1226
INDUSTRIAL A N D ENGINEERING CHEMISTRY
various knock inducers and suppressors a t particular stages in the cycle. This is now being done in this laboratory, utilizing an engine with quartz window and synchronous shutter. (0) Laboratory experiments upon the activation and decomposition of hydrocarbon mixtures by radiation in the spectrum from X-rays to radiant heat, utilizing also radiation from the flame itself, as analyzed spectrometrically and spectrobolometrically, reflected by highly polished walls. (7) Adsorption and catalysis studies of fuel mixtures on metal surfaces of various textures and possible catalytic poisoning by knock suppressors. Bibliography
54-Wendt and Grimm, Ind. Eng. Chem., 16, 890 (1924). 55-Weston, Proc. ROY. SOC.(London), 109A, 176 (1925). 56-Wheeler, J . Chem. SOL. (London), ( a ) 106, 81 (1914); (b) lS, 152 (1918); (c) 116,90 (1919); ( d ) 117,903 (1920); (e) 126, 1858, 1869 (1924); tg) 141, 14 (1925). 57-White, Ibid , ( a ) 121, 2561 (1922); (b) 126, 2387 (1924); (c) 117, 48, ( d J 672 (1925). 58-White and Price, Zbid., (a) 116, 1448, ( b ) 1462 (1919). 59-Woodbury, Lewis, and Canby. J. SOC.Automofioe Eng., 11, 209 (1922). 6O--Yamaga, J Faculty Eng. Tokyo I m p . Uniw., 16, 20 (1924). 61-Young and Holloway, J. SOL.Aufomotiwe Eng., 16, 315 (1924). ~~
The following references, almost entirely to the most recent work, together with ninety others listed in a paper by Berl and Fixher,*cover practically completely the more important literature up to July, 1925, upon the experimental side of the phenomena of explosion and detonation: 1-Audibert, Comfit. rend., 178, 1275 (1924). d% B eand Fischer, 2. Elektrochem., 30, 29 (1924). 3-Bone and Wheeler, J. Chem. Soc. (London), (oJ 81, 536 (1902): ( b ) 88, 1074 (1903); (c) Bone and Haward, Proc. Roy. SOL.(London), 101A, 67 (1921). 4-Boyd, Ind. Eng. Chem., 16, 893 (1924). &Brooks, Zbid., 17, 752 (1925). &Brown, Leslie, and Hunn, Zbid., 17, 397 (1925). 7-Bunset1, Ann. Physik., 131, 164 (1867). &Burgess and Wheeler, J. Chem. SOL.(London), ( a ) 99, 2013 (1911); (b) 105, 2596 (1914). 9-Campbell, Zbid., 121, 2454 (1922). IC--Coward and Brinsley, Ibid.. 106. 1859 (1914). 11-Coward, Carpenter, and Payman, Ibid., 106, 27 11914). 12-C.rouch and Carver, Znd. Eng. Chem., 17, 641 (1925). 13-Crowe and Newey, Phil. Mag., 49, 1112 (1925). 14--DeHemptinne, Bull. sci. acad. Toy. Belg., 11, 761 (1902). I b D i x o n , Phil. Trans., ( a ) 184, 97 (1893); ( b ) 200, 326 (1903). 16-Diron and Crofts, J . Chem. .SOL. (London), 106, 2036 (1914). 17-Dixon and Walls; Ibid., 128, 1026 (1923). l&Dixon and Wheeler, Zbid., 111, 1048 (1917). 19-Ellis, Ibid , 123, 1435 (1923). ZO-Ellis and Robinson, Ibid., 147, 760 (1925). 21-Ellis and Stubbs, Ibid., 126, 1957 (1924). 22-Ellis and Wheeler, Zbid., 127, 764 (1925). 23-French, J. SOL.Automolive Eng., 11, 182 (1922). (London), 147, 77 (1925). 24-Garner and Saunders, J . Chem. SOC. 2 6 H a b e r and Hodsman, 2. physik. Chem., 67, 343 (1889). ZB-Hemsalach and Watteville, Compl. r e n d , 146, 748 (1908). 27--Horning, J. SOC.Automotive Eng., 14, 144 (1924). Z&Jorissen and Meuwissen, R C C .trau. chim., (a) 43, 591 (1924); ( b ) 44, 132 (1925). 29-Jorissen and Valisek, Ibid., 48, 80 (1924). 