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NATURE 



[June 21, 1917 



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 anonytnous corhmunicatioris.] 



The Origin of Flint. 



The interesting letter of Sir E. Ray Lankester in 

 Nature of June 7 induces me to offer a brief account 

 of certain experiments I have been making .on this 

 subject. 



My interest in the origin of flint was aroused by 

 the many fantastically shaped flints like gnarled roots 

 that one comes across when walking over the Downs. 

 These are of many curious shapes, but an interesting 

 pciint is that when complete there is often one spot 

 which looks like the gutter of a mould. 



This suggests that the silica might have trickled 

 through an opening in the chalk held up in colloidal 

 solution by carbonic acid, and then the carbonic acid, 

 combining with the calcium carbonate of the chalk, 

 formed soluble calcium bicarbonate, thus at the same 

 tiine enlarging the cavity and producing the conditions 

 for the deposit of the silica, which is no longer held 

 up by the carbonic acid, and is precipitated by the 

 crystalloidal calcium bicarbonate now gone into solu- 

 tion. This view is, moreover, supported by the tabular 

 flints referred to by Sir E. Ray Lankester. 



If a very dilute solution of sodium silicate be pre- 

 pared by diluting ordinary waterglass with about 200 

 times its volume of water, this may be saturated with 

 carbon dioxide without any precipitation, and the solu- 

 tion can be dialysed, so yielding a colloidal solution of 

 silicic acid in carbonic acid. 



'When a piece of chalk is dipped in this solution 

 nothing happens immediately, but after twenty-four 

 •hours a silicic gej appears on 'the chalk ; in the absence 

 of chalk the silicic acid solution keeps for some weeks 

 before passing from its metastable solution over into 

 a gel. 



, An attempt was then made to simulate natural flint 

 formation by percolating colloidal silicic acid charged 

 with carbonic acid through chalk. - 



The chalk used was obtained' from the South Downs 

 near Jevington. A cubical block was sawn out 

 about 3 in. in the side, a hole was drilled 

 about J in. in width and 2 in. in depth, and then a 

 little chalk was scooped away to leave a shallow pit 

 surrounding the hole. Several times daily this was 

 filled up with the dilute silicic acid solution during a 

 period of more than three months, until there was a 

 solid core of silica in the place of the former hole in 

 the chalk. At first the percolation is rapid, but after 

 dbbut a week becomes slower. The silicic acid jelly 

 first formed is very porous, and takes up water readily. 

 Even when a silicic jelly has hardened until it is as 

 hard as and more brittle than glass, it will go on taking 

 up a remarkable amount of dilute silicic acid into its 

 pores, and giving ofT air-bubbles with a singing noise. 

 At the end of the experiment the bulk of the chalk 

 was sawn away, and the part containing the deposited 

 silica placed in dilute hydrochloric acid until nearly 

 all the chalk had been removed, leaving just a thin 

 layer at places to show contrast in colour; there 're- 

 mained a cast of the tube and pit at the top resemblinsf 

 in shape a small toadstool with a concave depressed 

 top. 



This was insoluble in' acid, dark brown in colour, 

 and semi-translucent. It looked like a flint, especial^' 

 when wet, when it showed ut> dark brown and semi- 

 transoarent against the small amount of chalk that 

 had been left. The onlv thing lacking was the ex- 

 NO. 2486, VOL. 99] 



treme hardness of flint, for although harder than the 

 chalk and glassy, it crushed under pressure more 

 readily than flint. It still absorbed water, as do 

 flints, and if there were many years of time to spare, 

 it seemed as if it mi"-ht be possible by such a process 

 to arrive at tr;ue flinty hardness. 



The process was tedious, and for some reason which 

 I cannot at present understand did not always suc- 

 ceed ; an attempt to feed the growth with a wick of 

 cotton threads failed because of an interesting silicifi. 

 cation on the fibres which stopped the flow. 



Benjamin Moore. 



Sir E. Ray Lankester's difficulties as to the origin 

 of flint (Nature, vol. xcix., p. 283) would be largelv 

 removed if it were more generally recognised that the 

 vast majority of flints in all formations, excluding die 

 occasional examples deposited along fissures, are. 

 chemical replacements of the limestones in which they 

 occur. Microscopic observation of thin sections has, 

 of course, furnished the most powerful confirmation of 

 this view. The difficulties as to the cause of such 

 replacement are similar in the case of all "concre- 

 tions " where the original . rock-substance has been 

 removed and new material has been substituted. We 

 now know that even iron pyrites may thus replace sili- 

 cates or quartz, and that massive crystalline ores need 

 not represent the infilling of cavities. 



May I refer to some views which would dissociate 

 flints from any special abundance of siliceous sponges 

 along the horizons at which they occur ("The Rhyth- 

 mic Deposition of Flint," Geological Magazine, 1917, 

 p. 64)? The traces of sponges found in flint seem 

 due to the fact that the deposition of the flint has 

 preserved them, while they have been dissolved away 

 from other zones. The paper above referred to mdy 

 be regarded as a supplement to the general discussion 

 of work on flints in my " Rocks and their Origins " 

 (1Q12), pp. 38-42. Grenville^A J. Cole. 



June 18. 



With reference to Sir E. Rav Lankester's in- 

 teresting notes on "The Origin of" Flint" in Nature 

 of June_ 7, it is worthy of remark that the structure df 

 black flints, referred to as consisting of minute crystals 

 of silica embedded in colloid silica, may indicate tWe 

 formation of such flints from the gradual crystaillisa- 

 tion of silicic acid gels. Many cases of the produc- 

 tion of micro-crystals in artificial inorganic and organic 

 gels are known; indeed, these usually break up even- 

 tually with the development of such micro- or macro- 

 crvstals. The very slow crvstallisation of gelatinous 

 silica appears to be due largely to its small diff'usion 

 constant and insolubility. S. C. Bradford. 



The Science Museum'. South Kensington, 

 London, S.W., June 14. 



Electric Discharge from Scythe. 



On the afternoon of June 4 I was mowing a heavy ' 

 crop of grass with the scvthe when I noticed a sharp 

 crack occurring during the cutting strokes. The noist 

 did not occur at every stroke, but was sometimes heard 

 three times during a stroke. The noise exactly -re- 

 sembled a high-tension discharge, and I can think of 

 no explanation other than that the blade became ; 

 charged, due to the friction on the very dry grass. < 

 It would be interesting to repeat the experiment in the^ 

 dark, but I fear the grass would not be sufficiently 

 dry. I_ may add that I am quite satisfied that the' 

 noise did not arise from the snapping of dry stems dr| 

 from the scythe hitting stones, etc. ' 



I should be interested to hear if anv of your readers, 

 have had a similar experience. J. R. Pannell. 



Twickenham, Middlesex. 



