SPECULUM. 



65. Copper 20Z., tin i oz., iron-filings 1 dr., regulus of 

 antimony l dr.; too blue a colour, and rough-grained. 



66. Regulus of antimony and tin, equal parts ; fparry, and 

 not fit for mirrors. 



67. Cail-fteel; will not polifh upon pitch, either with putty, 

 or colcothar of vitriol. 



68. Steel I, tin ;J ; very rough-grained and blueifh, and not 

 much different from Reel itfelf. 



69. Steel I, tin 1 ; rough-grained, and of a blueifh colour. 



70. Steel I, and forty-feventhcompofition 20 parts; rough- 

 grained, and not near fo good as forty-feven. 



71. Steel I, and forty-feventh compofition 30; not much 

 different from the forty-feventh compofition, but not lo 

 beautiful and clofe-grained. 



" Many other mixtures," fays Mr. Edwards, " were 

 tried, by combining the foregoing compcfitions in more than 

 3. hundred varieties of proportions ; but none of them were 

 found equal to N' 47, as that mixture forms a metal that is 

 the whitefl, hardefl, moft refleftive, and takes the higheft 

 luflre of any tiling I have yet feen." 



The mixture, therefore, wiiich was at laft chofen from 

 thefe various experiments, was 32 ounces of pure copper, 

 with 15 or 16 of grain-tin, juil enough to faturate the 

 copper, I of brafs, and I of arfenic ; which compofition 

 will be ftill further improved, it is faid in a note, but we 

 believe not correflly, by i of filvcr. The \ife of the copper 

 was to leffen the extreme brittlenefs of the mixture ; the 

 arfenic rendered the metal clofe and free from pores ; and 

 the filver, it was thought, improved the brightnefs, with- 

 out injuring the hardnefs, when ufed in a fmall quantity. 

 This mixture, it will be remarked, very much refembles 

 Mr. Mudge's, who ufed 14^ of grain-tin to 32 of copper, 

 but without arfenic, or other addition : he found 15 too 

 hard for the blue ilone to take hold of ; but Mr. Edwards 

 availed himfelf of another itone, procured at Edgedon, in 

 Shropfhire, fituatcd between Ludlow and Bifhop's CalUe, 

 which was keen enough to bite a metal with from 15 to 16 

 of tin to 32 of copper, which compofition Mr. Mudge con- 

 fiders to poffefs ^'maximum of whitenefs as well as of iiard- 

 nefs. Mr. Mudge, however, fays, that one of his beil work- 

 men, called Jackfon, found a flone that would cut a metal 

 of one-third tin and two-thirds copper, but the difcovery of 

 fuch ftone, which he always kept a fecret, colt h:m the ex- 

 pence of a journey of two hundred miles ; which dillance 

 exceeds th.it of Edgedon from London, by about fifty miles ; 

 and, therefore, the Rones of Jackfon and of Mr. Edwards 

 were probably not from the fame quarrv, if they were fimi- 

 lar in kind. Edwards's mode of caftmg is nearly the fame 

 as Mudge's, both as to a firll fufing for the ingot, with a 

 fecond fufing for the fpeculum at a lower heat ; and as to 

 his flaflcs and fand (procured from Highgate), a fmall por- 

 tion of the tin was alfo referved to be put into the crucible 

 at the fecond melting, when alfo the arfenic, tied up in 

 paper, was put in ; and after the, fumes of this ingredient 

 had ceafed to afcend, the mixture, itirred with awooden 

 fpatula, while no breath was inhaled into the lungs, was 

 poured into the flafks, the moment that about an ounce of 

 powdered refin, or refin and nitre, had been thrown in and 

 melted, to give a good face, as Mudge did with the char- 

 coal dull. The principal difference from Mudge's procefs 

 is, that in the firit melting;, after the brafs is made as fluid 

 as poffible, the brafs and filver mufl be added with the com- 

 mon black flux, of two parts of tartar to one of nitre, or 

 by ftirring the melted mixture with a fpatula of bircli ; after 

 which the tin mufl be added, and the whole poured out into 

 an ingot, as foon as it is found to be in a hquid ftate ; 

 and after the fecond melting, when the arfenic has Lieen 



added, and alfo the refin, the flaflcs receive tht metal, witli 

 the face downwards ; the ingate having been made on the 

 back of the metal near its edge, and its breadth increafing 

 to half the diameter of the metal at the place of joining. 

