155 



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 be 

 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 affect 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 porphyry 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. 



An Analysis of the magnetical Pyrites ; with Remarks on some of the 

 other Sulphurets of Iron. By Charles Hatchett, Esq. F.R.S. Read 

 May 17, 1804. [Phil. Trans. 1804, p. 315.] 



The substance which is the subject of this paper was, till lately, 

 found only in some parts of Norway and Germany ; but it now ap- 

 pears, 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 chiefly 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 in- 

 stituted 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- 



