ight. Let us suppose
that this play of light had always existed, so far as those two objects
were concerned. The marble would appear to be permanently green, and not
white; and if we had not a simple way of removing the light, we should
certainly say it was green marble. Could we as effectually change the
play of light which causes grass to appear green, we should at once
demonstrate as readily, that its color was an appearance to the eye, not
a part of the grass itself. It is very probable that we are extensively
deceived in this way,--that many appearances in nature are only
simulations which we have no means of detecting. Isomerism in minerals
has been discovered,--a state in which quite different physical
properties are coexistent with identity of component parts. What we
always see, and what seems to be permanent, we naturally accept as a
physical fact; and yet we can understand that our senses may, in many
instances, be the sport of appearances which, because permanent, we
conceive to be reality. Thus color is a cerebral sensation only, and
grass is not green.

Is sugar sweet? That sugar has certain chemical constituents which go to
make up a saccharine compound we know. But what evidence have we of its
sweetness, except that the nerves of taste are peculiarly affected when
brought in contact with it. Its sweetness is not measurable in the
chemist's scales. It can be analyzed, and its constituent elements
accurately defined. But sweetness is not one of those elements. The test
of that is the tongue. Pure sugar of milk has scarce any sweetness at
all; nevertheless, it is pure sugar. The influence which it has on the
nerves of taste is only different from that of cane-sugar. Destroy the
nice nervous connection between the tongue and the brain, and sweetness
disappears. A severe cold will accomplish this, and while the touch of
the sugar is felt, the delicate sympathy which is awakened by the sugar
and is felt in the brain as sweetness is destroyed. The sweetness, like
the color, is a nervous sensation. We can conceive of a development of
the nerves of taste which might receive a host of new impressions from
contact with objects now tasteless. The saccharine compound does exist
as a chemical quantity, and has a special effect on the nerves of taste,
exciting them peculiarly, the result of the excitement being the idea of
sweetness.

Is the rose fragrant? The sense of smell is indeed only a continuation
of that of t