Page:The New International Encyclopædia 1st ed. v. 16.djvu/124

* PLASMOLYSIS. »6 PLASTERS. by the osmotic presjuie of the substances dis- solved in the water occupying the vacuoles that it is usually stretched until its resilience equals the osmotic' pressure within (Fig. 1). If such a cell be surrounded by a solution having an os- motic pressure greater than that of the cell sap, water will be withdrawn from the latter, the pro- toplasm will shrink, and the cell wall, by its elasticity, will follow the shrinking protoplasm until it "has reached its unstretchcd dimensions. If the withdrawal of water still continues, the proto- plasm will continue to shrink, but the cell wall may he strong enough to maintain its form. Thus the" protoplasm will he withdruwn from contact with the wall, a condition ordinarily taken to indicate the beginning of plasmolysis, but in reality a later stage (Fig. 2). If the cell wall he very thin it may collapse upon the shrinking protoplasm, folding and wrinkling irregularly. Should withdrawal of water continue, the proto- plasm may shrink up into a small mass, the vacuole disappearing completely. When the amount of protoplasm in the plasmolyzed cell is relatively small, the protoplasm may happen to divide into two or more parts, a possibility which is taken advantage of for experimentation upon the functions of the nucleus. See Osmosis; TfR(ii>l{. PLASSEY, plas'sf, or PLASSI. A small town of liritish India, on the Bhagirathi River, 75 miles north of Calcutta (Map: India. E 4). It is celebrated for the great victory gained by Clive ((j.v.) oyer the numerically superior forces of Sirajud-Daula. Nawal) of Bengal. .June 23, 1757, a victory which laid the foundation of Brit- ish supremacy in India. The actual site of the battle has been washed away by the river. PLASTER. See Ovr.siM ; Blildikg. PLASTER, PLASTERING. See Building, section on 1,'illiiiifi mnl I'lustiriiiy. PLASTER OF PARIS. A cementing sub- stance which is lucpnii d Ijy |);irtial calcination or dehydration of gypsum. This mineral is chemi- cally a hydra ted sulphate of lime and contains 4(5.0 per cent, of sul|)luiric acid, .32.5 per cent, of lime, and 20.9 per cent, of water, correspond- ing to the formula CaSO,+ 2HjO. When heated to a temperature above I28''C. but less than 300°, gvpsum loses a definite portion of its water corresponding to one and one-half equiva- lents, and then has the composition (CaSOj + HjO. The material thus treated is known as plaster of Paris, or commonly plaster, and has the property of hardening or setting when water is added. If the calcination is conducted at a heat greater than 300°, however, the gj'psum loses all of its water and becomes inert. The set of plaster of Paris is due to its uniting with water again to form gypsum which assumes a crystalline form. In the manufacture of plaster of Paris, the gypsum may be quarried and burned directly in kilns, which is the method usually practiced in Europe, or the g^'psum may be ground to a fine powder and then calcined in kettles, this being the general practice in the United States. The improved form of kiln consists of a cylindrical chamber with brick or stone walls and a fire-pit at the bottom. The gypsum blocks are loosely placed around and above the fircpit so that the gases may circulate through all parts of the chambers before passing out of the top of the kiln. Coal is used as fuel. In the plaster mills of the United States the gypsum is ground in a bulirstone mill or some form of disintegrator and is then run into a calcining kettle set in stonework. The kettle has a diameter of about eight feet and holds about seven tons of ground gj'psum. During the process of calcinati(m the powder is stirred by revolving arms so that the whole mass may be l)urned evenly. Coal and oil are used as fuel. For the purpose of delaying the set of gypsum plaster, which ordinarily re- quires from six to ten minutes, it is mi.xed with some form of 'retarder." such as glue, soda, sor- ghum, etc., and is then sold as 'cement wall plas- ter.' The time re<iuired for hardening these spe- cial plasters varies from two to six hours. The principal uses of plaster of Paris are for the manufacture of stafl, which is employed in the construction of temporary buildings, and for Avail plaster. Owing to its greater hardness and durability, it is much superior to lime for con- struction purposes. The jiroduction of plaster of Paris in the United States in I'.lOl was 412.1)08 short tons, valued at .$1,317,570, the greater part being made in Kansas, Iowa, Michigan, and New- York. The domestic industry is exceeded only liy that of France. Consult (Jrimsley, "Technology of Gypsum," in The Mineral Industry, vol. vii. (Xew York, 1899). See Gypsum. PLASTERS ( AS. plaster, OHG. pflastar, Ger. r/laslir, from OF. piastre, plaietre, emplastre, Fr. pU'itrc, rmpldtre, from Lat. eiiiplaslnnn, from Gk. f/xTrXaiTTpoi', c»i pUislron, fiLTrXaarov, empUistim, plaster, from i/i-Xi^aeiu, eniplassein, to plaster, from in, en, in + Tr.a<rtLv, phissein, to form). Tenacious preparations for external application, solid at ordinary temperatures, but pliable and adhesive at the temperature of the body. ^lost plasters have as their base a compound of olive oil and litharge, constituting the lead plaster of the pharmacopoeia ; others owe their consistence and adhesiveness to resinous substances, or- to a mixture of these with wax and fats. These sub- stances, alone or impregnated Avith medicinally active materials, are spread in a thin layer upon linen, muslin, or leather. Formerly this spread- ing was done by hand, but of late years the work is accomplished entirely by machinery, which dis- tributes the plaster mass with great evenness, and perforates or makes porous the finished product. Since the introduction of machinery india-rubber has been largely employed in the adhesive composition. There are thirteen plas- ters recognized by the United States Pharmaco- poeia : lead, ammonia and mercury, iron, mercury, opium, soap, resin, arnica, capsicum, belladonna, pitch, isinglass, and cantharides, besides many unolhcial varieties. Besides their generally pro- tective properties, these plasters have special therapeutic actions dependent upon the active ingredient. Opium plaster, for example, is ano- dyne, as is also belladonna; lead and arnica are soothing and astringent; mercury, alterative; capsicum is used as a counter-irritant, and can- tharides as a blistering agent. Court plaster consists of a solution of isinglass thinly spread upon silk, previously coated with tincture of ben- zoin. It is employed as a dressing to protect slight cuts or abrasions and must be moistened before application. Gold-beaters' skin is simi- larly used. In surgery adhesive pla.ster is exten- sively used for its mechanical cflfect to reenforeo