Book excerpt: Research on the Nature of Mineral-Forming Solutions is the first book on the subject of fluid inclusions. This book contains observational data and studies of mineral-forming solutions done in the Soviet Union. The description and natural classification of inclusion in minerals according to their composition and state are discussed. Gaseous, liquefied, and solidified inclusions that are found in minerals and their significance are considered important in determining the presence and availability of the mineral. For example, any earlier or contemporaneous minerals that are found only in their host crystals can be determined by analyzing the presence of solid inclusions. The origin and genetic classifications of liquid and gaseous inclusions, being both abundant in hypogene ore deposits, are explained. Other less common methods in the study of inclusions, besides homogenization of inclusions by heating under the microscope, are forwarded. The authors believe that exact measurements of the homogenization temperature are possible and therefore can serve as a precise indicator in understanding the process of formation of individual crystals and hydrothermal deposits. Other studies of the All-Union Research Institute of Piezo-optical Mineral Raw Materials are also discussed. This collection of monographs will prove invaluable to mineralogists, geologists, and research-chemists studying minerals and ore deposits.