VOL. XXII.] PHILOSOPHICAL TRANSACTIONS. 553 



radius r = 1. Now this equation is of such a form, as always to be explicable 

 by one affirmative and one negative root, one of which, being the lesser, is 

 the tangent of the angle of refraction in the regress to the sun, that is, when 

 the purple colours are the nearer to the sun. But the greater root is the tan- 

 gent of the angle of refraction, in the iris going from the sun, as we have 

 observed above, that is in a fluid of lesser ratio. In oil of turpentine the dis- 

 tance of this iris from the point opposite to the sun, is observed to be 8 1° 30'. 

 Whence the curious reader may derive the roots 0,80822, and — 2,98131, the 

 tangents of the refracted angles. Hence is computed the ratio of greater ine- 

 quality, as 1 to 0,67995. Such it is in oil of turpentine. But from the greater 

 root proceeds the lesser ratio, as I to 0,9540 nearly. Such it would be in a 

 fluid exhibiting the secondary iris of the same diameter, but which would look. 

 towards the sun with its red colours, after the manner of a primary rainbow. 



The Way of Colouring Marble. N° 208, p. 72/. Translated from the Latin. 



1. The marble should be smooth, without any stain; the harder the better, 

 to bear the heat of the fire, and therefore alabaster is not fit for this purpose. 



2. Fire must be used to open its pores, yet in such a degree, as that it may 

 not be red hot; for then the colours are burnt ; nor in too little a degree, for 

 though it receives the colours, yet they are less fixed; and even cold marble will 

 imbibe some colours: as saffron, stone-blue, (for a sky colour,) &c.; but these 

 are soon dissipated by the least degree of heat; therefore let the degree of heat 

 be such as suffices gently to boil the liquor poured upon the marble. 



3. The menstrua are various according to the diversity of the ingredients to 

 be dissolved, as a lixivium made of horse's urine, though dog's urine be better, 

 and pot-ashes four parts, together with one part of quick-lime ; also spirits of 

 wine, common lixivium, wine, and some oleaginous substances. 



4. The colours that are laid on with vehicles are the following: 1. Stone- 

 blue, dissolved either in spirits of wine or in a lixivium of unslaked lime. 2. 

 Lacmus in the common lixivium. 3. Safi^ron or sap green, dissolved in a lixi- 

 vium of urine and unslaked lime, or in spirits of wine. 4. Vermilion or cochi- 

 neal dissolved as above. 5. Sanguis draconis dissolved in spirits of wine s. a. 

 6. Brasil wood dissolved in spirits of wine. 7- An extract of alcanna root with 

 oil of turpentine; for it cannot be dissolved in any other menstruum; neither 

 in spirits of wine, nor in a lixivium of unslaked lime as above ; there is another 

 kind of sanguis draconis, called lachryma, which mixed with urine strikes a 

 pretty agreeable colour, but with difficulty ; colours mixed with urine answer 

 best. 