 The upper part of the ingate fhould contain as much metal 

 in weight as the fpeculum and the flaflcs fhould be opened, 

 and the fand cleared out, while it remains red-hot, with the 

 face ftill downwards, and then removed by a pair of tongs 

 into an iron pot, containing a large quantity of hot afhes or 

 fmall coals, fufficient to bury the metal, where it mufl re- 

 main to cool by degrees with the aflies. We mufl not, 

 however, omit to mention one precaution, on which fome 

 ftrefs feems to be laid, viz. that when the pattern, with its 

 ingate, or git, is taken out of the fand, ten or a dozen fmall 

 holes muft be made through the fand at the back of the 

 mould, with a fmall wire, or common knitting-needle, to 

 permit the air to efcape while the liquid metal is fiUing the 

 mould, which are fafer than one large hole : alfo at the 

 fecond melting, the flaflcs mufl be jogged with a flight ftroke 

 or fhake, as foon as tiie fluid metal is poured in, to prevent 

 flakes or air-bubbles being formed on the face of the metal. 

 The git may be taken off, when the metal is quite cool, 

 with a common fine half-round file, to make an incifion, 

 after which a gentle blow will remove it. If, however, the 

 metal i> cafl from a mould too thick, it will not take the 

 parabolic figure. For a fpeculum of 4^ inches diameter, 

 and 18 inches focal length, the thicknefs fhould be -r\,ths of 

 an inch at the edge, and the back lliould be convex, fo as to 

 allow the whole metal to be of uniform thicknefs when 

 ground, in order that it may not yield to the preffure, or in- 

 cumbent weight, during the grinding and polifliing. The 

 handle fhould be made of the fame concavity and convexity 

 as the metal, but double its thicknefs, and only three-fourths 

 of its diameter ; it fiiould alfo have a hole through the 

 centre, to receive a fcrcw going into the chuck of a lathe, 

 in order that the edge of the metal may be formed, firll by 

 a file, and then by a flone from Edgedon, while it is re- 

 volving. 



The fpeculum being cafl by thefe diretlions, is next 

 rough-ground on a common grinding-flone, formed by a 

 bar of iteel, while revolving, to the radius of convexity 

 fuitable for making the defired concavity of the m.etal ; after 

 this rough-grinding, two tools are all that are requifite ; a 

 convex tool made of lead and tin, or elfe of pewter, and a 

 bed of hones, of the fort already mentioned ; for the con- 

 vex tool ferves alfo for a pol;fher, in conjundlion with its coat 

 of pitch and colcothar of vitriol, which is preferred to putty : 

 the polifh from the latter is indeed of a white filvery hue, 

 but that from the former gives a fine dark luflre, like the 

 complexion of polifhed fleel, which is deemed preferable. 

 The colcothar, to be good for this purpofe, mud be of a 

 deep red or purple colour, mufl diffolve m the mouth with- 

 out gritty lumps, and feel foft and oily to the touch. It 

 may be levigated with a little water between two bars of 

 polifhed fleel, and worked with two or three additions of 

 water, till it acquires a dark and nearly black colour, when 

 it will be fine enough to be put into a bottle with water for 

 ufe. We will not repeat all the proceffes to be attended to 

 in the ufe of the convex tool, and of the bed of hones, which 

 we have fufiiciently detailed under our article Grinding, 

 but remark only on their dimenfions and fhape, which form 

 cflential confiderations in Mr. Edwards's mode of grinding, 

 pohfhing, and particularly of giving the requifite figure with- 

 out a fucceffion of tentative operations. The bed of hones 

 fhould be of a circular figure at the circumference, and m 

 every refpeft the fegment of a fphere, with a diameter juil 

 i-Tjths of an inch wider than that of the metal, when it is 



four 



