Take these in order.  When a sunbeam enters a darkened room through a chink, it is commonly said to be rendered visible by the motes or dust particles dancing in it; but of course really it is not the motes which make the sunbeam visible, but the sunbeam the motes.  A dust particle is illuminated like any other solid screen, and is able to send a sufficient fraction of light to our eyes to render itself visible.  If there are no such particles in the beam—­nothing but clear, invisible air—­then of course nothing is seen, and the beam plunges on its way quite invisible to us unless we place our eyes in its course.  In other words, to be visible, light must enter the eye.  (A concentrated beam was passed through an empty tube, and then ordinary air let in.)

The other test, that of Mr. Aitken, depends on the condensation of steam.  When a jet of steam finds itself in dusty air, it condenses around each dust particle as a nucleus, and forms the white visible cloud popularly called steam.  In the absence of nuclei Mr. Aitken has shown that the steam cannot condense until it is highly supersaturated, and that when it does it condenses straight into rain—­that is, into large drops which fall.  The condensation of steam is a more delicate test for dust than is a beam of light.  A curious illustration of the action of nuclei in condensing moisture has just occurred to me, in the experiment—­well known to children—­of writing on a reasonably clean window-pane with, say, a blunt wooden point, and then breathing on the glass; the condensation of the breath renders the writing legible.  No doubt the nuclei are partially wiped away by the writing, and the moisture will condense into larger drops with less light-scattering power along the written lines than over the general surface of the pane where the nuclei are plentiful, and the drops therefore numerous and minute.  Mr. Aitken points out that if the air were ever quite dustless, vapor could not condense, but the air would gradually get into a horribly supersaturated condition, soaking all our walls and clothes, dripping from every leaf, and penetrating everywhere, instead of falling in an honest shower, against which umbrellas and slate roofs are some protection.  But let us understand what sort of dust it is which is necessary for this condensing process.  It is not the dust and smoke of towns, it is not the dust of a country road; all such particles as these are gross and large compared with those which are able to act as condensers of moisture.  The fine dust of Mr. Aitken exists everywhere, even in the upper regions of the atmosphere; many of its particles are of ultra-microscopic fineness, one of them must exist in every raindrop, nay, even in every spherule of a mist or cloud, but it is only occasionally that one can find them with the microscope.  It is to such particles as these that we owe the blue of the sky, and yet they are sufficiently gross and tangible to be capable of being filtered out of the