In particular, its low solubility in body tissues gives it a unique pharmacokinetic profile, with rapid washin and washout from the body. It has been shown to have a similar effect on the speed of uptake of accompanying agents like sevoflurane (the "second gas effect"), which have much slower pharmacokinetics. A recent study by us suggested that this promotes faster and smoother onset of anaesthesia, as measured using the standard monitor of depth of anaesthesia (the BIS monitor). This finding requires confirmation prospectively in a larger group of patients. The investigators further hypothesise that a similar effect also exists on washout of sevoflurane at the end of the procedure, promoting quicker recovery (emergence) from anaesthesia. This has never been previously demonstrated. This information will help better define the place of nitrous oxide in achieving optimal outcomes in modern anaesthetic practice. The investigators propose to conduct a simple study to measure the effects of nitrous oxide washin and washout on exhaled concentrations of accompanying sevoflurane during both induction of anaesthesia and emergence, and identify any accompanying effect on the rate of change in depth of anaesthesia using BIS. The investigators hypothesise that the rate of fall of exhaled sevoflurane concentration at the end of anaesthesia will be more rapid in the group of patients breathing a gas mixture containing nitrous oxide, and that the rate of fall of BIS on induction and the rate of rise of BIS on emergence will be faster in the nitrous oxide group.