Dust clouds were formed by blowing various pure chemical substances through tubes and the net electric charge imparted was determined as a function of the composition of the dust, tube material, area of contact between dust and tube during blowing process, velocity with which the dust moved through the tube, and length of path of the dust through the tube. An apparatus is described with which photographic records of the paths of particles in an electric field were obtained. Such photographs show the presence of positive, negative, and neutral particles in all dust clouds, even of very pure substances. The ratio of positive to negative electrification in a cloud is found to change as the larger particles in the cloud settle out but evidence is obtained which contradicts the hypothesis that the large particles carry an opposite charge from the small particles in a given cloud. The paper closes with a few remarks bearing on the relation of the present experimental data to the concept of a tribo-electric series.