30-La5tte, Compt. rend, ( a ) 176, 1392 (1923); ( b ) 178, 1277, (c) 2176 (1924); ( d ) 179, 1396 (1924). 31-LeChatelier, Ibid., 179, 971 (1924). (0) Trans. A m . Electrochem. Soc., 44, 63 (1923); ( b ) J . A m . 32-Lind, Chem. Soc., 41, 531 (1919). 33-I,upus, 2. ges. Schiess-Sgrengsloffw., 19, 155 (1924) 34--Malinovskii, J . chim. ghys., 21, 468 (1924). 36-Mallard, A n n . mines, 7, 355 (1875). 3&Mason, J. Chem. SOC.(London),123, 210 (1923). 37-Mason and Wheeler, Ibid., ( a ) 111, 1048 (1917); ( b ) 116, 578, (c) 2606 (1919); ( d ) 117, 1227 (1920); (e) 118,1920 (1920); V, 121, 2079 (1922); (g) 126, 1873 UQW. 3&Midgley, J. SOC.Automotioc Eng., 10, 218 (1922). 39-Midgley and Boyd, J . Ind. Eng. Chem., ( a ) 14, 849, ( b ) 894 (1922). 40-Midgley and Janeway, J . SOC.Automotioc Eng., 16, 458 (1923). 41--Morgan, Phil. Mag., 46, 968 (1923). 4 S M c K e n z i e and Honamun, J. SOL.Automotive Eng., (a) 11, 119, (a) 338 (1922). 43-Parker and Rhead, J . Chem. SOC.(London),106,3150 (1914). 4&Payman, Ibid., ( a ) 116, 1446, (b) 1454 (1910); (c) 117, 47 (1920); (a) 128, 415 (1923). d&Payman and Walls, Ibid., ( a ) 123, 421, (b)'434 (1923). 4&Payman and Wheeler, Ibid., (a) 106,36 (1914); ( b ) 113, 656 (1918); (6) 123, 426, (a) 1251 (1923). 4 7 4 h l e s i n g e r . J. SOC.Automolive Eng., 16, 433 (1925). 4 8 S p a r r o w s . Ibid.. 11, 129 (1922). 49-Stevens, R e p f . 176 Natl. Advisory Comm. Acronaufics, 1924. 50-Taffanel and Le Floch, Compf. rand., 166, 1544 (1913). 51-Thornton, Proc. Roy. Soc. (London), 101A, 272 (1924). 5 S V a u t i e r . Compf. rend., 119, 256 (1924). 53-Wendlandt, 2. 9hysik. Chcm.. (e) 110, 637 (1924); ( b ) 118, 277 (1925).
Vol. 17, No. 12
Effects of Knock Inducers and Suppressors upon Gaseous Ionization By G. L. Clark, E. W. B r u g m a n n , a n d W. C. T h e e MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASS.
T h e experiments of Wendt a n d G r i m m a r e repeated u n der carefully controlled conditions, using, however, a s an ionizing source, m o n o c h r o m a t i c molybdenum Kct X-rays of constant intensity a n d a very sensitive q u a d r a n t electrometer a n d a gold-leaf electroscope for m e a s u r e m e n t of ionization currents. T h e ionizations of air, air plus benzene, a n d air plus benzene plus various knock suppressors a n d inducers a r e determined, a n d f r o m t h e f u n d a m e n t a l information concerning gaseous ionization by X-rays, t h e relative ionizations of various vapors, gaseous catalysis, t h e effect of traces of tetraethyl lead, etc., i n absorbing electrons, a n d t h e r a t e of recombination of ions a r e obtained. T h e experiments indicate t h a t t h e theory of electron wave fronts i n explosions a n d t h e absorption of electrons by knock suppressors is n o t sufficient t o explain t h e practical operation of s u c h chemical substances in t h e control of detonation, in agreement with t h e negative results o n t h e in0uence of electric 5elds o n t h e propagation of explosive waves, a n d o n t h e effect of knock inducers i n increasing ionization. Eight theories of t h e action of these compounds a r e critically considered a n d new experiments suggested by this work a r e outlined.
