Volltext Seite (XML)
652 THE PHOTOGRAPHIC NEWS. [September 18, 1891. Other papers quickly followed by the same authors. M. Berthelot made the important discovery that the rate of explosion rapidity increases from its point of origin until it reaches a maximum which remains constant, how ever long the column of gases may be. This maximum M. Berthelot states to be independent of the pressure of the gases, of the material of the tube, and of its diameter above a small limit. The rate of explosion thus forms a new physico-chemical constant, having important theoretical and practical bearings. The name “ L’Onde Explosive ” is given by Berthelot to the flame when pro pagated through an explosive mixture of gases at the maximum velocity. While Berthelot, associated with Vieille, was measuring the rate of the “ explosion-wave” for various mixtures of gases, Mallard and Le Chatelier continued the study of the preliminary phenomena of explosion which precede the formation of the “wave.” They showed, by photo graphing on a revolving cylinder—(1) that when a mixture such as nitric oxide and carbon bisulphide is ignited at the open end of a tube, the flame travels a certain distance (depending on the diameter and length of the tube) at a uniform velocity ; (2) that at a certain point in the tube vibrations are set up which alter the character of the flame, and that these vibrations become more intense, the flame swinging backwards and forwards, with oscillations of increasing amplitude ; and (3) that the flame either goes out altogether, or that the rest of the gas detonates with extreme velocity. Again, when a mixture of gases was fired near the closed end of the tube, they found the velocity of the flame regularly increased, as far as their instruments were able to record the rapidly-increasing pace. Mixtures of coal-gas with air, and of fire-damp with air, show phenomena of the first and second kind. Ignited at the open end of a tube, these mixtures burn at a uniform rate for a certain distance, and then the flame begins to vibrate. The vibrations acquire greater orless velocity according to the nature of the mixture and the conditions of the experiment; but the third regime of uniform maximum velocity is not set up. In narrow tubes the explosion soon dies out. The phenomena studied by Mallard and Le Chatelier have been observed on a large scale in explosions in coal mines. It has been noticed that little damage was caused at the source of an explosion, and for a distance varying from fifty to eighty yards from the origin of the flame, while beyond that distance fall of roof, broken tubs, and blown-out stoppings have testified to the violence exerted by the explosion. Great as the destruction is which an explosion of fire-damp and air causes in a mine, it is for tunate that these mixtures do not detonate. Passing on to Berthelot’s researches on the regime of detonation, I will briefly summarise the results he has arrived at. The actual velocities of explosion are com pared by Berthelot with the mean velocity of translation of the gaseous products of combustion, supposing these products to contain all the heat that is developed in the reaction. For instance, we know the total heat given out when hydrogen and oxygen combine. If this heat is con tained in the steam produced, we can calculate what its temperature must be if we know its heat capacity ; and if we know the temperature of the steam, we can calculate the mean velocity with which the molecules must be mov ing. Now Berthelot supposes that the heat is all contained in the steam produced. He assumes that the heat capacity of steam is the same as the sum of those of its constituents; and he supposes, moreover, that the steam is heated at constant pressure. Making these assumptions, he calculates out the theoretical mean velocity of the products of combustion of various mixtures, and finds a close accordance between these numbers and the explosion rates of the same mixtures. He concludes that the ex plosive wave is propagated by the impact of the products of combustion of one layer upon the unburnt gases in the next layer, and so on to the end of the tube at the rate of movement of the products of combustion themselves. If his theory is true, it accounts not only for the extreme rapidity of explosion of gaseous mixtures, and gives us the means of calculating the maximum velocity obtainable with any mixture of gases, but it also affords us information on the specific heats of gases at very high temperatures, and it explains the phenomena of detonation, whether of gases or of solid or liquid explosives. Table I. shows the explosion rates found by Berthelot, compared with the theoretical velocity of the products of combustion. Two points in Table I. favoured the view that Berthelot might have here given the true theory of explosions: first, the close coincidence between the rates of explosion of hydrogen, both with oxygen and nitrous oxide, with the calculated mean velocities of the products of combustion; and secondly, the great discordance be tween the found and calculated rates for carbonic oxide with both oxygen and nitrous oxide. I had previously discovered that pure carbonic oxide cannot be exploded either with pure oxygen or pure nitrous oxide. The dis cordance found by Berthelot was what I should have expected from my own experiments. A consideration of Berthelot’s results, published in full in the Annales de Chemie, led me to think it would be use ful to repeat and extend these experiments. My objects were chiefly—(1) to determine as accurately as possible the rate of the explosion-wave for some well-known mixtures; (2) to measure the rate of the explosion-wave in carbonic oxide with different quantities of steam; and (3) to determine the influence of inert gases on the propagation of the wave. Table I.—Berthdot's Experiments. Gaseous Mixture. Velocity in Metres per Second. 2 Theoretical. Found. H, + 0 Hydrogen and oxygen. , 2830 2810 H, + N.O Hydrogen and nitrous oxide. | 2250 2284 CO +0 Carbonic oxide and oxygen. , 1940 1090 CO + N,0 Carbonic oxide and nitrous oxide. 1897 1106 CH, + 0, | 2427 2287 Marsh gas and oxygen. C,H, + o 6 Ethylene and oxygen. , 2517 2210 c,N, + 0, Cyanogen and oxygen. | 2490 2195 C,H, + o 5 Acetylene and oxygen. | 2660 2482 CO + H, + 0 2 Carbonic oxide, hydrogen, and oxygen. , 2236 2008 (To be continued.)