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On the densities of particles in smoke
April, 1927.]
CURRENT TOPICS.
60 5
The results given above held for sounds travelling in a general horizontal direction. Another set of observations was made on the sounds originating in an airplane flown after sunset. Such sounds came down in a roughly vertical direction through air in which the arrangement of physical properties is dii~erent from that in a horizontal path. " The range of audible transmission was good on every occasion that the general humidity of the atmosphere at all levels was high, and usually, but not always, poor when it was low." This is attributed to the close association of high humidity with homogeneity of the atmosphere. The only meteorological factor that varies with a rapidity equal to the rate at which audibility changes is humidity. The author adduces an instance when in 4 ~ minutes a hygrograph showed a change in relative humidity of 15 per cent., there being meanwhile no change of temperature or of other conditions. In this paper the effect of change of temperature receives less consideration than would be expected. In one airplane flight there were parts of its course from which the sound was audible. After the reading of this communication before the Royal Meteorological Society the discussion that followed brought out some interesting facts. Mr. Bonacina stated that in Essex during the war the sound of gun fire at a long distance would be heard only from May to August, while in some other parts of Europe audibility was confined to the winter months. He had noticed that firing was heard more distinctly inside a wood than outside it. G.F.S. On the Densities of Particles in Smoke. H. S. PATTERSON and R. W. GRAY. (Proc. Royal Soc., A 7 6 4 . ) - - I f the size of small particles is to be determined by their rate of fall in air or other gas, a knowledge of their density is needed. This is often taken to be the same as that of the substance in large quantities--an assumption probably correct for liquids and for solids in certain cases but not true for solid particles with a loose structure. Such a structure has been shown to exist for large particles in the cases of smokes made up of the oxides of cadmium, zinc and magnesium. " Further, it seems probable that the smaller particles of so-called ultra-microscopic size in these smokes also possess a similar structure, since they are formed in the same way and consist most likely of a number of primaries." In what follows the " density of a particle " means the density of a sphere of the same mass as the particle, which falls through air at the same rate as the particle. If the particle be composed of several parts irregularly grouped, the density as just defined would be expected to be less than that of a homogeneous sphere of the substance. Consider, for example, a gram of glass wool. Let the time be found that it takes it to fall to feet. Melt the wool into a single sphere of glass. This will fall through the IO feet in a less time than the wool, though its mass is the same. Thus to get a sphere of the same mass and the same time of fall, its density must
606
CURRENT T o r m s .
[J. F. I.
be reduced. Kohlschutter and Tuscher, working with electrically precipitated smokes, found their densities to be much less than that of the normal substance. The smoke of aluminium oxide, as an example, had only 5 per cent. of the density of the oxide in bulk. They further found that the density of smokes of the same substance varied greatly according to the method of production. The author attacks the problem of the density of smoke particles by the use of the method Millikan applied so successfully to the determination of unit electronic charges. The motion of smoke particles was observed while they were acted on hy a vertical electrostatic field as well as by gravity, or by the latter alone. Their electrical charges were varied by bringing radium near. From the data thus obtained the density can be calculated. " Cadmium oxide, silver and gold were produced by making an arc between poles of the various metals in a cubic metre chamber. Magnesium oxide was made by burning magnesium ribbon, and mercuric chloride and mercury by volatilization from an electrically heated boat." The calculated densities are remarkably small. For gold smokes they range from 8.o to .2I, though gold itself has the value I9. 3. In the case of cadmium oxide the smallest density was for a particle that was seen to consist of a chain of parts. " The simplest interpretation of the results is to assume that the smoke particles are loosely packed aggregates, made of primaries, which are in many cases ultramicroscopic." An additional conclusion is reached, that smoke particles are in reality considerably larger than has been supposed, indeed often being of microscopic and~ not of ultra-microscopic dimensions. This was confirmed by microscopic examination of the particles that settled upon slides. Their complex chain or ring structure was often visible. G.F.S.
The Thermal Conductivity of Vitreous Silica. G . W . C . KAYE and W. F. HmGIZqs. (Proc. Royal Soc., A764. ) While vitreous silica is growing more important, but little is known of its thermal conductivity. Eucken and Barratt both have determined it but their results are not in agreement. For instance, at Ioo ° Eucken's value is nearly twice that of Barratt. The method employed in this investigation is that of the " divided bar," due to Sir Oliver Lodge in I878. " A bar of metal heated at one end and cooled at the other is divided by a plane perpendicular to its axis, and a plate of the material under test is interposed. It follows that, under steady conditions of heat flow, the ratio of the conductivities of the specimen and of the bar is equal to the inverse ratio of the temperature gradients in the specimen and in the bar immediately adjacent to the specimen." Aluminum bars were used on account of the high conductivity of this metal and of its freedom from oxidation. Since the divided bar method gives only a ratio, it was necessary to measure the conductivity of aluminum. The pubfished values for this quantity differ by as much as I4 per cent. The
CURRENT TOPICS.
