1^,6 



SCIEiYCE-GOSSIP. 



troublesome to keep going. ^Vhile some such source 

 of current is essential for the induction coil, the work- 

 ing of the Wimshurst machine resolves itself into 

 turning a handle. There is a minimum limit to the 

 size of the plates of a Wimshurst, if good work is to be 

 done ; ihev should not be less than 20 inches diameter. 

 A compact six-plate machine of this size would how- 

 ever be more portable than, say, a 12-inch induction 

 coil, with its necessar}- batteries. 



In the earlier stages of the Avork small size induc- 

 tion coils were more generally used. These have, 

 however, given way to larger ones, in order to obtain 

 greater penetrative and other effects. More will be 

 said upon this point later in these articles. That 

 some useful work can be done with a small equip- 

 ment is nevertheless seen from Figs. 1 and 2, the shot 

 in the rabbit being clearly defined in the head and body. 



The short exposures of photographic plates neces- 

 sary at the present time compared with those required 

 formerly, are due to four difierent causes, (i) The 

 increase in the size of the induction coil used, {2\ 

 improvements in the form of the contact breakers for 

 the same, (3), increased efiiciency of the X-Ray vacuum 

 tubes, and (4) the help given by what are known as 

 intensifying screens. These latter are made of cal- 

 cium tungstate which fluoresces blue and is therefore 

 photographically active. The prepared surface is- 

 placed in contact with the film of the plate, so that 

 the image produced upon the screen assists the action 

 upon the plate. The time of exposure is by this 

 means sometimes reduced to one-fifth of what it would 

 be otherwise. 



[To he contiinted.^ 



MAN.GANESE IN RIVER-GRAVELS. 

 By Martin A. C. Hinton. 



T N a paper which I recently read before the 

 ■'■ Geologi.sts" Association of London ('), mention 

 was made of certain occurrences of manganese in the 

 Pleistocene deposits of the district therein described. 

 During the discussion following the reading of that 

 paper, Mr. Whitaker, F.R.S., alluded to the great 

 interest of such occurrences which he had himself fre- 

 quently observed when noting sections in the drift. 

 He added that he was quite at a loss to explain them. 



Manganese occurs in the gravels in the form of the 

 di-oxide, staining seams with a black coating, and 

 occasionally, on some of the larger stones, one finds 

 well-formed dendrites of this substance. The 

 remarkable point in connection with these stained 

 beds is, that their occurrence is not governed by the 

 porosity of the underlying strata, and hence in many 

 cases they do not owe their origin to filtration. 

 During the past few years I have made a great many 

 notes and observations on these peculiar seams, and 

 the following are the conclusions that may be deduced 

 from them. In the first place, perhaps it ought to 

 be mentioned, that in most cases chemical analysis of 

 the black encrusting material is desirable, because two 

 other substances occur viz., carbonaceous material 

 .and a black sulphide of iron, which may be con- 

 founded with manganese if field characters only are 

 relied upon. 



Gravels are almost invariably stained with a red or 

 Ijrown oxide of iron. Since manganese and iron 

 occur together, the question arises as to which of the 

 two oxides is the older. In nearly every case where 

 stained pebbles occur it will be found on examination 

 that dendrites of manganese overlie and rejjose upon 

 a crust formed of oxide of iron. The small fragments 

 of silica composing the .sand or matrix, when 

 encrusted with manganese will if examined show a 



^ " The Pleistocene Deposits of the Ilford and \\".'tnsteat' 

 District," by Martin A. C. Hinton. with an appendix on the 

 Molhisca, by A. S. Kennard and B. B. \\'oodward. F.L.S., 

 F.G.S. (In the pres.':.) 



similar state of aftairs. This evidence tends tcj prove 

 that iron occurring at the same horizon as manganese 

 is the older of the two minerals. 



There are apparently but two causes to which we 

 can possibl}' refer the origin of the seams of manganese 

 that occur in these beds, viz., (i) Filtration, and (2) 

 Contemporaneous Fluviatile Deposition. 



(l.) Filtration. Suppose water percolates down 

 through the gravel, or other deposits, and imagine that 

 this water carries a certain quantity of manganese in 

 solution as the di-oxide. Assume also, that somewhere 

 in this gravel there is a bed, which, from the character of 

 its constituents or from other conditions, will act as a 

 filter. When the water reaches this horizon, what hap- 

 pens ? If the bed is not absolutely impervious the water 

 will pass through it, but \vhether it does this or no it is 

 forced to give up the soluble manganese which is de- 

 positedupon the surroundingdetritus and thus givesrise 

 toablackseam. If thissuppositious case exists in nature, 

 then wehavean example of wfiat I should term ' ' Primary 

 Filtration.'' Let us assume, in another instance, that 

 we have a bed of gravel stained with di-oxide of man- 

 ganese. How this seam originated does not for a 

 moment concern us. If water is percolating through 

 the beds, and if this \\ ater is in the chemical condition 

 to take up the soluble di-oxide, on reaching the stained 

 seam it will do so, and the result will be that the 

 manganese is carried in solution to a lower level, 

 either wholly or in part. The exact depth it is con- 

 veyed will, of course, depend upon the presence or 

 absence of impervious material. In this suppositious 

 case we have an example of what I should term 

 " Secondarj Filtration." 



Does the field-evidence support the filtration theory 

 as being the cause of these occurrences? It may be 

 said with safety that it does not, and more particularly 

 so with regard to " Primar\- Filtration." To support 

 it, there is one condition that is fundamental, viz., the 

 bed immediatelv underlving the seam must be of such 



