568 OBSTETEICS pumice, which is like natural pumice in pro- portion to the amount of alkali in the mineral. If the obsidian is pulverized it does not swell, but merely turns brown. Obsidian heated above the" point at which pumice is formed melts into a greenish mass, so that obsidian is often said to be melted pumice ; but the obsid- ian may probably be formed without passing through the phase of pumice. The following table by Abich gives the analysis of two obsid- ians and two lavas : 1, obsidian from Teneriff e, sp. gr. 2-528; 2, pumice from Teneriffe, sp. gr. 2-477; 3, obsidian from Lipari, sp. gr. 2-370; 4, pumice from Lipari, sp. gr. 2'77 : CONSTITUENTS. I. 2. 3. 4. Silica... 61-18 62-25 74-05 78-70 Alumina Ferric oxide Manganic oxide Lime . 19-05 4-22 0-33 0-59 16-43 4-26 0-23 0-62 12-97 2-78 '6-12 12-27 2-81 6-65 Magrnesia 0'19 0*79 0-28 0*29 Soda 10-63 11-25 4-15 4-52 Potash 3-50 2-97 5-11 4-73 Chlorine 0'30 ) n KO 0-81 0-31 Water 0-04 > 0-53 0-22 1-22 Total 100-03 99-38 99-94 100-00 The characteristics of obsidian are its glassy lustre, susceptibility to high polish, and hard- ness, sufficient to scratch glass. The Greeks called it b^iav6q Woq, as is supposed by some from tyif, sight, in allusion to its translucence. Pliny derives its name from Obsidius, who is said to have brought it from Ethiopia. It was used by the ancients for mirrors, and for vari- ous ornamental purposes. The Mexicans used it, under the name of itzli, for knives, razors, and serrated weapons and implements. The pointed fragments were made into arrows. The stone is much used for ornamental purposes, particularly as mourning jewelry, but from its brittleness requires to be worked with great care. The iridescent variety, which has a pe- culiar greenish yellow color, and commands a high price, is sometimes cut in cdbochon and set in rings. The colors of obsidian are nu- merous, but each specimen commonly has but one shade. The characteristics of pumice are sponginess and lightness, so that, although the specific gravity of the material itself is as great as that of obsidian, it is often bulky enough to float on water. It is of grayish shades, passing into yellow and brown. It is employed in the arts, pulverized as a polishing material, and in the lump for grinding and smoothing surfaces. Its chief source in commerce is Campo Bianco, one of the Lipari islands, where it forms a hill nearly 1,000 ft. high. OBSTETRICS (Lat. obstetrfa, a midwife), the art and science of midwifery. It has a double mission : 1, to render possible, easy, and regu- lar, exempt from all abnormal suffering and all danger, the accomplishment of the numerous functions which directly or indirectly affect generation, from birth to the age when the procreative faculty becomes natural to the hu- man female ; 2, to direct and defend from all harm the delicate and precarious health of the infant during its early period of existence, and particularly during lactation. Labor is a natu- ral function, and happily the intervention of art is rarely demanded. But to foresee, pre- vent, or remove all possible dangers, to over- come serious obstacles when they exist, to re- solve the many difficult and trying questions that may arise, demand skill and judgment. The important part which woman takes in gen- eration consists in successive acts which are accomplished in her. One of the germs or ova, enclosed in the ovary, receives through impreg- nation the power to develop itself, and is then transported to the uterus. This organ retains the ovum, and furnishes it the materials for its growth. It becomes first an embryo, and next a foetus, when it acquires all the necessary powers for exterior life. This succession of phenomena constitutes gestation or pregnancy. When these powers are acquired, the new be- ing is expelled by a spontaneous action of the same organ which has contained, protected, and nourished it; and during this expulsion, known by the name of labor or parturition, in order to come to the light, it is obliged to trav- erse the pelvis, which is not accomplished with- out pain. But after its birth it still requires for some time the aid of the breasts, organs de- signed only for this function, which by the pro- cess of lactation furnish the necessary elements for its nutrition. The development of these phenomena is preceded and prepared for by particular modifications which the ovarian ves- icles undergo. A bloody periodical discharge, called menstruation, of which the uterus is the source, ordinarily coincides with this modifica- tion of the ovarian vesicles. Obstetrics then includes : 1, anatomy, the organs which con- cur to the execution of the acts above de- scribed ; 2, the physiological phenomena, the mode in which these acts are performed ; and 3, the pathology of these organs. As regards the anatomy, it is sufficient here to say that the female organs subservient to generation are the ovaries, the principal function of which is the production of the ovule or germ; the Fallopian tubes, designed to receive the ovule and conduct it into the cavity of the uterus ; the uterus or womb, a kind of receptacle, whose office it is to contain the fecundated germ du- ring its period of development, and to expel it immediately afterward ; and finally the va- gina, a membranous canal extending from the neck of the uterus to the external organs. Most of these organs are situated within a large cavity, the walls of which are composed of bones and soft parts ; this is termed the cavity of the pelvis. The functions of these organs are menstruation, conception, gestation or preg- nancy, and labor or parturition. These func- tions are limited to certain periods of life, usu- ally from about the 15th to the 45th or 48th year. The generative faculty in women coin- cides with the function of menstruation. The
