Spiculums of ReJltBing TeUfcopes, 494 



- fay that the proportion of tin is from 15, or 16 to 32 ounces of copper, I would be under- 

 ftood, that the proportion of tin will not always be accurately the fame, as copper will 

 take more or lefs tin to perfectly faturate it, acf ording to its purity. It might be of ufe 

 previoufly to purify the copper as much as polTible. A very little experience in thefe 

 matters, will enable any one to know exactly wlien the copper is compleatly faturated ; as 

 the compofition will, if broken, appear of a mod: beautiful bright and glofly nature, very 

 much refembling the fine face of quickfilver. My method to afcertain that point accu- 

 rately, is to melt 32 ounces of copper, and to add to it when fufficiently fufed, 15 ounces 

 of tin, and to pour the mixture into an ingot ; then to a certain known portion of this 

 compofition, I add a very fmall, but known portion of tin ; and thus by a few trials I can 

 eafily obtain the point of complete faturation, and the maximum of perfeftion. Having 

 then afcertained what portion of tin I added to the above known quantity of the compofi- 

 tion, I add the proportional quantity of tin to the whole when melted a fecond time. 

 Thus if I find that I muft add a quarter of an ounce of tin to one pound of the compofi- 

 tion, fo as to obtain the ne plus ultra of brilliancy*, then I know that when I fhall melt 

 the remainder of the metal a fecond time, in order to caft the fpeculum, I mufl add 

 one of grain tin to four pounds of the compofition, made according to the proportion of 

 ja ounces of copper to 15 ounces of tin. The arfenic muft be added in the fecond melting., 

 when the fpeculum is intended t to be caft, as the heat of the mixture in ihtfirjl melting 

 is fo great, as to render the moft part of the arfenic volatile, and in a great meafure pre- 

 vent its aftion upon the metals. It is fomewhat fingular that arfenic, though particularly 

 recommended by Sir Ifaac Newton % for this purpofe, Ihould be haftily thrown afide by the 

 founders, as well as paffed over unnoticed by the writers upon this fubjeft. This impru- 

 dent difufe I can only attribute to the difagrceable fumes, or vapours, which arife when 

 it is introduced into the crucible to the melted mixture, which may produce difagreeable 

 effefts upon the operator, if proper care is not taken to prevent them from being received 

 into the lungs §. All the precaution neceflary is, to bruife the arfenic coarfely, and in- 

 troduce it into the crucible with a pair of tongs, having tied it up in a piece of paper; 

 give it then a ftir with a wooden fpatula, retaining your breath, avoid it till you can fee 

 no more vapours arife from the crucible, when the metal will be ready to be poured into 

 the flaflcs to caft the fpeculum. The great ufe of arfenic in this compofition, is to render 



• If too much tin fliould be added ; viz. if 1 7 ounces of tin is put to 31 ounces of copper, the compofi- 

 tion is not brilliant when broken, but of a grey Hue and dull colour. If the quantity of tin be further, 

 increafed, the metal will be almoft black. 



■\ Sir Ifaac Newton melted, the copper firtt, then added the arfenic, and laftlythetin; as without doubt, 

 he knew that the tin (hould remain in a fluid ftate theftiorteft time poffibli. It is true that Sir Ifaac Newton 

 added the arfenic to the melted copper ; but as he well knew that a great part of it would be rendered 

 volatile, he therefore added a very large quantity of it; viz. arfenic i to copper 6. ^ 



X See Dr. David Gregory's optics, by Dr. Brown and Dr. Defagauliers, p. 219, or ?hilofophical 

 Tranfaftions, No.LXXXI. 



§ I have been affured, by two ingenious experimental philofophers, that the fumes of arfenic, even when 



the garlic fmell is very ftrong, are not in the leaft prejudicial to the lungs. 



Nkvil Maskelyne. 



4 the 