I
T HAS been pointed out that the presence of as little as one molecule of tetraethyl lead in 200,000 molecules of combustible mixture of kerosene and air exerts a marked effect in the suppression of detonation. A mechanism to explain this remarkable effect based upon a theory of many years' standing and experimental evidence in its support have been given by Wendt and Grimm.' Preliminary experiments in this laboratory with a modified form of their apparatus produce results that are not in accord with the suggested mechanism. Theoretical
It is a well-established fact that burning gases are good carriers of the electric current, and, therefore, that they are ionized. Part of the energy from the reaction of the molecules in the flame is believed to be used in the liberation of electrons, which precede the flame front and ionize the combustible mixture immediately ahead. This ionization activates the combustible mixture so that flame propagation is accelerated until detonation occurs. Increased temperature and pressure of the reacting gases would be accom"l'EnS]OURNAL,
16, 890 (1924).
December, 1925
INDUSTRIAL AND ENGINEERING CHElMIXTRY
panied by the liberation of electrons a t higher velocities, causing the flame propagation to be faster and a larger fraction of the charge to detonate. The function of the tetraethyl lead is to reduce the ionization of the combustible mixture adjacent to the flame front and thus to prevent the accelerated flame propagation which ends in detonation. I n the suggested mechanism for the action of tetraethyl lead this is accomplished by the attraction and absorption of the liberated electrons by the lead atoms, The negative charge which the lead atoms thus receive attracts the positively charged ions and discharges them. Thus a single lead atom, acting in the capacity of a charge carrier, is conceived to prevent many thousands of molecules from being sufficiently activated to lead to detonation. Of necessity this theory demands that knock inducers such as organic nitrites must act in an opposite sense by increasing ionization. Wendt and Grimm did not observe this effect. Furthermore, they found no effect upon the speed of propagation of explosive waves of electric and magnetic fields in accordance with the classical earlier experiments of Lind.*
1227
chromatic (KO of molybdenum) X-rays. The ionization chamber was blown in Pyrex, and had a very thin window through which the X-rays were received with little loss by absorption. The source of monochromatic X-rays was a self-rectifying molybdenum tube mounted in a lead-sheathed box to protect the experiment and the operator. A heavy lead slit placed before the window in the primary ionization chamber fixed the size of the X-ray beam. The position of the X-ray tube, the adjustment of the lead slit, and the position of the ionization chamber were unchanged during the series of runs. The intensity of the X-ray beam, and therefore of the ionization produced, was limited by the voltage fluctuation on'the 110-volt, a. c. power line. By connecting two 110/230OV, transformers in the line before the control the voltage fluctuation was reduced to a maximum of 2 to 3 per cent.
Work of Wendt and Grimm
It is extremely difficult to demonstrate the mechanism of the action of tetraethyl lead experimentally under the conditions of detonation. Wendt and Grimm have shown that when tetraethyl lead vapor a t room temperature and pressure is added to an ionized but not burning gas the rate of recombination of ions is increased by as much as 370 per cent. The apparatus used to obtain this result consisted essentially of a high-potential alternating current .arc as a source of ionization, a cooling coil, and a capillary safety tube through which air was drawn a t a constant rate. Tetraethyl lead vapor was then introduced into the ionized gas by passing it over a 1 per cent solution of tetraethyl lead in benzene. The ionized gas containing lead atoms was conducted into an ionization chamber a t two points along which the ionization was measured by the rate of discharge of a gold-leaf electroscope. A comparison of the gaseous ionization a t the two points in the ionization chamber with and without tetraethyl lead gave the effect of this catalyst on the rate of recombination of the ions in the gaseous mixture. Experimental
Method 1 The arrangement of the apparatus is shown in Figure 1. Air was drawn from the room through a bubbling bottle containing 98 per cent HzS04for the purpose of reducing the moisture content. It was then drawn through a drying tube filled with lumps of calcium chloride and soda lime in order to remove any entrained sulfuric acid and to bring the moisture content to a substantially constant value. Dust was removed from the air by a plug of cotton placed in the top of the drying tube. Air cleared in this manner was passed through a vaporizer in order to introduce benzene vapor, benzene plus butyl nitrite, or benzene plus tetraethyl lead vapor as the case required. The vaporizer was essentially the same as that used by Wendt and Grimm, and presented about a 1 x 10 cm. liquid surface to the air stream. Since the temperature of the liquid in the vaporizer remained constant a t room temperature within 2 degrees, and since the air rates were constant for all runs, the proportion of benzene introduced into the air stream was the same for all runs. The air-benzene mixture was then passed into a lead-sheathed ionization chamber in which the mixture was ionized by a beam of mono-
'
Malinovskii, J . chim. phys., 21, 468 (1924), reports that he has been able to check or stop the propagation of explosion waves b y transverse electric fields. These experiments are open to serious doubt.
