Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/117

X Y X Y 105 in the mile ; the length of channel from the town of Urganj near Khwaja-ili to the Caspian is about 600 miles, with a fall of (say) 300 feet, or also about 6 inches to the mile. Thus the degree of slope is much the same in both directions, and consequently the blocking of the channel towards one sea either naturally as by an accidental deposit of silt, or artificially by the construction of dams for the diversion of the river would most probably be soon followed by a flow of water towards the other sea. The writings of Strabo, Pliny, and Ptolemy indicate that from 500 B.C. to 600 A.D. the Oxus flowed into the Caspian. About 605 a great change is said to have taken place, which turned the full stream of the Oxus into the Aral. In subsequent years dams were constructed for irrigation purposes which prevented the stream from reverting to the Caspian. In 1221, during the siege of Urganj by the Turks, the dams were purposely broken down, and the stream was allowed to find its way back to the Uzboi, which had been deserted for several centuries. But by 1643 the Oxus is said to have been again debouching into the Aral, as at the present time. Authorities. Colonel Yule s " Essay " in Wood s Oxus, 2d ed. ; Id. , " Papers connected with the Upper Oxus Regions," in Jour. Roy. Geog. Soc., xlii. ; Sir Henry Rawlinson, England and Russia in the East ; Id., Review of Yule s " Marco Polo," in Edin. Rev., January 1872; Id., "Monograph on the Oxus," in Jour. Roy. Gcog. Soc., xlii.; Id., "Notes on the Oehus," in Proc. Roy. Geog. Soc., xx. ; Id., "Road to Merv," in Proc. Roy. Geog. Soc., March 1879; Price, Mahomedan History ; Lonz, Ancient Course of the Amu- Daria, translated from German by C. G. ; Arendarenko, Darwdz and Karateghin, translated from Russian Military Journal by R. M. ; General Walker, Map of Turkestan, 6th ed., 1883; "The Russian Pamir Expedition," in Proc. Roy. Gcog. Soc., March 1884. (J. T. W.) OXYGEN". See CHEMISTRY, vol. v. p. 479 sq. OXYHYDROGEN FLAME. Hydrogen gas readily burns in ox-ygen or air with formation of vapour of water. The quantity of heat evolved, according to Thomson, amounts to 34116 units for every unit of weight of hydrogen burned, which means that, supposing the two gases were originally at the temperature of, say, C., to bring the hot steam produced into the condition of liquid water of C., we must withdraw from it a quantity of heat equal to that necessary to raise 34116 units of weight of liquid water from to 1 C. This heat-disturbance is quite independent of the particular mode in which the process is conducted ; it is the same, for instance, whether pure oxygen or air be used as a reagent, being neither more or less than the balance of energy between 1 part of hydrogen plus 8 parts of oxygen on the one hand and 9 parts of liquid water on the other. The temperature of the flame, on the other hand, does depend on the circumstances under which the process takes place. It obviously attains its maximum in the case of the firing of pure " oxyhydrogen " gas (we mean a mixture of hydrogen with exactly half its volume of oxygen, the quantity it combines with in becoming water). It becomes less when the " oxyhydrogen " is mixed with excess of one or the other of the two co-reagents or an inert gas such as nitrogen, because in any such case the same amount of heat spreads over a larger quantity of matter. To calculate the "calorific effect," we may assume that, in any case, for every 1 grain of hydrogen burned 9x637 = 5733 units of heat are spent in the conversion of the 9 grains of liquid water (theoretically imagined to be) produced into steam of 100 C., and that only the rest of 341 16 - 5733 = 28383 units is available for heating up the products of combustion. Now the specific heat of steam (from 120 to 220 C.) has been found to be equal to 4805 units ; hence, on the basis of certain obvious (but bold) assumptions, in the firing of 9 grains of oxyhydrogen gas, as every 9 x 4805 units of heat correspond to an increase of 1 C. in temperature, the temperature of the flame should be by 28383 + 9 times 4805 (or 6564 C.) higher than 100, or equal to 6664 C. Let us now consider the case of 1 grain of hydrogen mixed with the quantity of air containing 8 grains of oxygen, i.e., the case of 1 grain hydrogen mixed with 8 grains of oxygen and 26 7S grains of nitrogen. Here the temperature t of the flame will be governed by the equa tion, 28383 = (t - 100) x 9 x 0-4805 + 1 x 26 78 x 0-2438, the last coefficient being the specific heat of nitrogen. Thus = 2655 C., as against the 6664 obtained with pure oxygen. But one of our tacit assumptions is obviously untenable ; ready-made vapour of water, if subjected to even the less of the two temperatures, would suffer far-going dissociation involving an absorption of heat and consequently a depression of tempera ture. Hence supposing a mass of oxyhydrogen gas to have been kindled, as soon as the temperature has passed a certain point the progress of the process of combina tion will be checked by that of the corresponding dis sociation, which latter, as the combustion progresses, will go on at a greater and greater rate, or until it just com pensates the effect of the process of combination. That is to say, as soon as through the combustion of a certain fraction of the oxyhydrogen a certain temperature (far less than 6664 3 C.) has been produced, there is no further increase of temperature, and the uncombined gas-residue would remain unchanged, if it were not for the practically unavoidable loss of heat by radiation and conduction, which enables it to become water. This interesting matter was inquired into experimentally by Bunsen. He exploded fulminating gas mixtures in a close vessel constructed so that the maximum tension attained by the gas-contents during the combustion could be observed and measured, and from this value and the analytical data he calculated the maximum temperature and the proportion of gas-mixture which had assumed the form of a chemical compound at the moment when the maximum temperature prevailed. He found () for the case of pure oxyhydrogen gas maximum temperature = 2844 C., fraction of burned gas at the respective moment 337 ; (b) for the case of a mixture of 1 volume of oxygen, 2 volumes of hydrogen, and 3 78 of nitrogen (very nearly the same as one volume of oxygen in the shape of air) maximum temperature = 2024 C., burned gas correspond ing =0 547 of the potential water. Hence we see that the temperature of a pure oxyhydrogen flame is not so much above that produced in the combustion of hydrogen by air as we should have concluded from our calculations. But, whatever the exact numerical value may be, it has long been known that the calorific effect of an oxyhydrogen flame exceeds that of any furnace, and the effect has long been put to practical use in the oxyhydrogen lamp. The most efficient form of this instrument is that which was given to it long ago by Newman, who pumps pure oxyhydrogen into a strong copper reservoir under 2 to 3 atmospheres pressure, lets the gas stream out of a narrow nozzle, and kindles it. The nozzle in the original apparatus consisted of a glass tube about 4 inches long and of ^-iiich bore. Newman worked long with this ap paratus without any accident occurring; but when he once came to substitute a tube of ^-inch bore the flame travelled back and the apparatus burst like a bomb-shell. Of the many safety arrangements suggested we will mention only that of Hare, who inserts a plug of (microscopically) porous copper between reservoir and nozzle, and forces the gas through this plug by applying a sufficient pressure. The plug of course acts on the principle of the Davy lamp, and offers protection as long as it has not got heated. But it may get hot without the operator noticing it, and probably has done so occasionally. At any rate, the use of ready mixed oxyhydrogen has long been given up in favour of the very oldest form of lamp, which was invented, before Newman s, by Hare. Hare s lamp, in all essential points, is our present gas-blowpipe as used for glass-blowing. The fuel (hydrogen, or coal-gas, which works as well) streams out of the annular space between two co- XVIII. --14