Page:Proceedings of the Royal Society of London Vol 1.djvu/175

Rh fibres of the spheroids, will produce convex articulations in the lower joints of the prisms. If the generating centres are not equidistant, the forms of the pillars will be irregular, and of different number of angles ; and as the compression of the ﬁbres will be greatest on the level of the generating centres, the lower part of the prisms will he most compact.

All the observations hitherto made on the great basaltic masses in nature, seem to conﬁrm this simple theory ; and the author bestows some pains in accounting for the appearances which seem at first sight not to agree perfectly with it. Those who shall peruse the paper will, if we are not much mistaken, be particularly gratified with the variety of information they will meet with in this part of the treatise.

Lastly, the author directs his attention to the many instances of other substances, besides basalt, which aﬁect a columnar form, and which afford convincing proofs that their configuration is not confined to either the aqueous or igneous formation. Such are, certain lavas, columns of porphy-ry found near Dresden, a bed of gypsum at Mont- martre, and other masses of various nature. Sandstone, clay, argil- laceous iron ore, and many other substances, become prismatic by torrefaction; and prisms of starch formed in drying have often been considered as illustrative of basaltic formation. Some of these are probably to be attributed solely to contraction; and it is shown that they do by no means contribute toward any explanation of the pro- cess here in contemplation.

The substance which is the subject of this paper was, till lately, found only in some parts of Norway and Germany; but it now appears, from some specimens in Mr. Greville’s collection, that it is likewise to be met with in considerable quantities near the foot of Snowdon in Caernarvonshire. The character by which this kind of sulphuret is chifly distinguished from the other martial pyrites, is its magnetic property, by which, especially if it have been placed some time between magnetical bars, it will turn a needle completely round, attract and take up abundance of iron ﬁlings, and retain this addition to its original power for a considerable length of time.

After an accurate description of the external characters of this ore, Mr. Hatchett enters into a full account of the processes he instituted in order to discover its nature and component parts. In hopes of discovering the cause of the magnetic property which is peculiar to this species, he has entered into an analysis of the other kinds of martial sulphurets, not only natural, but also artificial, and has also paid particular attention to the experiments of others on this subject, particularly those of Mr. Proust, the learned Professor of Chemistry at Madrid, who has taken considerable pains in ascertain-