), appears to settle 

 definitely against the view that osmotic pressure is due to im- 

 penetrability to the dissolved substance. A solution of propyl 

 alcohol and water was put into a porous pot and immersed in a 

 vessel of water ; the water passed through the pot to the solu- 

 tion, and this, according to the usual explanation, would show- 

 that the pot was impermeable to the propyl alcohol. The same 

 solution was then immersed in a vessel containing propyl alco- 

 hol, when the alcohol was found to pass through to the solution, 

 from which we should have to draw the diametrically opposite 

 conclusion that the pot is impermeable to the water. The true 

 conclusion obviously is that the pot is impermeable, neither to 

 the alcohol alone, nor to the water alone, but to the solution of 

 these in each other, and that the molecules composing this solu- 



NO. I 208. VOL. 47] 



