"It has, however, seriously been contended that the Martian polar caps are not snow at all, but frozen carbon dioxide--the poisonous dregs of what once was an atmosphere. Carbon dioxide, however, melts and becomes gaseous almost suddenly, but these polar snow-caps melt gradually, exactly as frozen snow would; so this theory fails altogether to fit the circumstances. "Moreover, the water which accumulates all round the base of the melting snow-cap has been carefully observed on many occasions, and in the early stage of melting it appears blue in tint, but later on, as upper layers of snow dissolve and those nearer the soil are reached, the water presents a turbid and muddy appearance; exactly what might be expected when water has been contaminated by the surface soil. "Dr. Alfred Russel Wallace declines to accept the blue tint as any proof that the liquid is water, and contends that shallow water would not appear that colour when viewed from a distance. You will, however, have observed that the water in all our shallow reservoirs appears intensely blue when observed from any distant and elevated point of view. It seems to me that when, as in the case of Mars, we have a very thin atmosphere laden with sand particles, we have exactly the conditions which would produce a very blue sky, and cause the water to appear a deep blue colour when viewed from a distance. "It is also contended that water cannot be present on Mars, because none of our skilled spectroscopists has yet been able to demonstrate by the spectroscope that there is any water vapour in the Martian atmosphere. "This, however, is generally acknowledged to be a very difficult and delicate operation; and, in any case, it is purely negative evidence, and cannot be accepted as final. I feel quite confident that sooner or later a means will be found of definitely proving the presence of water vapour upon Mars by the aid of the usual lines in the spectrum. There are too many evidences of its presence, such as clouds, hoarfrost, snow, and seasonal changes in vegetation, to warrant the rejection of the idea of its existence merely because it has not been detected by the particular means hitherto used by the spectroscopists. "Mr. Slipher, of Flagstaff Observatory, has made many experiments with specially sensitised photographic plates. He has taken several photographs of the spectrum of the moon and others of the spectrum of the planet Mars. The plates of the lunar spectrum show a darkening of the 'a' band, which indicates the presence of water vapour, and we know that is due to the water vapour in our own atmosphere. The plates of the spectrum of Mars show a much more definite darkening of the 'a' band, and Professor Lowell contends that this can only be due to water vapour in the atmosphere of Mars.