DRnffi TOWER
u +
IONIZATION CHAMBER
EQUALIZER
Figure 1-Typical
I! I4E li I
WRY ATION ABER
cHAL
EARTH 7 EARTH
Arrangement Experiments of Apparatus in Ionization
The ionization of the gas was measured a t two points, about 20 cm. (8 inches) apart, in the secondary ionization chamber. This chamber was made of 5-cm. (2-inch) brass tubing and was fitted with large amber insulators so that no appreciable leak could be detected with the gold-leaf electroscope. The two plates of the electroswpe were 112 volts from ground potential and the secondary ionization chamber was 225 volts above ground. The ionization current was measured by the rate a t which the gold leaf became charged. The air rate was determined by means of a capillary flowmeter placed immediately after the secondary ionization chamber. A screw clamp placed on a rubber connection between the flowmeter and the expansion equalizer served to regulate the flow and to hold it constant. With this arrangement the equalizer was under a high vacuum all the time and the pulsations of the electrically driven pump were barely perceptible on the flowmeter. Scrubbing bottles (not shown in the figure) were used to remove the tetraethyl lead vapor from the exhaust air. The first of these contained a saturated solution of bromine in carbon tetrachloride and the second a 6 N solution of sodium hydroxide. Measurements of the ionization current were made after the X-rays and suction had been on long enough to produce constant conditions. With only air entering the primary ionization chamber, the time required for the gold leaf to travel a given number of divisions was taken with a stopwatch and the current in divisions per minute was calculated. A simi-
INDUSTRIAL A N D ENGINEERI;VG CHEMISTRY
1228
lar observation was taken a t electrode 2 and the procedure was repeated to be sure that conditions were constant. Benzene and, subsequently, a 2 per cent solution of butyl nitrite were introduced into the vaporization chamber and observations were taken. The vaporization chamber was then rinsed with benzene and dried by the air current. Finally, a 3.4 per cent solution of tetraethyl lead in benzene was introduced into the vaporization chamber and after constant conditions had been reached, observations were taken.
Method 8
Results
ture. A summary of observations given by Method 1 is shown in Table I. Table I
IONIZATION CURRENT DIVISIONS PER MINUT&
Electrode 1 Air alone
Air Air
.
The experiments with the apparatus shown in Figure 1 show, first, the effect on the total ionization of the air-benzene mixture of small concentrations of tetraethyl lead and of butyl nitrite a t the time ionization occurs; and second, their effect on the rate of recombination of the ionized mix-
+ benzene + benzene + butyl nitrite
'
{;E: g::
Air alone
Air
The results given by the preceding method were corroborated by the use of a n ionization chamber (Figure 2) and a quadrant electrometer which are being developed for the accurate measurement of small gaseous ionization. The arrangement of the apparatus was essentially the same as that in Figure 1 except for the substitution of the secondary ionization chamber, and in some experiments the vaporization chamber was placed between the ionization chambers. By this means the effect of adding benzene vapor and a catalyst to the air after it had been ionized was studied. Although the ionization of the gaseous mixture was measured a t only a single place, the values obtained are indicative of the rates of recombination because the time element was the same in every case. A high rate of recombination would be accompanied by a small ionization current. I n still other experiments two of the precision ionization chambers were used in series. The ionization chamber, which was made of Pyrex glass was about 30 cm. (12 inches) long and 5 cm. (2 inches) in diameter. A very thin window was blown in one end to admit an X-ray beam with small loss by absorption. Since for this experiment the chamber was used only as a measuring instrument, it was entirely covered with sheet lead and earthed in order to shield it. Surface leakage between the oppositely charged electrodes was overcome by the steel collar and earthed mercury cup, which are shown in Figure 2. The quadrant electrometer was designed by the writers specially for the measurement of very small currents in spectrometric experiments of great precision. The 35-cm. (14-inch) suspension of phosphor bronze and silver ribbon held a very light aluminium needle with its vanes between the quadrant. A scale placed about 1.2 meters (4 feet) from the electrometer was read through a telescope sighted on a mirror fixed to the needle. The earthing key consisted of a point contact between a hardened steel ball and a hardened flat plate. Attempts to use a dipping wire and a mercury cup as a key led to a high induced electromotive force a t break. Because of the great sensitivity of the electrometer, it was necessary to shield it and the leads by earthed conductors, The use of brass tubing with the leads held in place on the axis by enameled fishline was finally adopted as the best means of shielding the leads. The electrometer was completely surrounded by a grounded shield made of fine copper gauze, which prevented electrostatic disturbance from the high-tension leads and from air ionized by X-rays. Once the mechanical and electrical zeros of the electrometer had been made to coincide, care was taken to keep the instrument a t a temperature constant within 5 degrees and readings were taken.
