268 



SCIENCE-GOSSIP. 



In the "Journal of the Chemical Society " (vol. 

 vi., i868, page 274), Mr. W. C. Roberts, now Mr. W. 

 Chandler Roberts-Austin,C.B.,F.R.S., of H.M. Mint, 

 described and figured plant-like forms found in col- 

 loid silica obtained by Dialysis. He says in that 

 article : " By bringing together 112 grains of silicate 

 of soda, 67-2 grains of dry hydrochloric acid, and 1 

 litre of water, and dialysing for four days, a solu- 

 tion of soluble silica (containing 4-9 per cent, of silicic 

 anhydrite) remains on the parchment paper, the 

 chloride of sodium and excess of hydrochloric acid 

 having diffused away. This solution becomes 

 pectous somewhat rapidly, forming a solid jelly, 

 which may be dried into a lustrous hydrate by two 

 days' exposure to vacuum over sulphuric acid, or 

 by a more protracted evaporation in air. The solid 

 thus obtained is remarkably similar to opal from 

 Zimapan, but contains 21-4 percent, of water. All 

 the specimens of jelly dried in the air contained 



Fractured Flint, with Dendrites. From Mr. W. Gamble's 

 collection. 



dendritic forms, varying in size from 0'2 millimetre 

 to 0-5 mm. At first it seemed probable that they 

 might be indications of the passage of colloid into 

 crystallized silica, but when magnified to 90 linear, 

 they appeared radiating fibres, and when magnified 

 700 times each fibre resolved itself into a series of 

 elongated bead-cells, with clusters of circular cells 

 at intervals." On further examination these forms 

 were found to be allied to certain common blue 

 moulds or mildew. Proceeding, the author says, 

 " It is possible that in natural agates many cellular 

 structures may have been produced as indicated 

 above and subsequently filled in with mineral 

 matter." 



In Science-Gossip, April, 1869, Dr. M. C. Cooke, 

 the editor at the time, wrote a three-column article 

 on " Spots on Paper," with an illustration, to which 



drawing there does not appear to be any reference 

 in the letterpress. Dr. Cooke dealt with two kinds 

 of spots — the dendritic crystals of metallic origin, 

 then not clearly understood, and several species of 

 true fungi — pointing out that there could be no 

 chance of confusing the two even without the aid 

 of a microscope. One species of fungus staining 

 papers, viz., Myxotrichum chartaricm, is described by 

 Mr. A. H. Church, F.C.S.,in "Annals and Magazine 

 of Natural History," January, 1862, with illustra- 

 tions, one of them being like a dendrite when drawn 

 natural size, but appearing very different when 

 magnified. 



Mr. Archibald Liversidge, in " Journal of Chemi- 

 cal Society" (vol.x., 1S72, page 646), mentions that 

 previous to his chemical examination in 1872, den- 

 dritic or plumose spots appearing on paper do not 

 appear to have received scientific treatment in view 

 of ascertaining their origin, though botanists and 

 microscopists had frequently examined them with- 

 out arriving at any satisfactory conclusion. Indeed, 

 Agardh and Lyngbye named the spots Conferva den- 

 dritica; and Schumacher placed them amongst 

 fungi under the name Dematium olivaceum. Mr. 

 Liversidge's experiments with the blowpipe, which 

 are set forth at length in his paper now referred to, 

 proved the dendritic spots on paper to be caused 

 by crystals of copper in combination with sulphur. 



Thus it appears that dendrites may have two 

 sources of origin. In some instances, possibly by 

 cryptogamic vegetable patterns being replaced or 

 filled up by the infiltration of some solution con- 

 taining metal oxide, which crystallizes in the cells 

 formed by the vegetation ; for instance, in some 

 Mocha-stones. Most commonly, however, they are 

 caused by a very slight fissure admitting of a thin 

 film of saturated solution containing a metal oxide, 

 which, on drying, leaves the crystals in tree-like 

 forms, after the manner of the beautiful ice patterns 

 on window-panes, made, as the children say, by 

 Jack Frost. 



To return now to the arborescent flints we 

 have figured in illustration of dendrites, which may 

 be readily obtained by any persons who have 

 access to flints. They make an exceptionally 

 pretty collection, and one of some interest. We 

 will, however, quote Mr. Gamble's own words 

 upon the subject : 



" The flints in question are the grey cherty flints, 

 often partly or completely enveloping nodular 

 black flints. These, with green-coated flints, form 

 the flint -bed on top of chalk, and base-bed of the 

 Thanet sands. This layer of flints is embedded in 

 dark brown earth. Over that is from two to three 

 feet of greensand, and probably Thanet sand and 

 glauconite. Above that is the ordinary Thanet 

 sand and brick earth. I give the strata because the 

 two questions I am anxious for answers to are — 

 whether the minerals or oxides producing the den- 



