MR. TAIT'S letter in NATURE of February 26 calls to mind an effective lecture illustration I have used in my classes to illustrate a fog or cloud produced by cooling air containing moisture. Instead of using an air-pump as described in â Heat, a mode of Motion,â take a flask of one or two litres capacity, rinse it out with distilled water, and attach to the neck a cork and glass tube of about twenty or thirty centimetres in length. Place the glass tube in the mouth and exhaust, when a dense cloud will be formed; then on blowing into the flask the cloud disappears. The cloud may be produced and dissolved as often as wished. and if a beam from the oxy-hydrogen light be sent through the flask, the experiment becomes very effective.