Answer from Radleys FAQ : Some very low boiling solvents are just too difficult to condense with any kind of air condenser. In such circumstances a water cooled condenser may be the only option. Diethyl Ether in particular is very difficult to condense. Findenser will cope with smaller volumes (up to 100ml in a 250ml flask) with a hotplate/heating block accurately controlled at no more than 5°C above boiling point.