Glass J tube fromed from 1mm bore thick walled capillary tube, with a brass screw and collar at the end of the longer limb. Used for sampling small quantities of gas held over water(e.g. in photosynthesis or repiration experiments). A bubble of gas is introduced into the short limb. This is then lowered into reagent solution and the bubble moved back and forth so that reagent diffuses through the end water seal. The change in length of the bubble indicates the amount of gas taken up by the reagent.