Page:Philosophical Transactions of the Royal Society A - Volume 184.djvu/106

Rh measurements of the rate were made at pressures varying from 200 mm. to 1500 mm. at a temperature close to 10° C. In the experiments at low pressures it was, of course, essential that the firing tube should be absolutely gas-tight; the tube was therefore tested before each experiment. An important fact concerning the develop- ment of the explosion-wave was detected in these experiments. As the pressure was reduced the flame was found to travel a greater distance before its rate became con- stant; so that, although four feet start was allowed before the first measurement was taken, this distance was found insufficient. Accordingly a tube 20 feet long was interposed between the firing point and the first bridge, after it had been shown that the flame acquired its maximum velocity in this distance under the lowest pressure used. Electrolytic gas can be exploded under lower pressures than 200 mm., but the flame travels irregularly. At 100 mm. pressure the flame went out in the tube, and at 150 mm. the rates found were not constant.

TABLE VI.- Pressure Experiments. H, + 0. Pressure. 200 mm 300 mm. 500 mm. 760 mm. 1100 mm. 1500 mm. Moan rate 2627 2705 2775 2821 2856 2872

These figures show that the rate of explosion increases rapidly with increase of pressure from 200 mm.; that the rate of increase diminishes, and that the velocity becomes nearly constant at two atmospheres pressure. This effect of pressure on the rate is plainly seen when the results are expressed graphically (see Plate 1).

BERTHELOT'S conclusion that the explosion-wave is independent of the initial pressure of the gases is, therefore, not strictly accurate. At lower pressures the rate falls off, but above a certain crucial pressure, which, in the case of hydrogen and oxygen, seems to be about two atmospheres, the velocity is independent of the pressure.

It will be observed that the mean rate of explosion of hydrogen and oxygen at 1500 mm. pressure is appreciably higher than BERTHELOTS 6—the calculated rate of translation of the steam molecules. But BERTHELOT, in his calculation, has not taken into account the fact that the gases are not at absolute zero to start with. Assuming that he worked at about 13° C., the theoretical rate would be 2900 metres per second.

Analogous results were found in experiments with other mixtures under varying pressures. With hydrogen and nitrous oxide the rate is slower at 500 mm. than at 760 mm., but no increase is found on raising the pressure to 1000 mm. The crucial pressure is in this instance below the atmospheric pressure. The same was found to be the case with marsh gas, ethylene, acetylene, and cyanogen exploded with oxygen.