ROWLEY RAG. 



fachufetts ; the inhabitants, amounting to the number of 

 1682, are moftly farmers; incorporated in 1639, and 

 fituated 4 miles N.W. by W. of Ipfwich. 



Rowley Rag, in Mineralogy, a bafaltic ftone from 

 Rowley, near Dudley, in Staffordlhirc. It is ufed for po- 

 lifhing in fome of the manufactures of Birmingham, and 

 has been particularly recommended for grinding the fpecula 

 of reflecting telefcopcs. On this variety of bafalt, the fine- 

 grained grunitein of Werner, fome molt interelting experi- 

 ments were made by the late Mr. Gregory Watt, to deter- 

 mine the important queltion refpecting the igneous or 

 aqueous origin of bafaltic rocks. Perhaps few experiments 

 in the laft fifty years throw more light on fome of the 

 mvfterious operations of nature in the mineral kingdom, 

 and have a better claim to the profound attention of philofo- 

 phers, or are more deferving of being repeated and diverfified. 

 (See Phil. Tranf. for 1804, p. 279.) Before a fhort account 

 of thefe experiments, it may be proper to give a delcription 

 of the ilone itlelf. Rowley rag is a fpecies of fine-grained 

 bafalt, of a confufed cryftallized texture ; its fracture, in 

 fmall pieces, is uneven ; in the large, conchoidal. Its 

 hardnefs is fuperior to common glafs, but inferior to felfpar ; 

 its tenacity is confiderable ; its action on the magnetic needle 

 is ftrong, but without finis of polarity ; its fpecific gravity 

 is 2.868. The general colour of Rowley rag is a very dark 

 grey, approaching to black : it reflects light from a variety 

 of brilliant points, fome cf which feem to be felfpar, the 

 others hornblende. According to the analyfis of Dr. 

 Withering, 1000 parts contain 475 filex, 325 argillaceous 

 earth, and 20c of the oxyd of iron. The magnetic pro- 

 perty of thele rocks was lirit obferved by Dr. Plott, who 

 fays they turned the needle 6° from its proper direction. 

 The fame power of affecting the magnet has fince been dif- 

 covered in feveral bafaltic mountains, particularly in the 

 Giant's Caufeway in Ireland. This ltone is fimilar to the 

 bafalt of the Clee hills, in Shropshire, and the bafaltic 

 Hones in various parts of Great Britain. In its characters 

 and chemical compolition it nearly reiembles fome of the 

 compact lavas from Ylitna and Teneriffe. It is eaiily fufible 

 into an opaque black glafs, which, however, tranfmits light 

 through very minute fragments. The texture of this glafs 

 is completely vitreous, with a tew air-bubbles. Its fracture 

 is conchoidal and undulated ; the hardnefs is fuperior to 

 felfpar, but inferior to quart/. This glals polleifes fcarcely 

 anv action on the magnetic needle. The fpecific gravity- 

 is 2.749. 



For the purpofe of afcertaining the effect of a high degree 

 of temperature on a confiderable quantity of this ltone, one 

 of the common reverberatory furnaces, ufed in iron foun- 

 deries for the fufion of pig-iron, was (trongly heated by a 

 fire maintained feveral hours. About (even hundred-weight 

 of amorphous Rowley rag was broken in fmall pieces, and 

 depofited gradually on 1 1 il part of the interior of 



the furnace, between the tire and the chimney ; from whence, 

 as it melted, it flowed into the deeper part, in which, in or- 

 dinary operations, the iron is collected. It was obferved 

 by the perfons attending, that it did not require half the 

 quantity of fuel to fufe the bafalt that would have been nc- 

 ceflary to melt an equal weight ot pig-iron. When the 

 whol [ted, it formed a liquid glafs, rather tenacious, 



from which a large ludleful was taken, which, being al- 

 lowed to cool, retained the characters of perfect glafs. Th 

 fire was maintained, though with gradual diminution, fur 

 more than fix hours; after which time, the draught of the 

 chimney was intercepted, the furface of the glai 

 vered with heated land, and the furnace was filled with 

 coals, which were confumed very flowly. It was eight 



days before the mais in the furnace was fufficiently cool to 

 be extracted, and even then it retained confiderable heat. 



