THALES. 180 THAMES. possessed sufficient astronomical knowledge to en- able him to predict the eclipse of the sun in the reign of the Lydian King Alj'attes, and was an excellent mathematician. He was famed also for his practical and political wisdom, but his chief renown is due to the fact that he was the first man among the Greeks to set aside the current explanations of the universe and to look for a first principle which might be grasped through reason. This first principle {apx'/) he regarded as water, which was the source of all things, and that into which all things were resolved. The importance of his speculation lies in the fact that he turned men to reasoning about the world and to an ettbrt to find the significant principle which lay behind the varied phe- nomena. Thales left no writings, and even among the Greeks considerable doubt prevailed as to his exact teachings. Consult: Ritter and Preller, Bistoria PhiloxophUe (jrcecw (7th ed., Gotha, 1888) ; Zeller, Philosophie der Griechen, vol. i. (4th ed., Berlin, 1900) ; Ueberweg, His- tory of Philosophy, Eng. trans., vol. i. (New York. 1887). THALI'A (Lat., from Gk. Qaleia, Thaleia, from tid'Aeia, thaleia, luxuriant, blooming, from OdlAeiv, thallein, to bloom ; connected with Alb. dal, I bloom ) . One of the nine iluses ( q.v. ) . In the late assignment of specific functions to the Muses, she became the IMuse of Comedy, and ■was represented as holding the comic mask. THALLIUM (Neo-Lat., from Gk. ftaU^, thallos, green shoot). A metallic element dis- covered by Crookes in 1861, and independently by Lamy in 1862. It was originally found by the bright green line which its compounds give when examined by the spectroscope. It occurs as the selenide in combination with copper and silver, in the rare mineral crookesite, in small quantities in copper and iron pyrites, and in the seleniferous deposits from sulphuric acid ; also in minute quantities in other minerals, and in certain mineral waters. It may be prepared from the flue-dust of sulphuric acid works in which pyrites containing thallium are burned, by treat- ing the dust with water acidulated with sul- phuric acid, concentrating the resulting solution, and precipitating the thallium by zinc. Thallium (symbol, Tl ; atomic weight, 204.15) is a soft, crystalline, lead-like metal, with a specific grav- ity of 11.8, and a melting point of 290° C. (554° F. ). When exposed to the air it tarnishes rapid- ly, becoming coated with a thin layer of oxide, which preserves the rest of the metal. It com- bines with oxygen to form a monoxide and a tri- oxide, which yield corresponding series of salts, known as the thalloiis and thallic compounds. The compounds of thallium are very poisonous, producing symptoms like those of lead poison- ing. Their presence in a given substance may be readily detected by the beautiful green color which they impart to a non-luminous gas flame. THALLOPHYTES (from Gk. dandc, thallos, green shoot, from 6a?i^civ, thallein, to bloom -|- i^vTov, phyton, plant). One of the four great divisions of the plant kingdom, containing the alg* and fungi (qq.v. ). In theory the thallo- phytes are distinguished from the other groups by having a thallus body. As a matter of fact, there are many thallophytes with upright stem- like axes, root-like regions of attachment, and lateral members which function as and resemble leaves, although not at all homologous with the leaves of higher plants. The best examples of these are found among the Phasophyce*, Rhodo- phyees, and Charales (qq.v.). The thallophytes are readily distinguished from the bryophytes aljove them by the absence of the peculiar fe- male sexual organ (archegonium. q.v.). and the sexless phase or plant ( sporophvte. q.v. ) char- acteristic of the bryophytes and pteridophytes. They are, therefore, defined more through the absence of structures foxinil in the higher plants than by an}- agreement among themselves. THALLUS (Lat.. from Gk. 8aA?.(5f, green shoot). A plant body whose vegetative struc- ture is not differentiated into such organs as .stem, leaf, etc. Most of the algae and fungi (thallophytes, q.v.) have thallus bodies. Many liverworts, the sexual phase (gametophyte) of ferns, and even certain reduced and simplified flowering plants such as the duckweed (Lerana), present a thalloid structure. On the other hand, certain thallophytes, especially among the brown and red alg*, have bodies differentiated far be- yond the limits of the definition of a thallus. THAMES, temz. The most important river in Great Britain, flowing with a southeastern trend through the southern portion of the Kingdom and passing through London (Map: England, G 5 ) . Its remotest source, the Churn, rises on the southeast slope of the Cotswold Hills, in Gloucester, five miles south of Cheltenham, and after flowing southeast for 20 miles receives the Thames, or Isis, from the west. The Thames then flows east-northeast for about 35 miles, and curving southeast, passes Oxford and flows on to Reading, where it again changes its course to a generally eastward trend. A few miles be- low CJravesend it expands into a wide estuar}', and enters the North Sea. The length of the Thames is about 250 miles. At London Bridge the width of the river is about 290 yards: at Woolwich, 490 yards ; at Gravesend pier, 800 yards: 3 miles below Gravesend, 1290 yards; at Nore Light, 6 miles ; and at its mouth, betw'eeu Whitstable and Foulness Point, about 8 miles below the Xore; the estuary is 18 miles across. The river is navigable for barges to Lechlade, up- ward of 200 miles above its mouth, and it is con- nected with several important canals, affording communication with the west and south coasts, and with the interior of the country. Its upper reaches and islands are much resorted to for their sylvan beauty, and pleasure steamers ply daily between Kingston, Henley, and Oxford. Vessels of 800 tons can reach Saint Katharine's docks, London, those of 4000 tons can ascend to Blackwall, while by the dredging operations that have been carried out, vessels of 22 feet draught are now accommodated in the Albert and Victoria docks. The volume of trattic on the river was estimated in 1900 at 30.000.000 tons. The Thames is also the main source of the water supply of London, as much as 130.000,000 gal- lons a day being withdrawn near Hampton for this purpose. The part of the river immediately below London Bridge is called the Pool, and the part between the bridge and Blackwall is called the Port. Two embankments have been formed, one on the north shore from Blackfriars Bridge to Westminster, and one on the south shore froin Westminster Bridge to Vauxhall. Among the
