CHAPTER III. 



PHYSICAL CHARACTERS OF IGNEOUS ROCKS. 



THE most important characters to which reference will be made in 

 the present chapter are specific gravity and texture. Fracture, 

 colour, and hardness, though of less importance, are often of considerable 

 use as furnishing diagnostic characters, and some account of them will 

 therefore be given. 



The methods by which specific gravity may be determined are described 

 in the ordinary text-books of physics, and need not, therefore, be explained 

 on the present occasion. In the case of a rock containing no hollow spaces 

 of any kind the specific gravity of any specimen, sufficiently large to 

 represent a fair average sample of the rock, may be determined by the 

 ordinary methods of weighing in air and in water. When hollow spaces 

 are present, it must be broken up into fragments so small as to leave no 

 cavities completely enclosed, and the determination must be made by 

 means of the specific gravity bottle. In using this method care must be 

 taken not to reduce the rock to the condition of too fine a powder, 

 otherwise the result obtained will be somewhat too high. Various methods 

 have been devised for the rapid determination of the specific gravity of 

 rocks, particulars of which together with a discussion of their relative 

 merits, will be found in a paper by Professor JUDD, published in the 

 "Proceedings of the Geologists' Association. " (1) 



The heavy solutions of bi-iodide of mercury and potassium (2) or barium (8) 

 and boro-tungstate of cadmium (4) are of great use for the determination of 

 the specific gravity of small particles. A solution is obtained, by suitable 

 dilution, in which the particle remains suspended, and then the specific 

 gravity of this solution is determined either by using a flask of known 

 capacity (25 c.c.s. is a convenient size) and weight ; or else by ' means of 

 a special apparatus such as that constructed by Gr. WESTPHAL, of Celle. (5) 

 An ingenious and original method of determining the specific gravity of 

 small particles by the use of the heavy solutions has been devised by Professor 

 SOLLAS, particulars of which will be found in the " Scientific Proceedings of 

 the Royal Dublin Society" for January 19th, 1885. 



(1) Vol. VIII., 1884, p. 277. 



(2) GOLDSCHMIDT. N.J., 1881. Beilage, Band I., p. 179. Maximum specific gravity = 3 196. 



(3) ROHBBACH. N.J., 1883. Band II., p. 186. Max. Sp. Gr. = 3'588. 



(4) D. KLEIN, B.S.M., 1884. p. 149. Max. Sp. Gr. =. 3-6. 



(5) For description of this apparatus, see N.J., 1883 Band II.. p. 87. 



