322 



two poles, and thence an increasing confidence in all the other ana- 

 logies conceived to exist between them. 



The points at which the needle is vertical are given by means of two 

 equations, one of the fifth and the other of the second degree, and 

 hence altogether there are ten such points theoretically possible. How 

 many of these may be simultaneously real the equations do not, in 

 their literal form, seem capable of determining ; but at all events they 

 will, in all cases, bean even number, either 0, 2, 4, 6, 8, or 10. One 

 having been determined, one other at least must exist in the actual 

 circumstances of the terrestrial two-poled magnet. How many so- 

 ever such simultaneous points there may be, they must all lie in the 

 same plane ; and hence, if the second point which must exist could 

 be determined, then the great circle in the plane of which the axis of 

 the magnet itself is situated would be determined ; and thus another 

 test would be afforded of the truth or error of the hypothesis itself. 

 Mr.Davies suggests that as this plane will be symmetrical with respect 

 to the phenomena taking place on each side of it, its position might 

 be tentatively assigned from a series of observations of those pheno- 

 mena, especially of the dip and intensity ; the variation being for ob- 

 vious geometrical reasons excluded. 



Though the resulting formula does not, in its literal form, appear to 

 be capable of decomposition into factors, yet from some considera- 

 tions, chiefly analogical, Mr. Davies is led to hazard the conjecture 

 that it is capable of such decomposition ; but as this is uncertain, he 

 builds no consequences upon it, but leaves those consequences which 

 would flow from it, open till it shall be discovered whether thev would 

 be justified by the conjecture itself being proved to be correct. 



A paper was also read, entitled, " On certain Peculiarities in the 

 double Refraction, and Absorption of Light, exhibited in the Oxalate of 

 Chromium and Potash." By Sir David Brewster, K.H., L.L.D., F.R.S. 



The crystals of the oxalate of chromium and potash are, generally 

 speaking, opake; for at thicknesses not much greater than the 25th 

 of an inch, they are absolutely impervious to the sun's rays, and their 

 colour, seen by reflected light, is nearly black ; but when powdered, 

 they are green ; and the colour of the smaller crystals, viewed either 

 by reflected or by transmitted daylight, is blue. One of the most 

 remarkable of the properties of this salt is the difference of colour in 

 the two images formed by double refraction. At a certain small thick- 

 ness, the least refracted image is bright blue, and the most refracted 

 image bright green. The blue is found by analysis with the prism to 

 contain an admixture of green, and the green an admixture of red ; 

 and by candlelight this red predominating over the green, gives the 

 crystal a pink hue. At greater thicknesses the blue becomes purer 

 and fainter, and the green passes into red; and at a certain thickness 

 the least refracted blue image disappears altogether, and the most 

 refracted image is alone seen. At still greater thicknesses this image 

 also disappears, and absolute opacity ensues. When the crystal is 

 exposed to polarized light, with its axis in the plane of polarization, 



