ground:  that the subject is not one that lends itself easily to experimental demonstration before an audience.  Many of the experiments can only be made on a small scale, and require to be watched closely.  However, by help of diagrams and by not confining myself too closely to our special investigation, but dealing somewhat with the wider subject of dust in general, I may hope to render myself and my subject intelligible if not very entertaining.

First of all, I draw no distinction between “dust” and “smoke.”  It would be possible to draw such a distinction, but it would hardly be in accordance with usage.  Dust might be defined as smoke which had settled, and the term smoke applied to solid particles still suspended in the air.  But at present the term “smoke” is applied to solid particles produced by combustion only, and “dust” to particles owing their floating existence to some other cause.  This is evidently an unessential distinction, and for the present I shall use either term without distinction, meaning by dust or smoke, solid particles floating in the air.  Then “fog”; this differs from smoke only in the fact that the particles are liquid instead of solid.  And the three terms dust, smoke, and fog, come to much the same thing, only that the latter term is applied when the suspended particles are liquid.  I do not think, however, that we usually apply the term “fog” when the liquid particles are pure water; we call it then mostly either mist or cloud.  The name “fog,” at any rate in towns, carries with it the idea of a hideous, greasy compound, consisting of smoke and mist and sulphur and filth, as unlike the mists on a Highland mountain as a country meadow is unlike a city slum.  Nevertheless, the finest cloud or mist that ever existed consists simply of little globules of water suspended in air, and thus for our present purpose differs in no important respect from fog, dust, and smoke.  A cloud or mist is, in fact, fine water-dust.  Rain is coarse water-dust formed by the aggregation of smaller globules, and varying in fineness from the Scotch mist to the tropical deluge.  It has often been asked how it is that clouds and mists are able to float about when water is so much heavier (800 times heavier) than air.  The answer to this is easy.  It depends on the resistance or viscosity of fluids, and on the smallness of the particles concerned.  Bodies falling far through fluids acquire a “terminal velocity,” at which they are in stable equilibrium—­their weight being exactly equal to the resistance—­and this terminal velocity is greater for large particles than for small; consequently we have all sorts of rain velocity, depending on the size of the drops; and large particles of dust settle more quickly than small.  Cloud-spherules are falling therefore, but falling very slowly.

To recognize the presence of dust in air there are two principal tests; the first is, the obvious one of looking at it with plenty of light, the way one is accustomed to look for anything else; the other is a method of Mr. John Aitken’s, viz., to observe the condensation of water vapor.