is described in the article on 

 Chlorates (q.v.). Potassium silicate 

 is dealt with in the article Water- 

 glass (q.v. ). 



Potassium cyanide, when pure, 

 is a white crystalline substance, 

 which will melt without decompos- 

 ing. It is soluble in water, and is a 

 compound of potassium with cy- 

 anogen according to the chemical 

 formula 4 KCN. It is most con- 

 veniently prepared from common 

 yellow prussiate of potash by heat- 

 ing with potassium carbonate. 

 Potassium cyanide has long been 

 known as a solvent of gold and 

 other metals, and has been, and is 

 still, largely used in the art of elec- 

 tro-plating. The successful appli- 

 cation in 1889-90 of the MacAr- 

 thur-Forrest process of extracting 

 gold from its ores by a weak solu- 

 tion of potassium cyanide led to an 

 immensely increased demand for 

 this chemical, with the result that 

 the production increased from 

 under 100 tons per annum to over 

 6,000 tons in a few years. The sub- 

 stance is still used in the gold 

 industry, but is being largely 

 displaced by the more conveni- 

 ently produced sodium cyanide. 

 Potassium cyanide is also used in 

 photography. It is a powerful 

 poison. See Carnallite ; Cyanide 

 Process ; Nitrate ; Pearl Ash ; 

 Saltpetre. 



Potato. (Solarium tuberosum). 

 Tuberous rooted perennial plant 

 of the natural order Solanaceae. 

 It is a native of S. America ; it is 

 said to have been introduced from 

 Peru into Europe by Spaniards 

 early in the 16th century and into 

 Britain by Sir Walter Raleigh 

 about 1585. Potatoes require a soil 

 which is rich and deep, but at the 

 same time contains sufficient sand, 

 or other light material, to ensure 

 efficient drainage. They should 

 never be grown two years in suc- 

 cession upon the same piece oJ 

 ground, but, in the kitchen garden, 

 should take their place in the 

 three-year rotation with the other 

 root crops. This system of rotation 

 will help to prevent potato disease. 



Animal manure should not b<j 

 dug into ground intended for pota- 

 toes, but a little superphosphate of 

 lime may be applied with advan- 

 tage previous to planting. Wher- 

 ever land is available near the sea- 

 side, potatoes should be planted 

 freely, as seaweed, which is easily 

 obtainable, forms an excellent and 

 inexpensive manure. A S. aspect 

 is the best for early varieties of 

 potatoes, but any ordinary position 

 will suit the main crop varieties. 



Seed potatoes should be planted 

 in Feb., March, and April, about 12 

 ins. apart and 4 ins. deep, with a 

 distance of from 18 ins. to 2 ft. 



between the rows. The later vari- 

 eties, planted in April, may have a 

 little extra space allotted to them. 

 The seed potatoes should be cut up 

 into sections, each containing an 

 eye, and not planted whole, which 

 is a wasteful habit. As soon as they 

 are about 6 ins. in height, the 

 young plants should be earthed up, 

 and the ground should be kept free 

 from weeds by a liberal use of the 

 hoe. The potatoes will be ready to 

 lift as soon as the foliage, or haulm, 

 dies down, and the process of lift- 

 ing should be carried out on a dry 

 day. The tubers should be stored 

 in a cool shed to shelter them from 

 light, air, and frost. 



Potato. 1. Under side, and 2, upper side of affected potato 

 leaf illustrated in healthy condition in 3. 4. Diseased tuber. 



5. Highly magnified portion ol under surface of diseased 

 potato leaf : A, Stomata or pores through which protrude 

 filaments, B, of Phytophthora, bearing sporangia, C. 



6. Highly magnified portion of diseased leaf, similarly 

 lettered to 5, showing phytophtbora in tissues of leaf. 



7. Tuber affected with Spongospora scabies 



There have been over one thou- 

 sand different varieties of potatoes, 

 but, owing to their susceptibility 

 to potato disease, very few sorts 

 are of cultural worth, or market- 

 able value. The secretary of the 

 board of agriculture, Whitehall, 

 S/W., issues from time to time a 

 list of varieties of potatoes which 

 have been tested and found to be 

 disease-resisting up to the time of 

 the publication of the list. 



Potato disease (Phytophthora 



POT-BOILER 



infestans) is a mould-like fungus 

 of the division Phycomyceteae, 

 which in some districts in certain 

 years causes the ruin of the potato 

 crop. The mycelium or vegetative 

 part of the plant consists of minute 

 threads, not jointed or divided 

 into cells, which permeate the cell 

 structure of the potato plant, ex- 

 tending from leaf to tuber. 



In plants hitherto entirely frer 

 from the disease inoculation 

 usually takes place by a spore of 

 the phytophthora settling upon a 

 leaf and, on germination, piercing 

 the green tissue, which then turns 

 black and shows on both surfaces 

 of the leaf. The black patch in- 

 creases rapidly, and around its 

 margin a delicate white mildew 

 forms, from which spores are liber- 

 ated to infect neighbouring plants. 

 The mycelium passes into and 

 right down the stem, entering all 

 the tubers formed and forming at 

 its base. As a rule, but not in- 

 variably, the presence of the disease 

 in the tuber is manifested by 

 brownish patches on the surface 

 and in the flesh. By means of such 

 infected tubers the disease is per- 

 petuated from year to year, and 

 spread from place to place by the 

 sale or exchange of seed-tubers. 

 All suspected tubers should be 

 destroyed by fire, and infected 

 haulm should be at once burnt. 

 See Agriculture. 



-^ Potato Spirit. 



E t h yl alcohol 

 made from 

 potatoes. The 

 potatoes are 

 heated by means 

 of steam to a 

 temperature of 

 140-150 C., 

 under a pressure 

 of from two to 

 three atmo- 

 spheres. The 

 mass is then 

 removed and 

 mashed with 

 malt at a tem- 

 perature of from 

 57-60 C., dur- 

 ing which pro- 

 cess the starch 

 of the potatoes 

 i s converted 



into sugar. The mash is then fer- 

 mented with yeast to form ethyl 

 alcohol, which is separated by 

 distillation. See Distilling. 



Pot-boiler. Slang term for a 

 picture painted, or an article or 

 book, written simply for the sake 

 of obtaining money. It is a work 

 that deliberately falls short of the 

 artist's or writer's standard, but 

 will fetch a price in a particular 

 market, and so help, figuratively 

 speaking, to " boil the pot." 



