62 BRITISH ASSOCIATION FOR THE ADVANCEMENT OP SCIENCE. 



values transferred to the other side of the equation. They "were all made by 

 means of the colour-top, and were verified by repetition at different times. 



" ON SOME DICHROMATIC PHENOMENA AMONG SOLUTIONS, AND THE MEANS OF REPRESENT- 

 ING THEM," Br DR. GLADSTONE. 



This paper was an extension of Sir John Herschel's observations on dichroma- 

 tism, that property whereby certain bodies appear of a different colour according 

 to the quantity seen through. It depends generally on the less rapid absorption 

 of the red ray as it penetrates a substance. A dichromatic solution was examined 

 by placing it in a wedge-shaped glass-trough, held in such a position that a slit in 

 a window-shutter was seen traversing the varying thicknesses of the liquid. The 

 diversely coloured line of light thus produced was analyzed by a prism ; and the 

 resulting spectrum was represented in a diagram by means of coloured chalks on 

 black paper, the true position of the apparent colours being determined by the 

 fixed lines of the spectrum. In this way the citrate and comenamate of iron, sul- 

 phate of indigo, litmus in various conditions, cochineal, and chromium, and cobalt 

 salts where examined and represented. Among the more notable results were the 

 following : — A base, such as chromic oxide, produces very nearly the same spec- 

 tral image with whatever acid it may be combined, although the salts may appear 

 very different in colour to the unaided eye. Citrate of ire n appears green, brown, 

 or red, according to the quantity seen through. It transmits the red ray most 

 easily, then the orange, then the green, which covers the space usually occupied 

 by the yellow ; it cuts off entirely the more refrangible half of the spectrum. Neu- 

 tral litmus appears blue or red, according to the strength or depth of the solution. 

 Alkalies cause a great development of the blue ray ; acids cause a like increase 

 of the orange, while the minimum of luminosity is altered to a position much 

 nearer the blue. Boracic acid causes a development of the violet. Alkaline lit- 

 mus was exhibited so strong that it appeared red, and slightly acid litmus so 

 dilute that it looked bluish purple ; indeed, on account of the easy transmissibility 

 of the orange ray through an acid solution, the apparent paradox was maintained 

 that a large amount of alkaline litmus is of a purer red than acid litmus itself. 

 Another kind of dichromatism was examined, dependent not on the actual quan- 

 tity of coloured material, but on the relative proportion of the solvent. Diagrams 

 of the changing appearances of sulphocyanide of iron, of chloride of copper, and 

 of chloride of cobalt were exhibited. 



"ON A METHOD OF DRAWING THE THF.ORKTICAL FORMS OF FARADAY'S LINES OF FORCE 

 WITHOUT CALCULATION," BY MR. J. C. MAXWELL. 



The method applies more particularly to those cases in which the lines are en- 

 tirely parallel to one plane, such as the lines of electric currents in a thin plate, 

 or those round a system of parallel electric currents. In such cases, if we know 

 the forms of the lines of force in any two cases, we may combine them by simple 

 addition of the functions on which the equations of the lines depend. Thus the 

 svstem of lines in a uniform magnetic field is a series of parallel straight lines at 

 equal intervals, and that for an infinite straight electric current perpendicular to 

 the paper is a series of concentric circles whose radii are in geometric progression. 

 Having drawn then two sets of lines on two separate sheets of paper and laid a 

 third piece above, draw a third set of lines through the intersections of the first 

 and second sets. This will be the system of lines in a uniform field disturbed by 

 an electric current. The most interesting cases are those of uniform fields die- 



