608 HORSE-POWER 



many trachytes. The basaltic hornblende of Werner includes the black crystallised 

 specimens usually associated with lavas and other volcanic rocks. Noble hornblende, 

 or pargasite, is a dark green, somewhat translucent mineral, occurring in thick, stout 

 crystals, or in granular masses, embedded in calcareous spar, at Ersby, near Pargas, 

 in Finland. The following is Rammelsberg's analysis of a Pargas hornblende : silica, 

 41-26; magnesia, 13-49; lime, 11-95; protoxide of iron, 9*92; peroxide of iron, 4'83; 

 alumina, 11-92; potash, 270; soda, 1-44; and water, 0'52 = 9973. 



In the hornblende called arfvcdsonite, soda occurs in considerable quantity, as shown 

 by the following analysis : silica, 5T22 ; soda, 10'58 ; protoxide of iron, 7'80 ; peroxide 

 of iron, 2375; lime, 2-08; magnesia, 0'90; and potash, 0'68 = 98-13 (Rammelsbcrg). 

 Arfvedsonite is an exceedingly fusible black mineral found in Greenland ; and often 

 separated as a distinct species. 



The mineral called uralite, discovered by the late G. Rose in the greenstones of the 

 Ural range, is commonly regarded as a pseudomorph, since it presents the external 

 form of augite with the cleavage and chemical composition of hornblende ; thus 

 establishing a relation between the two minerals. This relation is indeed so intimate 

 that it has been proposed to unite them in one species. Hornblende differs, however, 

 from augito in the angle of the prism obtained by cleavage being much more obtuse ; 

 but it has been shown that the two forms are easily derivable the one from the other. 

 Hornblende, again, is more fusible than augito ; it lias a somewhat lower range in 

 specific gravity ; it often contains fluorine, an element which has never been found in 

 augite. Hornblende, moreover, occurs in granitic and other highly -silicated rocks ; 

 whilst augite is confined to the more basic rocks, such as basalt : occasionally, how- 

 ever, the two minerals occur together. Experiments on the artificial formation of 

 these minerals have rendered it probable that the same chemical compound may form 

 augite when rapidly cooled from a state of fusion, and hornblende when slowly cooled ; 

 indeed, hornblende has been converted into augite by fusion and rapid solidification. 



It has been shown by Tschermak, and independently by Mr. Allport, that hornblende 

 may be distinguished from augite in sections of rock examined under the microscope, 

 by the fact that hornblende is dichroic, whilst augite is not : the intensity of the 

 dichroism varies, however, to a great extent in different hornblendes. 



The hornblendic minerals, although highly important in a geological point of view, 

 have little or no economic value. Certain Swedish hornblendes have been employed 

 in smelting magnetic ores : and the fibrous varieties known as asbestus have received 

 various applications. See ASBESTUS. F.W.K. 



HORNBLENDE-ROCK. (Amphibole, Fr. ; Hornblendfels, Ger.) A rock, con- 

 sisting chiefly of crystalline-granular, dark green, or black hornblende, associated 

 commonly, but by no means necessarily, with felspar, quartz, and mica in small 

 quantity. The rock often assumes a schistose structure, and then passes into Horn- 

 blende slate (Schiste amphibolithique, Fr. ; Hornblendschiefer, Ger.). This slate usually 

 occurs in connection with mica-schist, gneiss, and other metamorphic rocks. When 

 the hornblende belongs to the bright green variety called actinolite, the rock becomes 

 an actinolite schist, or Strahlsteinschiefer of the Germans. Hornblende also occurs, 

 although to a less extent, in other rocks ; such as syenite (a granite in which horn- 

 blende replaces mica), and greenstone and diorite (rocks consisting of hornblende and 

 felspar). The timazite of Hungary and Transylvania is a trachytic greenstone, con- 

 taining the peculiar hornblende called gamisgradite. 



On exposure to atmospheric influences, hornblendic rocks easily decompose ; the 

 protoxide of iron passing into the condition of peroxide. Soils resting on such rocks 

 are frequently remarkable for their fertility. 



HORK-XiEAD, or Phosgenite. Native chlorocarbonato of lead. See LEAD. 



HORN-QUICKSILVER. Native subchlorido of mercury. See CALOMEL. 



HORN-SILVER, or Luna Cornea. Both these names were given by the alche- 

 mists to fused chloride of silver. Horn-silver is found native as kerargyrite. See 

 SILVER. 



HORNSTONE. A variety of quartz, resembling flint, but more brittle, and 

 breaking with a more splintery fracture. It sometimes occurs imbedded in limestone. 

 See CHERT. 



HORSE. In mining, any large isolated mass of rock occurring in a vein, or 

 splitting a vein or seam into two or more parts. 



HORSE CHESTNUT. (Marronnicr tflndc, Fr. ; Gemeine Rosskastanic, Ger.) 

 The wood of this well-known tree is used by the Tunbridgo turner ; otherwise it is 

 only employed for some large varnished works. Seo HIPPOCASTANUM. 



HORSE-FLESH ORE. A Cornish name for purple copper-ore. See COPPER. 



HORSE-POWER, in steam-engines, was estimated by Watt at 32,000 Ibs. 

 avoirdupois lifted one foot high per minute, for one horse. M. D'Aubuisson, from an 

 examination of the work done by horses in the whims, or gigs (machines a moieties) 



