POSTING 326 POTATO DISEASE tain truths, the more successful Post- Impressionists have remained faithful to the traditions of the old masters, blend- ing as far as they can what is new and true to the older principles of the arts. POSTING, traveling by means of horses hired at different stations on the line of journey, a system established in England as early as the reign of Ed- ward II. POSTMASTER-GENERAL, the chief of the postoffice department. POST MILL, a form of wind mill so constructed that the whole fabric rests on a vertical axis, and can be turned by means of a lever. POST MORTEM, after death, as a post-mortem examination, i. e., one made after the death of a person, in order to ascertain the cause of death either in the interests of science, or for the ends of justice. POST OBIT, a bond given as security for the repayment of a sum of money to a lender on the death of some speci- fied person, from whom the borrower has expectations. Such loans in almost every case carry high, if not usurious, rates of interest, and generally the bor- rower binds himself to pay a much larger sum than he receives, in consideration of the risk which the lender runs in case of the borrower dying before the person from whom he has expectations. POST-PLIOCENE, in geology, in the etymological sense, more modern than the Pliocene, i. e., embracing all the de- posits from the end of the Pliocene till now; but Lyell, who introduced the term, restricts it to the older of these, applying the term Recent to the others. POSTULANT, one who asks, demands, or requests; a candidate; specifically, in the Roman Church, one seeking admission to a religious order or congregation. The postulant is bound by the rules of the order to which he or she is seeking admission, but does not wear its dis- tinctive dress till the habit is conferred. POSTULATE, a position, supposition, or proposition assumed without proof, as being self-evident or too plain to re- luire proof or illustration; a thing as- sumed for the purpose of future reason- ing; an _ assumption. In geometry, the enunciation of a self-evident problem. POTASH, a term applied to the hy- drate of potassium, KHO, either in the liquid or solid state, but sometimes used to denote potassium oxide and also crude carbonate of potassium. Potash salts are essential constituents in the human body, but if, when wasted, they are sup- plied directly to the blood they are very poisonous. A much diluted solution of potash is antacid and sedative in dys- pepsia and cutaneous diseases, also in pleuritis, pericarditis, scrofula, etc. Caustic potash is used externally as a caustic in ulcers, etc.; carbonate of potash has been given in whooping cough; acetate of potash, nitrate of potash, and, in small doses, tartrate of potash are diuretics; acid tartrate of potash is purgative and used in dropsy; citrate of potash is diuretic and febrif- ugal; sulphate of potash is a mild pur- gative generally given with rhubarb, etc.; nitrate of potash and chlorate of potash are refrigerants and diuretics. POTASH LIME, a mixture of dry hy- drate of potassium and quicklime em- ployed in estimating the nitrogen con- tained in organic substances. At a high temperature, it liberates the nitrogen in the form of ammonia. POTASH WATER, an artificial aerated water containing a minute quan- tity of potassic bicarbonate. POTASSIUM, symbol, K; at. wt., 39, a monad metallic element, discovered by Da'y in 1807, and very widely diffused through the vegetable, mineral and ani- mal kingdoms. It may be obtained by electrolysis, but is now produced in large quantity by distilling in an iron retort an intimate mixture of charcoal and car- bonate of potassium, a condition readily obtained by igniting crude tartar in a covered crucible. It can only be pre- served in the metallic state by immersing it in rock oil. POTATO, or POTATOE, Solamim tuberosum, a well-known plant, the tubers (dilated branches) of which are eaten. It is a native of Chile and Peru. Some think that it was first brought to Spain from the mountains near Quito early in the 16th century. Thence it spread to Italy and Austria. Sir Walter Raleigh is supposed to have taken it to England in July, 1586. For the next century and a half they were regarded as garden plants only. They gradually made way to the important position which they now occupy in agriculture. POTATO DISEASE, a disease or mur- rain produced by a fungus, Peronospora infestans. It generally first attacks the leaves and stems of the plant, forming brown spots on them in July and August. By this time the fungus, which first penetrated the tissue of the leaf, has thrust forth through the stomates its conidia-bearing filaments. The leaves soon afterward die. Next the tubers
