337 



while boiling ; but they are nevertheless both perfectly soluble in 

 oxalic acid, in tartaric acid, or in citric acid. 



They are both precipitated of an orange colour by infusion of galls, 

 but are not precipitated by that re-agent if a considerable excess either 

 of alkali or acid prevail in the solution. 



As a further agreement in their properties, it is added, that neither 

 of them is precipitated by prussiate of potash or by hydrosulphuret 

 of potash. 



From these experiments, although a great difference which sub- 

 sists between the specific gravities of the two minerals cannot be 

 very satisfactorily explained, the author is satisfied that the Ameri- 

 can and Swedish minerals, in fact, contain the same metal. 



Description of a reflective Goniometer. By William Hyde Wollaston, 

 M.D. Sec. R.S. Read June 8, 1809. [Phil. Trans. 1809, .p. 253 ] 



The instrument here described by the author is designed to obviate 

 the inconvenience which has been found in attempting to measure 

 any small crystals by the instruments hitherto used for that purpose. 



When a surface is so small as one fiftieth of an inch in breadth, it 

 becomes extremely difficult to apply the short radius of a goniometer 

 to it with correctness. But since a surface of that magnitude may 

 reflect a very brilliant light, the reflected ray may be employed as 

 radius, and may at pleasure be taken of such a length that the angles 

 of small crystals can be known with as much precision as those of 

 the largest surfaces. 



The crystal being attached to a horizontal axis, with its edge in 

 the line of the axis, one of the surfaces is made to reflect some bright 

 light to the eye ; and, while the eye is retained steadily in the same 

 place, the axis is turned till the second surface reflects the same light, 

 and is consequently in the same position. The number of degrees 

 through which the axis has turned being the supplement to the re- 

 quired angle, the angle itself is indicated by the graduations of a 

 circle which moves with the axis ; but the complete construction of 

 the instrument cannot be distinctly understood without reference to 

 a figure that accompanies the paper. 



Since any inaccuracy in placing the crystal would occasion some 

 error by parallax in this method of using the instrument, the author 

 describes a second method, by which all error may be entirely obviated. 



By placing the crystal so that the image of some distant object is 

 brought to correspond with some other object by one of its surfaces, 

 the position of that surface is determined with precision, and the 

 second surface may be brought round to the same position with the 

 utmost accuracy. 



With this instrument the author has remarked an error in the 

 supposed angle of the primitive crystal of carbonate of lime, which, 

 instead of being 104 28' 40", as it is now considered by writers on 

 crystallography, appears to the author to be correctly 105, as it was 

 formerly measured by Huygens and by Sir Isaac Newton. 



