698 EPSOM EPSOM SALT drives them frantic to the nearest pool. Birds, too, are nearly all infested by lice, each species generally supporting its own species of para- site, and sometimes more, which lives upon 1. CEstrus Bovis. 2. Larva. 3. Chrysalis. the feathers and blood of its host. Insects also, fortunately, are made a dwelling place by other insects, and thus their rapid growth and the consequent destruction of vegetation held in check. Ichneumon is the name given to these unnatural parasites. They are small flies with slender bodies, and there are many species known, probably as many as of cater- pillars and moths. The female deposits her eggs in the larvae, pupse, or eggs of other in- sects and spiders. When she has found her proper host, a caterpillar for instance, she seizes it, and deposits her egg in the skin be- hind the head. The larva, soon emerging from the egg, eats its way along within the caterpillar, avoiding those parts essential to life, and by the time the latter has become a chrysalis the former is nearly mature. It lies quiet for a time to undergo metamorphosis, and awaking once again a perfect animal, bores its way out from the cocoon of its murdered host, and flies forth in quest of fresh victims. Thus it is that nature keeps iu check its most destructive creatures by means so insignificant and unseen. See Baird, "British Entomos- traca" (Ray society, London, 1850); Brant and Katzeburg, Medicinische Zoologie (1833); Burmeister, "Manual of Entomology," trans- lated by Shuckard (London, 1836), and Genera Insectorum (Berlin, 1833-'46); Newman, "His- tory of Insects" (London, 1839); Westwood on "Insects" (2 vols., London, 1839), and bibliography therein contained ; Denny, Mono- graphia Anoplurorum Britannia (London, 1842); Siebold, "Anatomy of Invertebrate," translated by Burnett (Boston, 1854) ; WedPs Grundzuge der pathologischen Anatomie (Vi- enna, 1854); and Kiichenmeister, "Manual of Parasites," translated for the Sydenham society (London, 1857). EPSOM (Sax. Ebt>sham a market town of Surrey, England, on the margin of Banstead downs, 13 m. S. W. of London; pop. in 1871, 6,276. At one time it promised to become a prominent watering place, in consequence of the discovery of medicinal springs impregnated with sulphate of magnesia, from which the celebrated Epsom salt was manufactured. The springs are no longer visited, but the town has gained another attraction in the great annual races held during the week preceding "Whit- suntide on the neighboring downs. They are attended by nearly 100,000 persons of every class of society, and the grand stand on the race course, erected in 1829-'30, will hold 7,500 persons. The chief excitement centres in the race for the Derby stakes, instituted in 1780, which takes place on Wednesday. EPSOM SALT, the name given in pharmacy to the hydrated sulphate of magnesia, which was obtained as far back as the year 1675 by evaporating the waters of some mineral springs at Epsom, England. Sea water was afterward found to contain it, the brine remaining after the separation of the common salt consisting of the sulphate of magnesia and the chlorides of magnesium and calcium. It was readily obtained by collecting the first crystals which formed, and washing them with a strong solu- tion of the same salt. An excellent quality is manufactured at Baltimore and Philadelphia, from the mineral magnesite, a silicious hydrate of magnesia, which is found in the serpentine of that region. The mineral, reduced to pow- der, is dissolved in sulphuric acid. The pro- duct after drying is calcined in order to de- compose the sulphate of iron, and convert it into the peroxide of iron. It is then dissolved in water, and any iron present is precipitated by sulphuret of lime. The crystals of sulphate of magnesia are separated and dissolved again to complete their purification. This salt, and calcined magnesia also, have been prepared from the dolomite or magnesian carbonate of lime, by the process of Mr. William Henry of Manchester. The mineral was calcined, and the lime and magnesia were then converted into hydrates by sprinkling with water ; the former was dissolved out by a minimum quan- tity of hydrochloric acid, and the latter was converted into a sulphate by sulphuric acid. Epsom salt is also found as a mineral sub- stance, incrusting the walls of caves, in the form of an efflorescence, and also in silky fibres. In the Mammoth cave, Kentucky, loose masses of it adhere to the roof like snow balls, and in many other caves of the western states it is found upon the walls or mixed with the earth upon the floor. It occurs in some of the gypsum quarries near Paris, and in other parts of France ; and wherever water becomes charged with gypsum or sulphate of lime, and flows over rocks containing carbon- ate of magnesia, the sulphate of magnesia is likely to appear from the result of mutual de- composition of the two salts. Hydrated sul- phate of magnesia consists of 1 equivalent of magnesia, 20 ; 1 of sulphuric acid, 40 ; and 7 of water, 63 = 123 ; or magnesia 16-26 per cent., acid 32'52, and water 51-22. It crys- tallizes in four-sided prisms with reversed di- hedral summits, or four-sided pyramids, of hardness 2-25, and specific gravity 1'75. The crystals effloresce slightly in the air, and if
