Several statements by Campbell, as well as Slipher’s observations, indicate that a difference ought to have appeared between the spectrum of Mars and that of the Moon if the water content in Mars’ atmosphere had been the same as in the Earth’s at the time of the observation. This content, as stated before, was about 3 times smaller on Mount Whitney than at Flagstaff. At the latter place, the measurements gave 1.75 as the ratio of water vapour on Mars to that on the Earth. The amount of water vapour with the Sun in zenith on Mars should therefore, according to Campbell’s observations, only reach 0.4 gramme (6.1 grains) per cubic meter (1.3 cu. yds.) corresponding to a dew-point of -28° C. (-18.4° F.) or to an actual temperature of -17° C. (+1.4° F.) allowing also for a desert climate with only 31 per cent. saturation. This temperature is probably higher than the mean for a summer day as the observations were made at noon on Mars. It must now be evident that we should consider Mars as unfit to harbour living beings. There is possibly a slight amount of oxygen in the thin air but the extremely low temperature and the scant supply of water vapour form insurmountable obstacles to the subsistence of even the simplest forms of life in the equatorial regions on Mars. The temperature difference between day and night must be enormous on account of the desert climate. Even if life could develop during the day, which has nearly the same duration as with us--Lowell fixed it at 24 hours, 37 minutes, 22.6 seconds--and during which the temperature possibly might rise above the freezing point, it would nevertheless be destroyed without mercy by the bitter frost at night.