ally olivine; but
the boundary lines between diorites and gabbros are admittedly somewhat
vague, e.g. some authors would call rocks gabbro which others would
regard as augite-diorite. The hornblendites differ from the diorites in
containing little felspar, and consist principally of hornblende. Among
varietal designations given to rocks of the diorite family are
"banatite" for an augite-diorite with or without quartz (from the
Schemnitz district), "granodiorite" for a quartz-hornblende-diorite
(essentially the same as tonalite) from California, &c., "adamellite"
for the quartz-mica-diorite or tonalite of Monte Adamello (Alps),
"ornite" for a hornblende-diorite rich in felspar, from Sweden.
(J. S. F.)

DIP (Old Eng. _dyppan_, connected with the common Teutonic root seen in
"deep"), the angle which the magnetic needle makes with the horizon. A
freely suspended magnetic needle will not maintain a horizontal position
except at the magnetic equator. Over the N. magnetic pole the
north-seeking end of the needle points directly downwards and dips at an
intermediate angle at intermediate distances between the magnetic poles
and equator. There are secular progressive variations of dip as well as
of declination and the maxima are independent of each other. In 1576
the dip at London was 71 deg. 50', in 1720 (max.) 74 deg. 42', in 1900
67 deg. 9'. (For Dip Circle see INCLINOMETER.)

DIPHENYL (phenyl benzene), C6H5.C6H5, a hydrocarbon found in that
fraction of the coal-tar distillate boiling between 240-300 deg. C.,
from which it may be obtained by warming with sulphuric acid, separating
the acid layer and strongly cooling the undissolved oil. It may be
artificially prepared by passing benzene vapour through a red-hot tube;
by the action of sodium on brombenzene dissolved in ether; by the action
of stannous chloride on phenyldiazonium chloride; or by the addition of
solid phenyldiazonium sulphate to warm benzene (R. Mohlau, _Berichte,
1893, 26_, 1997) C6H5N2.HSO4 + C6H6 = H2SO4 + N2 + C6H5.C6H5. L.
Gattermann (_Berichte, 1890, 23_, 1226) has also prepared it by the
decomposition of a solution of phenyldiazonium sulphate with alcohol and
copper powder. It crystallizes in plates (from alcohol) melting at 70-71
deg. C. and boiling at 254 deg. C. It is oxidized by chromic acid in
glacial acetic acid solution to benzoic acid, dilute nitric acid and
chromic acid mixture being without effect. It is not reduced by
hydriod