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SCIENTIFIC NEWS. 



[Dec. 21, 1888. 



colloidal silica between particles of silicified white chalk 

 might make it assume a grey tint, and pass from the 

 opaque to the translucent condition. On the other hand, 

 on the outside of flints the action was continually going 

 on. Percolating water was eating out the colloidal silica, 

 which filled up the interspaces between the silicified 

 particles, and then the mass became white, in just the 

 same way as a piece of white blotting paper becomes 

 white and opaque on the water being driven out of it, 

 and on its being held before a fire. Such were the 

 changes which went on in the mass of silicified chalk 

 mud constituting flints, and that so far as had yet been 

 made out was the process of flint-making. Now, he 

 must call their attention to another process that was 

 going on in nature — the destruction of flints. At first 

 sight flints seemed the most indestructible of bodies. 

 The flint instruments he would have to point out in the 

 next lecture seemed to be the most imperishable of 

 objects. Objects made of iron, bronze, gold, and 

 silver even, got lost and perished, but articles 

 chipped out of flint remained apparently unchanged even 

 from very remote periods ; but although, speaking in the 

 historical sense, flints were very indestructible, geologic- 

 ally, when dealing with not thousands but millions of 

 years, flints were extremely perishable, and the reason 

 of that was that they consisted in part of this unstable 

 substance, colloidal silica. Quartz, on the other hand — 

 perfectly crystalline quartz — was almost the most im- 

 perishable of substances. It had no tendency to change 

 its state. It was in its stable ' condition, and it only 

 passed into solution with the greatest difficulty, so that 

 grains of sand which built up the sandstone and quartzite 

 and other rocks, were many of them of inconceivable 

 antiquity. Many little grains of sand could be proved 

 to have been washed out of one formation and gone into 

 another, and to have been washed out of that and gone 

 into another, and the process repeated again and again, 

 perhaps a little being worn from the outside of the sand 

 grain on each occasion, but nevertheless the quartz grain 

 retained its identity. Now flints were continually rubbing 

 against one another, and being worn down, first into sub- 

 angular flints, and then at last into perfectly rounded 

 flints or pebbles, and it might seem that they ought to 

 find among the rocks of the globe large quantities of 

 little fragments broken off the flints, and when, as at Black- 

 heath and many other places about London, they found 

 beds wholly made up of rounded flint-pebbles, they 

 might well expect the sand in which they were embedded 

 to be made up of flint-particles. But they would find it 

 was not sand composed of flint, but sand composed of 

 little bits of quartz. He would see if he could make 

 them understand why that was. First he must call their 

 attention to the fact that if they wanted to dissolve a 

 thing quickly they must reduce it to powder. The 

 reason of that was very obvious. The action of 

 the acid could only take place in a solid substance 

 at the surface where the two met together. The 

 consequence was when they had a mass with an 

 increased surface, the action between the acid and 

 the substance took place immediately, and solution was 

 very rapidly effected. That was the way in which they 

 always proceeded in chemical laboratories ; if they 

 wanted to proceed quickly they pounded the mass up. 

 They reduced it to powder so as to increase the surface. 

 Nature did not chose to reduce flints to powder, but did 

 the thing in a slightly different way. They would 

 remember what he said the other day about the 



conchoidal fracture of flint, and they would recol- 

 lect those beautiful specimens which he put be- 

 fore them abstracted from a church which he did 

 not chose to mention, the surfaces exhibiting the taps 

 that fell from the old mason's hammer ; the hand was 

 dead long ago, but the force of the blow, and the weight 

 of each particular blow, could be calculated to a nicety 

 from the way the cracks had been produced in these 

 flints. They were like the negative of a photograph, 

 quite invisiblewhenfirststruck.buttheyhad beenrevealed 

 by the weathering process going on in nature, and told 

 their tale. Nature was continually doing things of that 

 kind in ordinary flints. Here were some flints taken 

 up on the shore during his summer vacation. It 

 was one of the sorrows of geologists that they could 

 never let their trade alone ; it followed them every- 

 where — even to their holiday excursions to the sea- 

 side — and these flints, when the surfaces were ex- 

 amined, seemed to be covered with most beautiful curved 

 cracks. If the flint was of fine grain, the fracture would 

 be a fine one ; if of course grain, a course one. If they 

 picked up flints on the shore, they would find that a 

 wonderful change had gone on — that along the line of 

 cracks there had been a process of change. White flint 

 had been developed along both sides of the crack, and 

 the reason of that was that solvent action had penetrated 

 along those cracks, eaten out the soluble silica on 

 each side, and rendered the flint on both sides white and 

 opaque. If they made a microscopic section of the flint, 

 they could see how that process was going on. The white 

 flint became opaque, the black clear and translucent, 

 and they would see how this whitening of flint by solu- 

 tion of colloid silica between the particles was going on. 

 In that way the whole mass of the flint was gradually 

 broken up. The crystalline parts came off in the mud, 

 and in time the whole flint was destroyed. If they took 

 any specimens of those instruments which had been 

 fashioned out of flint, they saw the beginning of the 

 change. The original black flint was covered with a 

 skin of a light colour. The process went on much more 

 rapidly wherever the flints were rattling against one 

 another on the sea shore. Wherever that process was 

 going on they knew that nature was doing her work of 

 making fresh fractures, producing new surfaces by which 

 the solvent action of the water could operate on the 

 flint. He had brought that fact before them, because it 

 was a very characteristic way in which nature did her 

 work. 



CORRESPONDENCE. 



The Editor does not hold himself responsible for opinions expressed 

 by his correspondents, nor can hetake notice of anonymous com- 

 munications. All letters must be accompanied by the name and 

 address of the writer, not necessarily for publication, but as a 

 guarantee of good faith. 



THE SPARROW AGAIN. 



Having noticed in your valuable paper a short controversy 

 on the sparrow, I beg to add a few remarks concerning its 

 behaviour to other birds. It has latterly been observed to 

 seize upon the nests of the bank-swallow or sand-martin just 

 as it does upon that of the common martin. Whether this 

 is a novel habit or an old one, which has previously escaped 

 observation, I am unable to say. 



Sparrows habitually monopolise crumbs thrown out in the 

 winter for the relief of starving birds. Being always in the 

 majority, they attack and drive away the robin. 



Sandy. 



