A further study of the characteristics of cloud chamber expansions and of track growths in the chamber was conducted. The chamber had a long sensitive time and the process of multiple photography per expansion was adopted. 40% of the total expansion were found to be free from back-ground effect. On the whole, conditions were attained in such a way as to record a background level two orders of magnitude lower than attained before. Detailed explanations of the importance of long sensitive time and of very low back ground level are given. Discussions are made of the factors which limit the useful portion of the sensitive time per expansion. The basic factor for selecting those photographic records which may clearly indicate the presence of an interesting low ionizing particle are emphasized. Curves showing track-load track-droplet density, and background density, all against expansion time are shown. Discussions are also made showing clear distinction between different classes of chamber expansion.