The electric light was known in the laboratory from the time of Sir Humphry Davy in the early years of the nineteenth century, but until cheaper methods of producing electricity than by the use of primary batteries had been invented no commercial application was possible. In few matters is the steady growth in efficiency so strikingly shown as in the improvements in electric lighting. If a high-tension current of electricity is passed through a partially exhausted glass tube, the air or other gas within glows with a soft light, the colour depending on the gas or gases present. When a current of electricity passes through a conductor heat is produced, and the greater the resistance offered to the passage of the current the greater is the proportion of the electricity which is converted into heat. Among the numerous domestic heating appliances to be seen to-day in the shop windows are electrically heated irons for the laundry.