Vol. 17, No. 12
+
+
benzene tetraethyl lead
59.8
Electrode 2 Ratio, 1:2
2.52 4.67 7.42 7.95 8.83 8.50 7.37 8.10 11.8 10.1 10.6
9:2 9.1 6.8 6.4 6.1 6.4
7.2 6.2 5.3 5.9 5.7
The values of the ionization current at electrode 1 give an indication of the total ionization produced, because the ionization is measured immediately after the gas leaves the ionization chamber. From Table I it will be seen that the presence of butyl nitrite slightly increased the total ionization of the benzene-air mixture, which is in accord with the suggested mechanism; but that the presence of tetraethyl lead gave a still lafger total ionization, which is in opposition to predictions based on the theory. The relative rates of recombination of the ions formed in the benzene-air mixture are shown by the ratio of the ionization currents at the two electrodes. A large value of the ratio of the ionization current a t electrode 1 to that a t electrode 2 signifies a large rate of recombination. It will be seen that the presence of butyl nitrite slightly decreased the rate of recombination-which is in accord with the theory. On the other hand, the presence of tetraethyl lead decreased the rate of recombination still more. This is in accord with neither the theory nor the results of Wendt and Grimm.
XUM A -4
Figure 2-Detail
of Pyrex Glass Ionization Chamber for Use with Quadrant Electrometer
The experiments in which the quadrant electrometer and ionization chamber were used to measure the ionization show no marked effect due to the addition of tetraethyl lead to the ionized air. The ionization currents for the benzene-air mixture alone were actually slightly less than that when tetraethyl lead was present, exactly as was found with the vaporizer on the intake side of the primary ionization chamber. The results obtained with the use of X-rays as a source of ionization have been checked by S. C. Lind, who used radium as the ionizing source. Discussion
The use of monochromatic X-radiation as a source of ionization is recommended because the intensity of radiation
December, 1926
INDUSTRIAE &$B ENGINEERING CHEMISTRY
can be maintained very steady. This is in contrast with the alternating current arc, which is difficult to hold steady and from which the gases come hot. Not only does this obviate the simultaneous ionization of air, hydrocarbon, and catalyst vapor, but the passage of the ionized gas through cooling coils undoubtedly reduces the ionization. The removal from between the primary and secondary ionization chambers (Figure 1) of a 20-cm. (%inch) length of 10-cm. glass tubing reduced for 25 mm. (1 inch) of length a t each end increased the ionization fivefold. It should be mentioned that the presence of X-radiation has practically no effect on most of the atoms over which it passes, but that its effect is concentrated on the particular atoms which it ionizes. The development of the technic of accurate measurement of gaseous ionization by means of the ionization chamber and electrometer suggests new experiments and applications. Of scientific interest are the studies of ionization potential of various gases, and the effects on ionization of changes in temperature and pressure and the presence of a gaseous catalyst. A very important application is the measurement of X-ray intensities for dosage in X-ray therapy. Present Status
The fact that many theories have been advanced to explain detonation and the action of antiknock compounds is an indication of the lack of knowledge of these phenomena. Detonation has been considered in detail in another paper in this symposium by Clark and Thee. The theory of “knock” that is subject to least objection is the detonation wave theory, according to which high pressures in the wave front give rise to arbitrary motions producing sound.
