The investigations described in the present paper have already been the subject of two short preliminary notes published in collaboration with Prof. Kamerlingh Onnes. It was intended that this communication should also be a joint paper, but, as it has been used by the author in an amplified and somewhat altered form as his doctoral dissertation (Leiden 1923), it was decided, with the concurrence of Prof. Onnes, to present it for publication in its present form. The work is intended as a contribution to the study of the paramagnetism of solids at low temperatures in continuation of the previous investigations already carried out at Leiden. It falls naturally into two parts, the first dealing with measurements of the susceptibilities of powdered substances and the second in which measurements on crystals are described. It was decided to investigate the following series of compounds in the powder form, viz.: the anhydrous and heptahydrated sulphates and the ammonium double sulphates of ferrous iron, nickel and cobalt. Of these compounds, data for the anhydrous and heptahydrated ferrous sulphates have already been published. They are considered here for the sake of completion in the series.