The Eclectic and general dispensatory : comprehending a system of pharmacy, materia medica, the formulae of the London, Edinburgh, and Dublin pharmacopoeias, prescriptions of many eminent physicians, and receipts for the most common empirical medicines / collated from the best authorities, by an American physician.

Date:: 1827

Credit: The Eclectic and general dispensatory : comprehending a system of pharmacy, materia medica, the formulae of the London, Edinburgh, and Dublin pharmacopoeias, prescriptions of many eminent physicians, and receipts for the most common empirical medicines / collated from the best authorities, by an American physician. Source: Wellcome Collection.

Provider: This material has been provided by the Harvey Cushing/John Hay Whitney Medical Library at Yale University, through the Medical Heritage Library. The original may be consulted at the Harvey Cushing/John Hay Whitney Medical Library at Yale University.

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$A ray of light fulling obliquely upon a polished surface is refected from it at an angle equal to the angle of its incidence. But when a ray of light falls at an angle of 35° 25' on a polished plate of glass ; and in being reflected from it falls upon another plate of glass so placed that its angle of incidence is also 35° 25'; the second plate may be turned round its axis without vary- ing the angle which it makes with the ray that falls upon it. If the two planes of re- flection be parallel to each other, the ray of. light is reflected in the same manner from both plates of glass ; but if the se- cond plate be turned round a quadrant of u circle, so as to make the plane of reflec- tion perpendicular, the whole ray will pass through it, and none of it be reflected ; yet if this plate be turned round another quadrant of a circle, so as to make the re- flecting planes again parallel, the ray will be now reflected by it as at first. The light can penetrate through the glass only when the reflecting planes are perpendicular, but is reflected when they are parallel. This property of light has been termed by Ma- lus, by whom it was first discovered, its po- larization. When a ray of light moving in a straight line passes within a certain distance of a body parallel to its direction, it bends to- wards the body, or is inflected,• but when the body parallel to its course is at a greater distance, the ray is bent from it, or deflected. When it passes obliquely from one medium toanotherof a different density, it is bent a little from the line of its former direction, and assumes a new one, or is refracted. In passing into a denser medium it is refract- ed towards the perpendicular; but it is re- fracted from the perpendicular when pas- sing into a rarer medium. The refraction is proportional to the density of the me- dium, but in that of a combustible the re- fraction is greater than the ratio of its den- sity ; and when a liquid is converted into vapour, its refractive power diminishes at a greater ratio than its density diminishes.* Every ray of light is resolvable into seven other distinct rays, each possessing a diffe- rent degree of refrangibility; and conse- quently divisible from each other by the prism. The ultimate or component rays are distinguishable by the impression of colours they excite on the eye; and are arranged in the following order, red, orange, yellow, green, blue, indigo, violet. The red is the least refrangible, reflexible, and in- flexible ; the violet the most; and that of the others follow in the order in which they are placed. The colour of bodies depends on their transmitting or reflecting those * See the experiments of Arago and Petit, in the Ann. tie C/iem. et Phys. toni.i. I. rays only which excite the impressions of their colour. The reflection of the whole prismatic rays constitutes white ; the ab- sorption or suffocation of all or the greater part of these occasions black, which is the total absence of light. The illuminating power of the rays of light differs. Those towards the middle of the prismatic spectrum, as above arranged, possess the greatest illuminating power; but this diminishes as the rays approach to- wards the extremities. Light enters into combination with bo- dies ; and in some cases is again extricated without any change being produced, as in pyrophori, or substances which absorb light, and emit it again when carried into a dark place. In some cases, however, the ab- sorption of light by bodies occasions very sensible changes in them: the colour of plants, for example, their taste and odour, and the quantity of combustible matter they contain, depend on light: for a plant rear- ed in the dark is nearly colourless, insipid, inodorous, and contains a very small pro- portion of combustible matter. The natural sources of light are the sun and fixed stars; but it is also artificially produced by combustion, chemical combi- nation, heat, and percussion. The sun's rays, the greatest source of light, have been found to be composed of three different species of rays : 1. rays which produce light and colour; 2. rays of mere heat; and, 3. rays which produce neither light nor co- lour, nor affect the thermometer; but which have the power of deoxidizing: and, thus constituted, it produces very important che- mical effects. Light partially deoxidizes metallic ox- ides and salts. Thus it blackens muriate of silver; and as this takes place when the salt is placed beyond the violet ray, or out of the prismatic spectrum, the effect is ap- parently to be attributed to the action of the third species of rays. It also reduces the nitro-muriatic solution of gold, when it is placed in contact with charcoal, or any other vegetable or animal matter; and the red oxides of mercury and of lead become much paler when exposed to the sun ; the rays that produce these effects are the least refrangible. Dr. Wollaston, however, has pointed out one exception to this effect of these rays, in guaiacum, which becomes green or oxidized in the least refrangible rays; and is again changed to yellow, or deoxidized in the most refrangible. Light has a powerful tendency to decom- pose nitric acid, which it renders red and fuming, even when it is contained in ves- sels accurately closed. Almost all the ve- getable and animal colouring matters have their brilliancy and colour much impaired by long exposure to the sun's rays: and$

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