342 tween MA and MF will be reflected by the furface FD; for, rad. : S of the angle CBF:: CB : CF CB CD (BD being the direction of MB after refraction) :: S. of the angle CDB S. of the angle CBD :: S. of the angle EDB: S. of the angle CBD :: S. of the angle CBF -rad. x S. of the angle CBD but S. of the angle S. of the angle EDB CBF S. of the angle CBD :: S. of incid. S. of rad. x S. of the angle CBD refract, hence S. of the angle EDB : S. of the angle CBD :: S. of incid. : S. of refract, or rad. S. of the angle EDB:: S. of incid. : S. of refract. Confequently, the angle EDB is the leaft angle of incidence on the second surface that the rays can have, and be reflected. Thefe rays will be emergent in different directions, and the nearer they are to MB (fig. 3.) the less will be the inclination of thofe in the fame pencil to each other and to GO. Hence it appears that the lefs refrangible rays will be nearly parallel to the more refrangible effectual rays which come from the fame drop, and by a mixture of the different colours, may be produced the whitenefs in the interior part of the firft halo. These rays will be too much difperfed to produce any effect in the fecond and third. It may eafily be conceived, from the manner in which Sir Ifaac Newton has expreffed himself, that had the diameters of the halos he mentions been been accurately measured, they would have agreed fufficiently well with this theory. It is not however confidered as free from objections, but by future obfervations muft its truth or fallacy be afcertained: fhould it be found to agree with fuch obfervations, it may throw some light on the doctrine of the afcent of vapours, a matter hitherto fo much difputed. CONSIDERATIONS relative to the NATURE of WOOL, SILK, and COTTON, as OBJECTS of the ART of DYING; on the various PREPARATIONS, and MORDANTS, requifite for thefe different SUBSTANCES; and on the NATURE and PROPERTIES of COLOURING MATTER. Together with fome OBSERVATIONS on the THEORY of DYING in general, and particularly the TURKEY RED; by THOMAS HENRY, F.R.S. and of the Amer. Philof. Society, held at Philadelphia. IN PART FIRST. READ DECEMBER 20, 1786. N the following Memoir, on a fubject to which too by English writers, my principal intention is to prefent, at one view, the preparatory and other little attention has been directed proceffes for the dying of wool, filk, and cotton, and to endeavour to deduce a theory better fuited to account for the refults, than has hitherto been advanced. In the execution of this undertaking it will be neceffary to repeat feveral things from Hellot, Macquer, and d'Apligny, who have written feparate treatifes on the dying of these materials; and I am to confess great obligations to that very celebrated chemist M. Berthollet, whose researches are, every day, affording new and important information in the several departments of the hermetic art. The art of dying, or of imparting to different materials, employed for the fabrication of garments and furniture, thofe beautiful colours which are afforded by many articles of the vegetable, animal, and mineral kingdoms, appears to have been of high antiquity*. As most of these materials are, of themselves, either of dark and difagreeable colours, or else devoid of all colour, it is probable, that, even in the very earliest ages, the love of ornament, which is natural to mankind, would induce them to ftain their vestments with various colouring ingredients, especially with vegetable juices. But the art of imparting permanent dyes to cloth, and affixing to its fibres fuch colouring materials, as could not easily be washed out by water, or obliterated * Delaval on Light and Colours, or or greatly changed by the action of air, or of certain faline fubftances, to which they are liable to be exposed, and which are necessary to render them clean, when foiled ;-this was an art which required the knowledge of principles not within the reach of untutored men, and only to be obtained by gradual investigation, and in long process of time. It has been proved by our ingenious affociate, Mr Delaval, that the Egyptians were poffeffed of the art of dying, and even of that of printing on cloths. In a paffage, which he has quoted from Pliny, that author relates that the Egyptians having befmeared, or drawn on white cloth, with various substances, which were in themselves colourless, but capable of abforbing colouring matter, threw them into a caldron of hot liquor, tinged with dying materials; and that, though the parts, thus drawn upon, were not distinguishable, before the cloth was immersed in the colouring liquor, and though this liquor contained only one colour, it was furprizing to fee the cloth taken out, stained with several different colours, according as the different parts of it had been impregnated with the various fubftances, capable of receiving and altering the nature of the pigment. This is fo plain a defcription of the art of what is now called callico printing, that though it is my intention to refer thofe gentlemen, who wish to be more intimately acquainted with the ancient history history of dying, to that written by Mr. Delaval, and prefixed to his ingenious treatise on light and colours, yet I could not refrain from relating it on this occafion. Permit me alfo to mention another hiftorical fact from the ancient hiftory of this art. The Phoenicians held a decided pre-eminence in the tinctorian art, for many ages: their purple and fcarlet cloths were fought after by every civilized nation, and the city of Tyre, enriched by its commerce, increased to an amazing extent. But her career was ftopped by the vanity and folly of the Eaftern, emperors; under whofe dominion, this opulent city had unfortunately fallen. Defirous of monopolifing the wearing of the beautiful cloths of Tyre, thefe misjudging tyrants iffued most severe edicts, prohibiting any one from appearing in the Tyrian blue, purple, or scarlet, except themselves, and their great officers of state. The enacting and enforcing of fumptuary laws requires great judgment, and delicacy; and much caution fhould be used, lest, in curbing exceffive luxury, the arts, which are fupported by its moderate indulgence, fhould be deftroyed. Such however was the fate of the Tyrian dyes. Under the impolitic restraint imposed on the confumption of the Phoenician cloths, the manufacturers and dyers were no longer able to carry on their trade, it grew lan |