These lines in the solar spectrum are there because something is at work cutting out those rays of light which are wanting, and they explained this want by showing to us that around the sun and all the stars there are absorbing atmospheres containing the vapours of certain substances cooler than the interior of the sun or of the stars. These philosophers also showed us, that we can divide radiation and absorption into four classes, and that we can have general radiation and selective radiation, and general absorption and selective absorption, so that the phenomena that we see in our chemical and physical laboratories and our observatories may all be classed as general and selective radiation, or general and selective absorption. Let us explain these terms more fully. Kirchhoff showed us that from incandescent solid and liquid bodies we get a continuous spectrum; thus from the carbon poles of an electric lamp we get a complete spectrum. That is called a continuous spectrum, and it is an instance of continuous radiation, which we get from the molecular complexity of solids or liquids, and likewise, from dense gases or vapours. When we examine vapours or gases which are not very dense we get an indication of selective radiation—that is to say, the light one gets from these substances, instead of being spread broadcast from the red to the violet, will simply fall here and there on the spectrum; in the case of one vapour we may get a yellow line—a yellow image of the slit—and in the case of another vapour, we may get a green one; the light selects its point of appearance, and does not appear all along the spectrum. [Illustration: FIG. 179.—Electric Lamp. _y_, _z_, wires connecting battery of 50 Grove or Bunsen elements; G, H, carbon holders; K, rod, which stops a clockwork movement, which when going makes the poles approach until the current passes; A, armature of a magnet which by means of K frees the clockwork when not in contact; E, electro-magnet round which the current passes when the poles are at the proper distance apart, causing it to attract the armature A. ] This selective radiation is due to a simplification of the molecular structure of the vapours, the simpler states are less rich in vibrations, and therefore instead of getting rays of _all_ refrangibilities we only get rays of _some_. [Illustration: FIG. 180.—Electric Lamp arranged for throwing a spectrum on a screen. D, lens; E E´, bisulphide of carbon prisms. ]