Theories of Action of Knock Suppressors Some of the theories of the action of knock suppressors are as follows: 1-According to a French theory, colloidal lead formed in the vapor by decomposition of lead tetraethyl deposits on points, corners, and edges, which may help to accelerate reaction velocity in the flame to the detonation stage. This does not explain why knocking begins immediately if fuel alone is substituted for the mixture containing the suppressor, though the lead and its oxides continue to cling to the walls; nor why particular compounds of lead exhibiting organic valences are required; nor why organic amines, etc., are also effective. 2-The lead particles become incandescent by chemical decomposition of the compound and act as multiple miniature spark plugs producing uniform combustion throughout the mixture. This is subject to the objections that i t does not explain why Pb(CzH&,CI and Pb(C2H6)2Cllr respectively, are threefourths and one-half as effective as Pb(CzHs)r, nor does i t explain the nature of the decomposition, the action of the amines, and of knock inducers. 3-The lead atom absorbs electrons in the wave front, thus retarding velocity of propagation. This theory has been proved incorrect experimentally in this paper. 4-Knock suppressors catalyze one of two or more possible oxidation reactions-e. g., hydroxylation of hydrocarbonswhich is least likely to result in detonating velocities. &The simple positive catalyst theory postulates that suppressors lower the ignition temperature to the point a t which the heat of adiabatic compression could cause appreciable but relatively slow chemical action to occur ahead of the flame so that the reaction wave would pass through partly burned air. The objection is that there is no relationship between ignition temperature and tendency to detonate. 6-The simple negative catalyst postulates that the substances as gases decrease the reaction velocity of the burning fuel (Midgley). 7-The theory of poisonings of the metal walls of the combustion chamber which are acting as positive contact catalysts of reaction velocity has the advantage that organic amines are known to be mild poisons and iodine, selenium, lead, etc., pronounced ones. The objections are that mercury and sulfur, which are catalyst poisons, are slight knock inducers, that no
* Schlesinger,
J. SOC.Automoliuc Eng., 16, 441 (1925).
iiig
appreciable lag exists between abwtac@and presence of knock when fuel without suppressor is suddenly introduced, and that detonation occurs as well in glass as in metal vessels. 8-The radiation theory is that the gaseous knock suppressors absorb radiations from the initial flame, which otherwise may activate and accelerate reactions by splitting hydrocarbons into a more reactive condition. This theory is without experimental proof, and will be difficult to test. It would seem inadequate to explain inducers.
Suggestions for Further Work
It is clearly evident that much more experimental work is necessary before a really satisfactory mechanism of the action of knock inducers and suppressors may be devised. Some of the fundamental experiments suggested by the various theories which have been proposed are as follows: (1) Effects of knock inducers and suppressors on speed of propagation of flames photographically registered. (2) Laboratory experiments on reactions of knock suppressors such as lead tetraethyl with hydrocarbons and oxygen, for light bearing upon the nature of organic valences, which must be an important factor in the antiknock reaction. (3). Accurate spectrometric studies of combustion and detonation of various fuels in actual engines and in presence of various knock inducers and suppressors a t particular points in the cycle. This is now being done utilizing an engine with quartz window and a synchronous shutter. (4) Laboratory experiments upon the activation and decomposition of hydrocarbon mixtures by radiation in the spectrum from X-rays to radiant heat, utilizing radiation from flame itself as analyzed spectrometrically and spectrobolometrically, in the presence and absence of knock inducers and suppressors. ( 5 ) Absorption spectra of tetraethyl lead and related compounds. (6) Adsorption and catalysis studies of fuel mixtures on metal surfaces and possible catalytic poisoning by knock suppressors. (7) Effect upon ignition temperatures of knock inducers and suppressors, using both the secondary discharge and heated surfaces. (8) Relative efficiencies of introduction of knock suppressor in gasoline or in lubricating oil to test whether existence in gaseous phase is essential or whether the action is a surface catalysis or catalytic poisoning.
Gaseous Explosions 11-Homogeneous and Heterogeneous Reactions Defined and Classified By George Granger Brown UNIVERSITY OF
MICHIGAN. ANN ARBOR.MICR.
Gaseous reactions m a y be divided into t w o classes, homogeneous reactions wholly confined to a single gas phase a n d heterogeneous reactions occurring at the interface between two phases. Homogeneous reactions are continuously homogeneous if all parts of the gas phase are in the same stage of reaction at the same time, progressive homogeneous reactions if the zone of reaction advances continuously through the gas, o r hetero-homogeneous if indirectly modified by another phase. Heterogeneous reactions are gas-gas if the reaction occurs at the interface between two gas phases, gas-liquid if the reaction occurs at the interface between a gas phase and a liquid phase, or gas-solid if the reaction occurs at the interface between a gas phase and a solid phase.
HE properties of gaseous reactions and explosions occurring under different conditions vary in an apparently inconsistent manner. I n a previous paper’ the conflicting evidence concerning the effect of initial temperature upon the rate of rise of pressure of a gaseous explosion was briefly reviewed, and all the apparently conflicting evidence
T 1
Brown, Leslie, and Hunn, THISJ O U R N A L , 17, 397 (1926).
