POTASSIUM

1537

POTENTIAL

money-order may include any amount from one cent to $100. During the fiscal year ending June 30, 1907, the number of money-orders issued was 65,709,919, covering the sum of $77,252,907.

The number of post-offices in the United States authorized to issue money-order in 1908 was between 38,000 and 40,000, of which number 10,000 also were international money-order offices.

The one-cent postal card was first issued in 1873, large numbers of which are now used for brief communications. The two-cent card, with a reply-card attached for the return of an answer, was first issued during 1892.

In 1864 the free-delivery system was introduced into the larger cities, and it has since been extended to all towns and cities having over io,~oo 'nhabitants or doing a post-office business of over $10,000. Provision has since been made for the extension of this system to villages and rural communities, and this has been accomplished to such an extent that there are more than 16,000 rural free-delivery routes. All post-offices whose gross receipts amount to $1,000 or more are divided into three classes, first, second and third, and are filled by presidential appointment; those whose receipts are less than $1,000 constitute the fourth class, and are filled by the appointment of the fourth assistant-postmaster. The salaries of the postmasters appointed by the President vary from $1,000 to $6,000, according to the amount of business transacted. The salaries ˛f fourthclass postmasters are derived from the percentage allowed on the sale of stamps etc.

By the action of an international postal convention, which met at Berne, Switzerland, in 1874, the postage on all letters between the countries included in that union, as well as those that have sin-e joined it, was fixed at the uniform r .pi of five cents on each letter weighing one half-ounce. On Oct. 1, 1908, the letter postage between England and the United States became two cents the half-ounce.

Among other improvements in the United States postal service that have been discussed are postal savings-banks. The reduction of letter postage to one cent is still agitated.

Potassium (p˘-tăs's´-um), a bluish-white metal discovered by Sir Humphrey Davy in 1807. It is the lightest of all the metals except lithium, and, as its specific gravity is only .865, it readily floats on water. In so doing it reacts violently with the water, forming caustic potash and giving off hydrogen, which ignites spontaneously from the heat of the reaction. At 320 F. it is brittle, like glass; at 590 it is soft and malleable; at 144° it becomes liquid. Potassium is widely diffused in the vegetable, animal and mineral kingdoms. Some of the more important potassium compounds are caustic potash

(the hydroxide), the carbonate, the bicarbonate; also potassium chloride, bromide, iodide, nitrate (saltpeter), sulphate, chlorate, cyanide etc. Of these iodide, bromide and chlorate are used for medicines, the others being employed more or less in the arts.

Pota'to, the tubers of Solatium tuberosum, a genus of the nightshade family, which contains about 900 species, distributed almost everywhere, but most abundant in tropical America. Only about 2 5 species are found in North America. The potato, from which numerous cultivated varieties have originated, came from Chile, and is native in the mountains as far north as southern Colorado. The importance of these tubers as an article of food is well-known. It is sometimes called Irish or white potato, to distinguish it from the sweet pot-to, which belongs to a totally different plant The cultivated varieties of potatoes are exceedingly numerous, and additions to the list are being made yearly. That this tub^. is. a modified stem is apparent from the so-called "eyes," which are buds appearing in the axils of minute scales. As is well-known, these buds are able to develop into new plants, so that in planting the tubers care is taken in cutting them to include one or more eyes. In 1906 the acreage of po V - raising in the United States was 3,013,150 acres, and the value of the year'r ,;iop was estimated at $157,547,392. The ═ tal number of bushels in the gross yield was 308,038,382.

Potato=Bug or Bee'tle, a widely known beetle infesting potato-vines In 25 years it spread from the Colorado region to the Atlantic coast and from Virginia to Maine. Up to 1859 this insect fed upon the sand-bur, a wild plant related to the potato, but about that year it began to appear upon the potato vine. It multiplied rapidly and migrated eastward, causing extensive destruction. The second word in its Latin name (Dory-phoradecemlineata) refers to the ten dark lines on the wing-covers.

The orange-colored eggs are laid on the underside of a leaf in a cluster of "hout 50. The dark-red larvŠ hatch within a week, feed for two or three reeks, then crawl under rubbish or burrow ',r u the soil and there pass the pupa state ~; xi remain about ten days. About a month after hatching the adult beetle appears. Then are two or three broods a year, the last brood in the beetle state lying dormant in tho ground during the winter. When warn weather arrives, the beetles come forth in great numbers and fall greedily on the young potato-plants. The pest is successfully combatted by sprinkling plants with paris green mixed with water; or by dusting with this poison mixed with flour or pulverized plaster in the proportion of one pound to 20, used after a shower or a heavy dew.

Potential (pō-těn'shal), a term employed in physics, astronomy and electric engineer-