378 



are mixed together, a small portion of the latter salt adheres tenaci- 

 ously to the sulphate of baryta, which is precipitated, and escapes 

 decomposition. By employing different processes the author avoids 

 this source of fallacy ; first, from the chloride of barium, previously 

 dissolved in water, he throws down sulphate of baryta by adding 

 sulphuric acid ; and, secondly, he effects a precipitation from a simi- 

 lar solution of the chloride, by nitrate of silver, and infers the quantity 

 of chloride from that of the fused horn-silver obtained, having pre- 

 viously determined, by a separate series of experiments, the exact 

 composition of horn- silver. The conclusion he draws from his re- 

 searches is, that 100 parts of chloride of barium correspond to 13 7 '63 

 parts of the chloride of silver, which latter substance contains 34'016 

 parts of chlorine, and therefore leaves for the proportion of barium 

 65'984 parts. The real equivalent of barium, however, will depend 

 upon that of chlorine, which is itself not yet satisfactorily de- 

 termined. 



On a new Series of periodical Colours produced by the grooved Sur- 

 faces of Metallic and Transparent Bodies. By David Brewster, 

 LL.D. F.R.S. L.SfE. Read May 21, 1829. {Phil. Trans. 1829, 

 p. 301.] 



The author, having received from Mr. Barton in the year 1822 

 some fine specimens of his Iris ornaments, undertook a series of ex- 

 periments on the action of grooved surfaces upon light, of which he 

 communicated an account to the Royal Society of Edinburgh in the 

 following year. The investigation having since been taken up by 

 Mr. Fraunhofer, the author had desisted from pursuing it until lately, 

 when he learned that the phenomena which had principally occupied 

 his attention had escaped the notice of this philosopher. The image 

 of a candle seen by reflexion, from a flat and polished metallic sur- 

 face, covered with equal and equidistant grooves, the plane of re- 

 flexion being parallel to the grooves, is accompanied with a row of 

 prismatic images, arranged in a line perpendicular to the grooves. 

 The colourless image of the candle is formed by the original por- 

 tions of the metallic surface, which have been left between the 

 grooves, while the prismatic images are formed by the sides of the 

 grooves themselves. This is rendered evident to the eye by vary- 

 ing the proportion between these two parts of the surface. The 

 general phenomena of the prismatic images, such as their distance 

 from the ordinary image, and the dispersion of their colours, de- 

 pend entirely on the number of grooves and intervals which oc- 

 cupy a given breadth ; and the laws of these phenomena have been 

 accurately determined by Mr. Fraunhofer. Dr. Brewster, by ex- 

 amining the appearances with more attention, observed in some spe- 

 cimens a remarkable defalcation of particular colours, varying with 

 the angle of incidence, and sometimes affecting one of the images 

 and not the others ; in some cases even the image reflected from the 

 original surface of the steel was slightly coloured, its tint having a 



