When we inquire what these tints are, we find that they are tints which are _sometimes_ wanting even from direct sunlight. When the sun sinks very low and looks like a great red ball through the moisture-laden air, his spectrum is not the same exactly as that of the sun shining high in the mid heaven. It shows other gaps than those corresponding to the ordinary myriads of missing tints. Its red colour shows indeed that some thing has happened to the sunlight; but, oddly enough (at first sight at least), the gaps are chiefly in the red part of the spectrum, just what one would expect if the sun's light showed a want instead of an excess of ruddy light. The fact is, however, that the violet, indigo, and blue are weakened altogether, not by the mere abstraction of tints here and there. The red suffers under a few abstractions of tint, but remains on the whole little weakened. Now the same gaps which at such times appear in the spectrum of the sun are found (generally, if not always) in the spectrum of the planet Mars, even when he is shining high in the heavens, so that his light is _not_ at the time absorbed by the denser portions of our air. In fact the gaps have been seen in the spectrum of Mars when the planet has been shining higher in the heavens than the moon, whose spectrum was found on trial (at the time) not to show the same gaps,--as of course it must have done, and even more markedly, if the missing tints had been abstracted by our own air. No doubt can remain, then, that the sun's light, which reaches us after falling on Mars, has suffered _at Mars_ the same absorption which our own air produces on the rays of the sun when he is low down. But we know what it is in our air which causes this absorption. It is the aqueous vapour. We know this from several independent series of researches. It was proved first by an American physicist, Professor Cooke of Harvard, who found that these lines in the red are always darker when the air is moister. Then by Janssen, who observed the spectrum of great bonfires lit at a distance of many miles, on the Swiss mountains, finding these same lines in the spectrum of the fire-light when the air was heavily laden with moisture. Wherefore we know that the air of Mars must also contain the same substance--the vapour of water--which, in our own air, produces these dark lines. We can, indeed, understand that the ruddy colour of Mars is in part due to this moisture, which, precisely as in our own air it makes the sun and moon look red, would, in the air of a planet, make the planet itself look red.