In tracing the development of the methods em ployed in the liquefaction of gases, it is perhaps advisable to divide the work into four periods. The four chapters Of this volume correspond to these periods. The first chapter is concerned with the early history of the subject. In this period some of the earlier observations on the compression of gases, and likewise a few disconnected experiments on the liquefaction of gases, are considered. These investigations are only of historical interest, and hence are only briefly outlined.