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THERMOPYLÆ 

The principle of the mercurial thermometer is that mercury when heated expands more than glass. If, therefore, a glass-tube having a bulb blown at one end be partially filled with mercury and immersed in a bath at a higher temperature than its own, the mercury will rise in the tube. If the bath have a lower temperature, the mercury will fall. If the mercury neither rise nor fall we may fairly conclude that the mercury and the bath are each at the same temperature. The essential features of a good thermometer are that it must be easily portable; permanent; always give the same reading when subjected to the same temperature; render it possible for the user to test the correctness of its graduation and determine any errors in its graduation; and be relatively small, so that when placed in contact with a second body the temperature of the second body will not be seriously affected.

In making a thermometer which will satisfy these conditions, the peculiar advantages of mercury are that it is easily prepared in a pure state; does not wet glass or stick to it; expands rapidly with changes of temperature, so that its changes in volume are easily read; that to each particular volume corresponds a definite temperature, which is not the case with water; and that it does not freeze except at temperatures comparatively low, and does not boil except at temperatures comparatively high.

The accompanying figure illustrates the four principal steps in the manufacture of an

Four stages in the manufacture of a mercurial thermometer. ordinary mercurial thermometer. First, a piece of thick-walled, capillary tubing of uniform bore is selected; a bulb is then blown on one end  of this tube, after which the bulb is filled with mercury. It is then highly heated and “sealed off.” The last step is the graduation of the tube of the thermometer.

Now it happens that there are two temperatures which are easily reproduced in the laboratory. These are the temperatures of melting ice and boiling water. On the centigrade scale the first of these is called 0° and the second 100°. The apparent change of volume between these two points is divided into 100 equal parts, each indicating one degree. On the Fahrenheit scale the interval between melting ice and boiling water is divided into 180°, the lower temperature being called 32° and the higher temperature 212°. For the gas thermometer the student is referred to Preston's Theory of Heat, Sec. II. See.  Thermopylæ, a famous pass leading from northern to central Greece, between Mt. Œta and an impassable marsh on the edge of the Maliac Gulf. The pass narrows at each end to the width of a single wheel-track, forming the eastern and western gates. Here (q. v.) of Sparta made his famous stand against the mighty army of Xerxes in 480 B. C. Learning that a Thessalian had betrayed to the Persians a path over the mountains to his rear, he sent away all his troops, which numbered some 7,000, except 300 Spartans, 700 Thespians and 400 Thebans. This chosen band made a sally and perished to a man. But one Spartan who was too sick to fight, escaped, only to meet the scorn of his countrymen on his return home.  Ther′mostat, Elec′trical, is an instrument for giving an alarm when the temperature rises to a point where the instrument completes the circuit, or when it falls to a point to produce a similar result. They are used in stoves, hothouses, dwellings and wherever uniform temperature is of vital importance. They are also used as maximum thermometers or fire-alarms. Various devices are employed, the simplest being the ordinary mercurial thermometer, the upper end of the tube not being hermetically sealed, but closed by an airtight cork of rubber, through which passes a metallic rod that can be raised or lowered in the tube. Whenever the expanding mercury reaches the tip of the rod, an electric current is closed and the alarm sounded. This alarm-current may be arranged so as to set off an electromagnet which will open or close a register or regulate the draft in the furnace. See.  Thermotropism, the sensitiveness of a plant to heat coming from one side, in response to which the rate of growth is so unequally affected as to cause the part to bend toward or away from the source of heat; e. g., corn-seedlings and hyacinth-leaves bend toward a warm iron-plate (30°C) placed near them; cress-seedlings turn away from it. See and .  Theseus, a famous Greek hero, said to have been the son of Ægeus, king of Athens. His most noted exploit was his offering himself as one of the youths yearly delivered to the Cretan minotaur, for the purpose of killing that monster. Ariadne, daughter of King Minos of Crete, fell in love with him and gave him a sword and a clue of thread, with which he killed the minotaur and escaped from the famous labyrinth. The fatal ship which carried the tribute youths always carried black sails. Theseus had promised his father to hoist white sails if successful in his undertaking. This was forgotten, and the king, thinking his son killed, threw himself into the sea. Theseus became king. He is said to have waged war