27 



by no means so effectual in the 25reparation of beautiful and 

 instructive slides as the methods which I have attempted to des- 

 cribe. 



Captain McDalcin exhibited some foliated crystals of heavy 

 spar or sulphato of Barj-ta, from the interior of a septaria, 

 found in tlic London clay of the Isle of Sheppy. This mineral, 

 although not unknown in the fonuation, is not mentioned either 

 by Lyell or PliUlips, or by Farady in his analysis. The nodular 

 bodies called soptariro are so well known on the coast of Kent 

 from Whitstable to beyond Heme Bay, and especially on the 

 coast of Sheppey, that they need little comment. Off the coast 

 near Harwich they have been dredged up. for some time for the 

 pm-posoof making Roman cement. Their most striking cha- 

 racteristics arc the septa, or divisions of carbonate of lime, 

 which give them a rough cellular structure, becoming very appa- 

 rent when they are sa-^-n into slabs and polished, for the tops of 

 ornamental tables. They seem to be seggregations of carbonate 

 of lime from the surrounding beds, very frequently around some 

 organic remains, such as a shell, coral, teredo, bored wood, &c., 

 bearing some analogy to the flints in the chalk and hornstone in 

 other rocks, for as the flints or silica have separated out from 

 the surrounding chalk into nodules containing sponges, echini, 

 &c.,_in like manner have the septarias been formed by carbonate 

 of lime separating out from the surrounding silica and alumina 

 constituting the principal part of clay. No mention is made by 

 Farady in his analysis of London clay and septaria> of baryta 

 or its sidphate, which is one of the most insoluble of minerals, 

 being in most text books of chemistry called insoluble, but some 

 mineral waters contain as much as one part in seven hundred 

 thousand. Such a mineral stream flowing into the sea would be 

 so diluted that the molecules of sulphate of baryta would become 

 so far apart that we can but wonder at the strange property of 

 matter which chemists call aflinity that caused them to seek out 

 each other from among a multitude of particles of a different 

 nature, reminding one of tluit higher power of selection which 

 in the animal kingdom takes the form of instinct. From a 

 rough analysis of these crystals I find tliem to be pure sidphate 

 of baryta, the silica, alumina, and oxide of iron, arising from 

 particles of clay entangled among the crystals 



August 1th, 1878. 



Mr. Fullngar exhibited a specimen of the largo saw fly (Ten- 

 thredo), on which a few remarks wore made, explaining some- 

 thing of the wonderful instrmnent from which it derives its 

 popidar name of saw fly. The insect was caught by Mr. Bate- 

 man, St. Georgc's-strcet, in his studio. It can hardly be called 