60 5
The results given above held for sounds travelling in a general horizontal direction. Another set of observations was made on the sounds originating in an airplane flown after sunset. Such sounds came down in a roughly vertical direction through air in which the arrangement of physical properties is dii~erent from that in a horizontal path. " The range of audible transmission was good on every occasion that the general humidity of the atmosphere at all levels was high, and usually, but not always, poor when it was low." This is attributed to the close association of high humidity with homogeneity of the atmosphere. The only meteorological factor that varies with a rapidity equal to the rate at which audibility changes is humidity. The author adduces an instance when in 4 ~ minutes a hygrograph showed a change in relative humidity of 15 per cent., there being meanwhile no change of temperature or of other conditions. In this paper the effect of change of temperature receives less consideration than would be expected. In one airplane flight there were parts of its course from which the sound was audible. After the reading of this communication before the Royal Meteorological Society the discussion that followed brought out some interesting facts. Mr. Bonacina stated that in Essex during the war the sound of gun fire at a long distance would be heard only from May to August, while in some other parts of Europe audibility was confined to the winter months. He had noticed that firing was heard more distinctly inside a wood than outside it. G.F.S. On the Densities of Particles in Smoke. H. S. PATTERSON and R. W. GRAY. (Proc. Royal Soc., A 7 6 4 . ) - - I f the size of small particles is to be determined by their rate of fall in air or other gas, a knowledge of their density is needed. This is often taken to be the same as that of the substance in large quantities--an assumption probably correct for liquids and for solids in certain cases but not true for solid particles with a loose structure. Such a structure has been shown to exist for large particles in the cases of smokes made up of the oxides of cadmium, zinc and magnesium. " Further, it seems probable that the smaller particles of so-called ultra-microscopic size in these smokes also possess a similar structure, since they are formed in the same way and consist most likely of a number of primaries." In what follows the " density of a particle " means the density of a sphere of the same mass as the particle, which falls through air at the same rate as the particle. If the particle be composed of several parts irregularly grouped, the density as just defined would be expected to be less than that of a homogeneous sphere of the substance. Consider, for example, a gram of glass wool. Let the time be found that it takes it to fall to feet. Melt the wool into a single sphere of glass. This will fall through the IO feet in a less time than the wool, though its mass is the same. Thus to get a sphere of the same mass and the same time of fall, its density must
606
CURRENT T o r m s .
[J. F. I.
be reduced. Kohlschutter and Tuscher, working with electrically precipitated smokes, found their densities to be much less than that of the normal substance. The smoke of aluminium oxide, as an example, had only 5 per cent. of the density of the oxide in bulk. They further found that the density of smokes of the same substance varied greatly according to the method of production. The author attacks the problem of the density of smoke particles by the use of the method Millikan applied so successfully to the determination of unit electronic charges. The motion of smoke particles was observed while they were acted on hy a vertical electrostatic field as well as by gravity, or by the latter alone. Their electrical charges were varied by bringing radium near. From the data thus obtained the density can be calculated. " Cadmium oxide, silver and gold were produced by making an arc between poles of the various metals in a cubic metre chamber. Magnesium oxide was made by burning magnesium ribbon, and mercuric chloride and mercury by volatilization from an electrically heated boat." The calculated densities are remarkably small. For gold smokes they range from 8.o to .2I, though gold itself has the value I9. 3. In the case of cadmium oxide the smallest density was for a particle that was seen to consist of a chain of parts. " The simplest interpretation of the results is to assume that the smoke particles are loosely packed aggregates, made of primaries, which are in many cases ultramicroscopic." An additional conclusion is reached, that smoke particles are in reality considerably larger than has been supposed, indeed often being of microscopic and~ not of ultra-microscopic dimensions. This was confirmed by microscopic examination of the particles that settled upon slides. Their complex chain or ring structure was often visible. G.F.S.
The Thermal Conductivity of Vitreous Silica. G . W . C . KAYE and W. F. HmGIZqs. (Proc. Royal Soc., A764. ) While vitreous silica is growing more important, but little is known of its thermal conductivity. Eucken and Barratt both have determined it but their results are not in agreement. For instance, at Ioo ° Eucken's value is nearly twice that of Barratt. The method employed in this investigation is that of the " divided bar," due to Sir Oliver Lodge in I878. " A bar of metal heated at one end and cooled at the other is divided by a plane perpendicular to its axis, and a plate of the material under test is interposed. It follows that, under steady conditions of heat flow, the ratio of the conductivities of the specimen and of the bar is equal to the inverse ratio of the temperature gradients in the specimen and in the bar immediately adjacent to the specimen." Aluminum bars were used on account of the high conductivity of this metal and of its freedom from oxidation. Since the divided bar method gives only a ratio, it was necessary to measure the conductivity of aluminum. The pubfished values for this quantity differ by as much as I4 per cent. The