. The Philosophical magazine; a journal of theoretical, experimental and applied physics. on its isolated and still uncon-nected particles, may possibly suffice to produce such an efllect. T may here anticipate that a mere pulverulent aggregate havinga rotatory movement in space must necessarily also acquire a sphe-roidal form dependent upon rotation, exactly like a liquid (accord-ing to Professor Plateaus experiments) not acted on by terres-trial attraction, and consequently in a state of free suspension. A septaria, an object familiar to mineralogists and geologists,may serve to convey an id
. The Philosophical magazine; a journal of theoretical, experimental and applied physics. on its isolated and still uncon-nected particles, may possibly suffice to produce such an efllect. T may here anticipate that a mere pulverulent aggregate havinga rotatory movement in space must necessarily also acquire a sphe-roidal form dependent upon rotation, exactly like a liquid (accord-ing to Professor Plateaus experiments) not acted on by terres-trial attraction, and consequently in a state of free suspension. A septaria, an object familiar to mineralogists and geologists,may serve to convey an idea of the effects of pressure acting fromthe circumference to the centre. Septarise are spheroidal tuberi-form bodies, occasionally slightly compressed in one direction(see fig 3), consisting of an external solid shell or crust of com-pact argillaceous sphaerosiderite, filled up with the same sub-stance, and intersected by numerous and somewhat imperfectveins of calcareous and magnesio-calcareous spar. Fig 3 is anautotype, taken from a specimen in the Imperial Museum of Fig. 3. .11 n. Vienna. The formation of such a septaria may be explained asfollows :—within a stratum of clay, the particles richest in thecarbonate of oxide of iron agglomerate or coalesce: the clay- 446 M. Haidingev on the Original Formation of Aerolites. stratum, and with it the sphserosideritic agglomeration, under-goes pressure, which, if sufficient, leaves in the interior a softerportion, more impregnated with water than the external crustfrom which that element has been squeezed more completely sphjerosiderite is naturally inclined to assume throughoutthe consistence of the external crust, which, like a vault or arch,acts in every direction against further contraction. Contrac-tion ensues, and the fissures produced in consequence are subse-quently filled up with crystalline deposits of substances held insolution by liquids penetrating, or already contained within, theinterstices. At first magnes
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