6$ Chap. XlT. Appendix, Part 4. No. 1 .

one ounce, exclusive of stones and vegetable matter, containedcoarse-grained sand, partly fragments of white grains.

quartz, partly dark coloured, ... 320

Very fine-grained brown sand, - - 11

-331

Pure clay, technically called altrmine, - - - 42

Qxid of iron, - - - - - 107

480

The great proportion of this oxid appears to be the cause of theexcessive porosity of this soil, which prevented it from giving adue hold to the roots of plants ; since, without it, the other ingre-dients Would surely have lain compact. It. is this probably whichLord Dundonald calls Earth of Iron , and notices as an ingredi-ent in the soil. Wherever it is too prevalent, it is certainly unfa-vourable to fertility : for all soils much coloured with it are of asterile quality; and where it greatly abounds, no plant will thrive.It is doubtful if its injurious effects proceed from its fine particlesbeing conveyed with the ascending sap into the bodies of plants,or by their accumulation around the fibres of the roots block-ing up the mouths by which the ascent of vegetable food is ob-structed. It is likely, in part at least, owing to the latter cause:for when a plant growing in those ochrey soils is taken up, thefibrous roots appear all covered with this substance.

Spots, however, occur of a dull whitish soil, in which few tra-ces of these oxids are discoverable, which are of a faulty con-struction and sterile quality: from whence it seems probable,that though the excess of the orange oxid is hostile to fertility,the absence of all tile oxids would be equally, or more so. Butit has been long ago suspected that those bodies, called primitiveearths, might be oxids of metals; and it has been proved thatsome of them were so. It was reserved, however, to a celebratedphilosopher of the present times, * whose penetrating genius,persevering industry, and happy discoveries, have rendered hisname illustrious, more fully to elucidate this point. He, havingexamined some of the earths existing in the soil, has found themto be oxids of peculiar metals ; and from thence concludes thatall the earths found on the surface may probably be of metallicorigin. Hence vital air is the agent by which the soilis formed,and is a considerable ingredient in it. Other bodies, which ge-nerally exist iii an aeriform state, appear also to be combined orkept condensed in the soil: since several philosophers have ex-tracted a large volume of air of different characters from a smallbulk of these earths. Much common air likewise, forced into theinterstices of cultivated land, and kept condensed there, by thepressure of the atmosphere, must lodge in a concentrated state i*' the soil.

Dr Davy.