have felsitic bands alternating with others which are purely glassy.
In Arran there are pitchstone dikes, some of which are very completely
vitreous, while others are changed to spherulitic felsites more
or less silicified. The pitchstone of the Scuir of Eigg is at its margins
characterized by a dull semi-opaque matrix which seems to be the
result of secondary devitrification. In the same way artificial glass
can be devitrified if it be kept at a temperature slightly below the
fusing point for some days. Window glass exposed to alkaline
vapours often shows a thin iridescent surface film which is supposed
to be due to crystallization; the same change is found in pieces of
Roman glass which have been dug out of the ruins of Pompeii.
Obsidians occur in many parts of the world along with rhyolites and pumice. In Europe the best-known localities for them are the Lipari Islands, Pantellaria, Iceland and Hungary. Very fine obsidians are also obtained in Mexico, at the Yellowstone Park, in New Zealand, Ascension and in the Caucasus. Included in this group are some rocks which are more properly to be regarded as vitreous forms of trachyte than as glassy rhyolites (Iceland), but except by chemical analyses they cannot be separated. It is certain, however, that most obsidian's are very acid or rhyolitic. The dark, semi-opaque glassy forms of the basic igneous rocks are known as tachylytes. The typical obsidians exhibit the chemical peculiarities of the acid igneous rocks (viz. high percentage of silica, low iron, lime and magnesia, and a considerable amount of potash and soda).
The chemical composition of typical obsidian's is shown by the following analyses:—
| SiO2. | Al2O3. | FeO. | Fe2O3. | CaO. | MgO. | K2O. | Na2O. | H2O. | |||
|
74.70 |
13.72 |
0.62 |
1.01 |
0.78 |
0.14 |
4.02 |
3.90 |
0.62 |
Obsidian, when broken, shows a conchoidal fracture, like that of glass, and yields sharp-edged fragments, which have been used in many localities as arrow-points, spear-heads, knives and razors. For such purposes, as also for use as mirrors, masks and labrets, it was extensively employed, under the name of itztli, by the ancient Mexicans, who quarried it at the Cerro de las Navajas, or “ Hill of Knives,” near Timapan. The natives of the Admiralty Islands have used it for the heads of spears. By the ancient Greeks and Romans obsidian was worked as a gem-stone; and in consequence of its having been often imitated in glass there arose among collectors of gems in the 18th century the practice of calling all antique pastes “ obsidians.” At the present time obsidian is sometimes cut and polished as an ornamental stone, but its softness (H=5 to 5.5) detracts from its value. Certain varieties, notably some from Russia, possess a beautiful metallic sheen, referable to the presence of either microscopic fissures or enclosures. The substance known as moldavite, often regarded as an obsidian, and the so-called obsidian bombs, or obsidianites, are described under Moldavite. (J. S. F.)
OBSTETRICS, the science and art of midwifery (Lat. obstetrix, a midwife, from obstare, to stand before). Along with Medicine and Surgery, Obstetrics goes to form what has been called the Tripos of the medical profession, because every person desiring to be registered under the Medical Acts must pass a qualifying examination alike in medicine, surgery and midwifery. The term Gynaecology (q.v.), which has come to be applied to the study of the diseases of the female generative system, in its primary sense includes all that pertains to women both in health and disease. Obstetrics, or midwifery, is more specially that part of the science of gynaecology which deals with the care of a pregnant woman and the ushering of her child into the world.
Tokology—the doctrine of parturition—is the most distinctive sphere of interest for obstetricians, and here their activities bring them into a closer approximation to the work of surgeons. As a science it demands a study of the phenomena of labour, which in their ordered succession are seen to present three distinct stages: one of preparation, during which the uterus dilates sufficiently to allow of the escape of the infant; a second, of progress, during which the infant is expelled; and a third, of the extrusion of the after-birth or placenta. In each of the stages analysis of the phenomena reveals the presence of three elements which are known as the factors of labour, viz. the powers or forces which are engaged in the emptying of the uterus; the passages or canals through which the ovum is driven; and the passenger or body that is being extruded. The mechanism of labour depends on the balance of these factors as they become adjusted to each other in the varying phenomena of the several stages. The diversities that are met with in different labours even of the same woman have led to their being classified into different groups. A natural labour is commonly defined as one where the child presents by the head and the labour is terminated within twenty-four hours. From this it is obvious that no case of labour can be defined at its onset. The relation of the factors may warrant a favourable expectation; but until the labour is completed, and completed within a reasonably safe period, it cannot be classed as natural. The element of time has this importance, that it is found that, apart from all accidents and interferences, the mortality both to mother and child becomes greater the longer the duration of the labour. Hence lingering or tedious labours, in which the child still presents with the head, but is not expelled within twenty-four hours after the onset of labour-pains, are properly grouped in a separate class, although they are terminated without operative interference. In the class of preternatural labours, where the head comes last instead of first, there are two subdivisions, according as the child presents by the breech and feet, or lies transversely as a cross-birth, and has usually to be delivered artificially. Operative or instrumental labours vary according as the procedures adopted are safe in principle to mother and child, such as turning and the application of the midwifery forceps; or as they involve damage to the infant in the various forms of embryotomy; or are more dangerous to the mother, as in the Caesarean section and symphysiotomy. A final class of labours includes the cases where some complication or anomaly arises and becomes a source of danger, independently of disturbances of the mechanism or of any operative interference. These complex labours are due to complications that may be maternal, such as hemorrhage and convulsions; or foetal, such as twins or prolapse of the umbilical cord. To cope with these anomalies an obstetrician requires all the resource of a physician and all the dexterity of a surgeon.
The interest of obstetricians in their patients does not end with the birth of the children, even after natural labours. The puerpera is still a subject of care. The uterus, that during its nine months' evolution had been increasing enormously in all its elements, has in six weeks to undergo an involution that will restore it to its pregravid condition. The allied organs share in their measure in the change, all the systems of the body feel the influence, and especially the mammary glands take on their function of providing milk for the nutriment of the new-born infant. A patient with some latent flaw in her constitution may pass the test of pregnancy and labour with success, only to succumb during the puerperium. Of patients who become insane in connexion with child-bearing, a half manifest their mental disorder first during the days or weeks immediately succeeding their confinement, and numbers more whilst they are suckling their infants. A woman may have had an easy labour, and may have been thankful at the time for help from a hand that she did not know to be unclean; three days later germs left by that hand may have so multiplied within her that she is in mortal danger from septicaemia. The management of the puerperal patient requires not only the warding off of deleterious influences, but the watching of the normal processes, because slight deviations in these, undetected and uncorrected now, may become later a source of lifelong invalidism. It remains further to be noted that to obstetricians belong the earliest stages of pediatrics in their care of the new-born child. In some old works practitioners of this branch of the profession are described as δμφαλστόμοι, because their first business was to cut the umbilical cord. The causes of the high death-rate among infants, whether due to ante-natal, intra-natal or neo-natal conditions, come under their observation. They have charge of the whole wide field of the hygiene, pathology and therapeutics of infancy.
