Because they reflect back to us the solar beam _unchanged_. 532. _Why does the sky appear red at sunset?_ Because the light vapours of the air, which are condensed as the sun sets, refract the rays of light, and produce red rays. The refraction which produces _red_ requires only a _moderate degree of density_. 533. _Why do the clouds sometimes appear yellow?_ Because there is a larger amount of vapour in the air, which produces a different degree of refraction, _resulting in yellow_. 534. _Why does a yellow sunset foretell wet weather?_ Because it shows that the air is heavy with vapours. The refraction that produces _yellow_ requires a greater degree of density. [Verse: "When it is evening ye say it will be fair weather, for the sky is red."--MATT. XVI.] 535. _Why does a red sunset foretell fine weather?_ Because the redness shows that the vapours in the air _towards the West_, or wet quarter, are _light_, as is evidenced by the degree of refraction of the sun's rays. 536. _Why does a red sunrise foretell wet?_ Because it shows that _towards the East_, or dry quarter, the air is charged with vapour, and therefore probably at other points the air has reached _saturation_. 537. _Why does a grey sunrise foretell a dry day?_ Because it shows that the vapours in the air are _not_ very dense. 538. _Why is "a rainbow in the morning the shepherd's warning?"_ Because it shows that _in the West_, or wet quarter, the air is _saturated_ to the rain point. 539. _Why is "a rainbow at night the shepherd's delight?"_ Because it shows that the _rain is falling in the East_, and as that is a dry quarter, it will _soon be over_. Rainbows are always seen in opposition to the sun. CHAPTER XXVI. 540. _What is the difference between light and heat?_ The most obvious distinction is, that light acts upon _vision_, and heat upon _sensation_, or feeling. Another distinction is, that _heat expands all bodies_, and alters their atomic condition; while _light_, though usually attended by heat, does not display the same expansive force, but produces various effects which are _peculiar to itself_. [Verse: "Ye are the light of the world. A city that is set on a hill cannot be hid."--MATTHEW V.] 541. _Are light and heat combined in the solar ray?_ Yes. A ray of light, as well as containing elementary rays that produce colours under refraction, contains also _chemical rays_, and _heat rays_. 542. _How do we know that light and heat are separate elements?_ Because we have _heat rays_, as from dark hot iron, from various chemical actions, and from friction, which are _unattended by the development of light_. And we have light, or luminosity, such as that of _phosophoresence_, which is unaccompanied by any appreciable degree of heat.