642 LOWELL—MARS ON GLACIAL EPOCHS. [Nov. 16, 
3. The obvious simplicity of this explanation, however, has 
tempted some ingenious minds to see in the exhibit not frozen 
water, but frozen carbonic acid gas. For under great stress of 
cold, carbonic acid not only takes refuge in the solid form, but does 
so with all the delicate purity of snow. ‘The transformation de- 
mands, indeed, a very low temperature and to this end was the idea 
invented ; inasmuch as the distance of Mars from the sun seems 
hard to reconcile with a mean temperature comparable to that of 
our earth. Now as it is of the first importance to the inquiry 
before us to be as certain as possible whether it be really snow or 
ice that we gaze upon over there in space, I make no circumstance 
of confronting at once this hypothesis. 
Plausible as the suggestion of carbonic acid sounds, examination 
of the fitness of the substance for the place discloses one fatal 
defect. Carbonic acid and water agree well enough in their solid 
state and are not incompatible in their gaseous one, both being 
then invisible; but they are not at all at one upon their intermedi- 
ate condition. As fluids their behavior is quite diverse, and Mars 
chances to exhibit the very effect which this diversity should cause. 
It is a peculiarity of carbonic acid that it is not at home asa 
liquid, passing, except under great pressure, at all temperatures 
practically instantaneously from the solid into the gaseous state. 
Under one atmosphere or less the curves representing the melting 
and the evaporation points of this substance lie almost side by side. 
And they do this as conclusively at very low temperatures as at 
relatively high ones. Carbonic acid insists on volatilizing. Thus 
no place is left in the economy of its behavior for a permanent 
liquid, whatever the degree of cold. 
Now, one of the most striking features of the polar caps is the 
unmistakable exhibit of such a liquid. Seventy years ago, Beer 
and Madler noticed a dark band surrounding the northern cap, but. 
the full significance of the observation seems to have escaped 
deduction. Since then several observers have noted this band on 
one or other of the caps; and W. H. Pickering, in 1892, added a 
most significant detail, a large bay connected with the one about 
the southern cap. Farther study has brought out still more detail 
of the sort. Under this scrutiny the character of the phenomenon 
appears in so clear a light as to preclude mistaking its import. 
The state of things seems to be this: So soon as either cap begins 
to shrink, there proceeds to surround it a blue belt. The belt in- 
