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amount of sediment which subsides. If we wish to be more 

 accurate, we can weigh out a given quantity of the marl, then 

 pursue the above plan, and decant the clear water from the clay; 

 and after thoroughly drying the clay, weigh it to ascertain its 

 amount. Having got away most of the clay, we should spread 

 out the granular matter upon a sheet of paper and dry it, when 

 there will be no farther difficulty in distinguishing, by their colour 

 and lustre, the foreign impurities from the grains of true marl; 

 and also of estimating the relative abundance of each. When 

 the marl to be examined contains much clay, I would recom- 

 mend the experiments to be made upon a regularly weighed 

 quantity, weighing both the clayey and the granular portions. A 

 delicate apothecary's balance will commonly be found accurate 

 enough. Another more expeditious, though less accurate method, 

 is merely to dry the marl, spread it extremely thin upon a sheet 

 of white paper, and then hold it near a window or in the light, to 

 examine it carefully by the magnifier. The flinty sand, though 

 stained with clay, may then be clearly discerned in consequence 

 of its transparency; whereas when we inspect a solid lump, all the 

 panicles upon the surface are nearly alike dark. 



A useful suggestion is, to place a portion of the marl upon a 

 hot shovel or on the top of a stove, when all the granules will 

 change from their ordinary green tint to a light red or brick co- 

 lour, while the other materials of the mass sustain little alteration. 

 This will often render obvious to the naked eye the proportion of 

 the green grains. 



When there is a yellowish or whitish incrustation upon the 

 marl after the moist surface has remained for some time exposed 

 to the weather, it is indicative of the existence of a portion of 

 either copperas or sulphate of alumina, the hurtful nature of which 

 has already been explained. 



An astringent inky taste will very often detect the presence of 

 these noxious substances at times when no such efflorescence 

 shows itself. If the quantity be too small to betray them dis- 

 tinctly to the palate, and we are still in doubt as to their presence, 

 other more rigorous tests are within our reach ; and as these 

 astringent matters are so unquestionably pernicious in their action, 

 it is of importance to have the means of determining in what pro- 

 portion they abound in diflTerent marls. This can be effected with 