The form of the mafs being given by the bottom of the 

 furnace, approached to the fhape of a wedge. It was 

 nearly three feet and a half long, and two feet and a half 

 wide ; about four inches thick at one end, and above eighteen 

 inches at the other. Owing to this inequality of thicknefs, 

 and alfo to the unequal diltribution of heat in the furnace, 

 Mr. Watt Hates that the cooling of the mafs had been too 

 irregular to permit it to attain an homogenous texture ; 

 but this circumltance fortunately difclofed fome very re- 

 markable peculiarities in the arrangement of the particles of 

 bodies, palling from a vitreous to a ftony Itate. 



Thefe peculiar changes were discovered, by infpecting the 

 various parts of the wedge-fhaped mafs, which had pro- 

 grefiivcly cooled, as they were more remote from the fire, 

 and nearer to the fmall extremity. This circumltance, not 

 being very diltinctly itated by Mr. Watt, has led fome of 

 the readers of the paper to believe that the changes were 

 obferved by taking a portion of the fubltance out of the 

 furnace at different times, during the procefs : hence they 

 have not fufficiently attended to a mod important fact 

 which this experiment difclofes, namely, that the particles 

 of bodies in a folid llate, and at a temperature much below 

 that of fufion, are capable of a kind of internal motion, and 

 of aifuming a cryftalline arrangement, in all the various 

 ftages from fluidity to a perfectly folid ftate. 



The tendency towards arrangement in the particles of 

 the fluid glafs is firft developed by the formation of minute 

 globules, which are generally nearly fpherical, but fome- 

 times elongated, and which are thickly difleminated through 

 the mafs. The colour of thele globules is confiderably 

 lighter than that of the glafs ; they are commonly greyifh- 

 brown, fometimes inclining to chocolate-brown ; and when 

 they have been formed near the interior furface of the ca- 

 vities in the glafs, they project and refeniblc a clufter of 

 fmall feeds. Their diameter rarely exceeds a line, and fel- 

 dom attains that fize ; as, in general, they are fo near to 

 one another, that their furfaces touch before they can ac- 

 quire confiderable magnitude. In the procefs of cooling, 

 they adapt their form to their confined lituation, fill up 

 every interltice, and finally prefent a homogenous body, 

 wholly unlike glafs, and equally unlike the parent bafalt. 

 When the union of the little globules has been imperfectly 

 effected, the fracture of the mafs indicates its ltructure, by 

 numerous minute conchoidal fractures, which difplay the 

 form of each globule. But if the arrangement has extended 

 a little farther, all thefe fubdivilions are entirely loft ; the 

 mafs becomes perfectly compact, has an even or a flat con- 

 choidal fracture, is nearly of the fame hardnefs as the glafs, 

 is commonly of a chocolate colour, graduating into a 

 browmfh-hlack, and the intenlity of the colour iucreafes in 

 proportion to the degree to which the arrangement has ex- 

 tended. Its afpect is rather grcafy ; and it much referables 

 fome varieties of jafper, in the compactnefs ot its texture, 

 and its opacity. Its magnetic action is extremely feeble. 

 Its fpecific gravity appeal 73*- From this refem- 



blance to jafper, Mr. Wat) denominates the mafs in this 

 ifpideous. 



If the mafs were now rapidly cooled, il . lavs 



Mr. Watt, that the reiult would be the fubftance 1 have 



jult defcribed| but if the temperate- ! to the 



farther arrangement of its particles be continued, another 



change is immediately commenced ; by the progrefs of 

 which il acquires a moi Hire, much greater tena- 



city, <; 11 , olour deepens as the! ' . till it 



becomes ablolntely black. Sometimes this alteration is 



elleCted 



