When a sample of smoke is viewed in the ultra-microscope, it appears like the starry heavens, except that the stars are dancing violently about. At first little distinction is made between the particles, as there seems to be no order in their motion, but soon it becomes evident that the brighter particles are more sluggish than the dim ones. This is due to the greater mass of the bright particles, for they are larger. The particles are all moving slowly away from the source of light and eventually diffuse to the walls of the cell. When the electric field is turned on, about one-third of the particles immediately migrate, about equally in both directions, to the two electrodes. If the field is reversed, the direction of migration is reversed and if the commutator is used the particles oscillate regularly. Sometimes the particles may be seen to combine and become neutral, in which case oscillation ceases. CONCENTRATION OF SMOKE In measuring the concentration of smokes, the following terms are useful: =Density.= The density of a smoke is defined as the reciprocal of the thickness of the smoke layer in feet necessary to obscure a given filament. Thus six inches of a smoke of density 2.0 is required to obscure an electric light filament, whereas one requiring four feet would have a density of 4. Another way to show the significance of this definition is to point out that if a definite weight of a stable smoke is diluted with air after it is formed, the product of the volume by the density always remains constant. Any marked variation in this rule may be taken as evidence that the particles of smoke are undergoing a change, in most cases due to evaporation. =Total Obscuring Power.= The volume of smoke produced per unit weight of material used is the second factor in determining the value of a smoke. The product of this volume per unit weight by the density of the smoke is the real measure of effectiveness, and is called the total obscuring power (T. O. P.) of the smoke. If the volume is expressed in cubic feet per pound and the density in reciprocal feet, the unit of T. O. P. is square feet per pound. That is, it expresses the square feet of a smoke wall, thick enough to completely obscure a light filament behind it, which could be produced from a pound of the reacting substances. The total obscuring power of some typical smokes are as follows: Phosphorus 4600 NH₄Cl(NH₃ + HCl) 2500 SnCl₄ + NH₃ + H₂O 1590 Berger Mixture 1250 SnCl₄ + NH₃ 900 SO₂ + NH₃ 375