Crookes dark space
During the last decade of the nineteenth century a number of investigators began to seek an explanation of the discharge phenomena. Interest became centred on what happened inside the Crookes dark space, and various tubes were made which were
evacuated to such a pressure that the whole of the space between the electrodes was occupied by this space. Small pieces of certain minerals placed in these tubes were seen to fluoresce brilliantly with characteristic colours. We have already mentioned the green fluorescence of the glass tube itself.