infected vessel the sick may immediately be isolated. In large ports he should be provided with a sufficient staff to enable a thorough inspection of every vessel coming to our shores to be efficiently carried out, and he should therefore have a small steamer or other means of locomotion at his disposal. It well admits of argument whether the appointment of medical officers of health to ports ought not to be taken out of the hands of local authorities, as the importation of cholera is a national calamity, and therefore the necessary precaution should not be left to the unequal and sometimes unintelligent action of local authorities.

Port Wine-See WINE.

Post-Mortem Examinations - Any local authority may provide and maintain a proper place (otherwise than at a workhouse or at a mortuary) for the reception of dead bodies, for the purpose of undergoing authorised post-mortem examinations.-(P. H., s. 143.) See MORTUARY.

Potassium (K=39·1)—A metallic element discovered by Sir H. Davy in 1807. Specific gravity, 865; fusing-point, 1445° F. (62-5 C.) This remarkable substance is a bluish-white metal, which at the common temperature of the air is so soft that it can easily be cut by a knife, but at 32° F. it is crystalline and brittle. It has a most powerful affinity for oxygen, rapidly tarnishing in air, and decomposing water, with the production of flame, when thrown upon it.

The salts of potassium are very numerous. They are all soluble in water, and most of them are colourless. They may be recognised in a pure state by the violet hue they impart to the blowpipe flame when heated in platinum wire, by giving no precipitate either with sulphuretted hydrogen or with sulphide of ammonium, and by giving a yellow crystalline precipitate with bichloride of platinum, and a white crystalline precipitate with tartaric acid, when the latter is added in excess to moderately strong neutral or alkaline solutions. We can only here notice a few of the salts of potassium.

Potassium, Arseniate of (KH,AsO), is prepared with arsenious acid and nitrate of potash. It forms large crystals, which are soluble in about 4 parts of water, and insoluble in alcohol. It is employed medicinally, and also to form a resist paste in calico-printing and in the manufacture of cobalt blue. See ARSENIC.

Potassium, Carbonate of (KCO3), often contains an undue quantity of water, as well as silicic acid, sulphates, and chlorides. The

water may be detected by the loss of weight the salt suffers when heated; the silica, by adding to it hydrochloric acid in excess, evaporating to dryness, and igniting the residuum by which the contamination is rendered insoluble. The sulphates and chlorides may be detected by adding nitric acid in excess, and testing the liquid with nitrate of silver and chloride of barium. If the former produces a white precipitate, a chloride is present; and if the latter does the same, the contamination is a sulphate.

Potassium, Chromate of (K¿CrO4), prepared from chrome ore, a natural octahedral chromate of iron found in various parts of Europe and America, and the bichromate of potassium (K2CrO7), prepared from the above, have been extolled by Dr. Angus Smith as being powerful antiseptics, but their price is too great to allow of their being largely employed.

It

Potassium, Cyanide of (KCN or KCy), is a highly poisonous salt, extensively used in photography and gilding. Accidents from its employment are not uncommon. It has a local chemical action upon the skin, and if this be abraded or wounded, it may be absorbed, and produce serious effects. The symptoms produced by the salt are the same as those produced by prussic acid-2 grains being equal to 50 drops of medicinal prussic acid. has been found as an impurity in reduced iron. It may be detected by the tests previously given for potassium, and it gives a white precipitate with nitrate of silver, which, when dried and heated, possesses all the properties of cyanide of silver. If a solution of proto-sulphate of iron is added to a solution of the cyanide of potassium, and after agitation the mixture is treated with diluted sulphuric acid, Prussian blue will be produced.

Potassium, Nitrate of.-Saltpetre (KNO3). Specific gravity, 1925 to 1975.

The salt occurs as an efflorescence on the soil in the East Indies and elsewhere; it is also produced artificially on the Continent by exposing nitrogenous matters mixed with a calcareous earth to the atmosphere. A nitrate of calcium is slowly formed, lixiviated out and decomposed by wood ashes, the main result being that carbonate of lime is precipitated, and nitrate of potash remains in solution, from which it is recovered by evaporation and crystallisation.

Nitrate of potash is a dimorphous salt, its usual form being that of six-sided striated prisms, but also occurring in microscopic rhombohedra. It is soluble in about three and a half times its weight of cold water, and a third of its weight of boiling water; it is insoluble in alcohol. It fuses without decomposition

at 642-2° F. (339° C.), and may be cast into moulds (sal prunelle). If heated to redness, part of the oxygen is expelled, and a deliquescent mass of potassic nitrite is formed; by a yet stronger heat, nitrogen mixed with oxygen escapes, potash and peroxide of potash remaining.

Commercial nitre generally contains chlorides, sulphates, or calcareous salts; the first may be detected by its solution giving a cloudy white precipitate with nitrate of silver, the second by chlorides of barium or calcium giving a white precipitate, and the third by oxalate of ammonium giving a white precipitate.

In a hygienic point of view saltpetre is most valuable for its disinfectant and antiseptic properties; but it is employed in the arts for a variety of purposes, such as manufacture of fireworks, gunpowder, and nitric acid. This salt has on several occasions destroyed life, but only when taken in large doses. An ounce has proved fatal.

Potash, Permanganate of (KMnO4), is made by mixing together certain quantities of chlorate of potash, peroxide of manganese, caustic potash, and a small quantity of water, and afterwards evaporating, &c. This is a valuable disinfectant, but it possesses no antiseptic properties. Put into the foulest waters, it destroys almost instantaneously all disagreeable smell, and will quickly deodorise the most offensive substance. In sick-rooms, &c., where impurities cannot be removed with sufficient speed, permanganate of potash is invaluable. See DISINFECTANTS.

