WE are glad to be able to publish as a special supplement this week a survey of existing knowledge of the atmospheres of the planets, which formed the subject of the presidential address delivered by Prof. H. N. Russell at the recent Pittsburgh meeting of the American Association. Prof. Russell is known to all astronomers for his work on stellar development, and particularly by his division of stars into the totfo types of ‘giants' and ‘dwarfs' in which the temperature is rising and falling respectively. In recognition of this and other contributions to astrophysics, he was awarded the Gold Medal of the Royal Astronomical Society in 1921, and he has been similarly honoured by a number of other leading scientific societies. In his address Prof. Russell first points out that the presence of atmospheres on Jupiter, Saturn, Mars and Venus was proved, many years ago, by telescopic observations of clouds, polar snow-caps and twilight. The moon has no trace of atmosphere, nor has Mercury. The spectroscope enables information about the composition of these atmospheres to be obtained, but it will not detect hydrogen, nitrogen, helium or other inert gases in such atmospheres. Tests for oxygen and water-vapour are complicated by the presence of these substances in the earth's atmosphere. By taking advantage of the Doppler shift of the lines when a planet's distance is changing rapidly, this difficulty can be escaped. The latest observations at Mount Wilson show no traces of oxygen or water-vapour, either on Mars or Venus. The small amount of water required for the Martian polar caps might escape detection. Bands due to carbon dioxide have been discovered in Venus, and they indicate a layer of the gas at least two miles thick. The major planets show other bandsincreasing in strength from Jupiter to Neptuneall due to methane (CH4) the simplest hydrocarbon. Ammonia gas gives weaker bands in Jupiter and Saturn.