Other important salts of potassium are bromide of potassium (KB) (see BROMINE), iodide of potassium (KI) (see IODINE), and the ferro and ferri cyanides of potassium. These latter are important tests for IRON, &c., which see.

The composition of the ash is remarkable for the great disproportion which exists between the potash and soda salts. The amount of potash is indeed very large, and this disproportion perhaps affords an explanation of the fact that all who use the potato instinctively add salt to it.

Potatoes are deficient in fat, and they do not contain more than 23 per cent. of nitrogenous matter, so that, dietetically speaking, the potato is a carbo-hydrate or starch food, and requires the addition of meat and fat to render it a perfect article of food.

The potato has great antiscorbutic properties, so much so that the addition of potatoes to the diet has been found sufficient to arrest the prevalence of scurvy in prisons where it had before existed.

Potatoes may be preserved for a considerable time by thoroughly desiccating them in an oven or by steam heat. For this purpose the roots, either raw or three parts dressed, are generally first cut into dice of above inch square to facilitate the operation. Under a patent granted to Mr. Downes Edwards, August 1840, the boiled potatoes are mashed and granulated by forcing them through a perforated plate before drying them. The granulated product, beaten up with a little hot milk or hot water, forms an excellent extemporaneous dish of mashed potatoes.(COOLEY.) Parkes recommends that slices of potato be packed in sugar, and also gives the following methods for determining the amount of solids, starch, and the quality :

"The solids can be determined by taking the specific gravity and multiplying it by a factor taken from the subjoined table, the result is the percentage of solids.

"If the starch alone is to be determined, deduct seven from the factor and multiply the specific gravity by the number thus obtained; the result is the percentage of starch.

"If the specific gravity of the potato is below 1068, the quality is very bad; between 10681082, the quality is very inferior; between 1082-1105, the quality is rather poor; above 1105, the quality is very good; above 1110, the quality is best."

A poisonous principle, termed solanine, is said to become developed in the buds and shoots of potatoes that are allowed to grow out on keeping; but no case is recorded, notwithstanding the universal consumption, of poisonous effects arising from the use of such potatoes.

With regard to the cooking of potatoes, the best general method is, without doubt, either to bake or steam them in their skins. Dr. Letheby asserts that when potatoes are peeled and then boiled, the loss in cooking is 14 per cent.; but if cooked without peeling, it is only 3 per cent.

to

The Potato Disease.-In the United States in 1843 a disastrous disease appeared among the potato crops; in 1844 it had reached Canada, and before the end of 1855 it had shown itself in most European countries. Since that date this disease appears have been on the increase, and it resists all efforts to eradicate it. It usually appears in July, August, September, and October, but a few crops have been attacked in May. A mild and moist atmosphere appears to favour the spread of this malady, and no soil is exempt from its attacks, though sloping well-drained soils are always the least affected. An abundance of manure, especially if directly applied, often corresponds with the maximum intensity of the plague; and again, the potatoes only lightly covered by the soil have frequently been those most violently attacked. No variety has been able to resist this disease, though one or two have in a measure succeeded in escaping its influence. It commences in the leaves of the plant, and thence extends from the stem to the tubers. On the surface of the latter brown spots make their appearance, penetrate the substance, and eventually lead to decay.

The disease is caused by a minute fungus called Peronospora infestans. The life-history,

a large portion of which has long been known by the researches of Montague, Berkeley, De Bary, and others, has recently been completed by the discoveries and investigations of Worthington G. Smith. Fig. 68, reduced from a cut in the "Gardeners' Chronicle" (July 17, 1875) to one of Mr. Smith's original papers, will give an idea of the nature and method of reproduction of the fungus. It represents a very fine and successful section of the leaf of a diseased potato highly magnified. A A are the minute hairs always present; B B are the individual cells of the leaf. The former are structures belonging entirely to the healthy plant, whilst the threads and bodies shown at C, D, E, F, and G belong to the fungus, the parasite which preys upon the plant. The fine thread at C is a continuation of the spawn or mycelium living inside, and at the expense of the assimilated material of the leaf. Emerging into the air, the thread ramifies at the tips of the branches and bears fruit, D D. These fruits are termed simple spores, or conidia, because of their dust-like appearance. The conidia are capable of germinating and reproducing the species just in the same way as a seed. A second method of reproduction of the peronospora is shown in the "swarm-spores "E F. These, when moistened artificially, or in nature by dew or rain, set free fifteen or sixteen bodies known

as

zoospores," so named because they exhibit every phenomenon of animal and spermatozoa-like life. They are furnished with two lash-like tails, and move about for half an hour with great rapidity. The zoospores falling upon any portion of the plant, have an extraordinary power of instantly corroding and boring through the cellular epidermis. When movement ceases, the tails (cilia) disappear, the zoospores burst at one end, and protrude a tube which develops into mycelium, producing, as before, the perfect plant. These two asexual methods of reproduction have long been known, but as in both of them the structures are far too delicate to withstand the frosts of winter, it was difficult to account for its winter life until Mr. Smith showed that the third mode of reproduction, already made out in similar species of peronospora, was also to be found in the potato plant. The third form is a true sexual method, perfectly analogous to the reproduction of the higher flowering plants. This third method is the production of eggshaped bodies, about 1 of an inch in diameter, known as "oospores." The oospores are produced by the conjugation of two bodies the one, the male, known as the antheridium (see H, fig. 68), and analogous to the anther of a flower; and the other the