550 THE POPULAR SCIENCE MONTHLY. 



through it. At first nothing is visible. But very soon in the path of 

 the luminous ray we perceive a delicate blue color. It is because the 

 gas is decomposed by the luminous waves, and the invisible particles 

 of sulphur set free decompose the light in turn. The blue of the vapor 

 deepens, then it turns whitish, and at last a white cloud is produced. 

 The particles composing this cloud are still each by itself invisible, 

 even under strong microscopes, and yet they are infinitely more coarse 

 than the primitive atoms that occasioned the sky-blue tint at first 

 seen in the receiver. In this experiment we pass in steady progress 

 from the free atom of sulphur parted from the oxygen-atom by the 

 ether-waves to a mass apparent to the senses; but, if this mass is made 

 up of free molecules which defy the strongest magnifiers, what must 

 be the particles which have produced those very molecules ! 



A last instance of another kind will complete the proof as to the 

 minuteness of the elements of matter. When a clear solution of sul- 

 phate of aluminum is poured into an equally clear solution of sulphate 

 of potassa, the mixture at once grows turbid, and after a few seconds 

 myriads of little crystals, sparkling like diamonds, make their appear- 

 ance in the liquid, which are nothing else than crystals of alum. If 

 we suppose the diameter of these crystals to be ^ of an inch, it will 

 follow from this experiment that in the lapse of a few seconds crystals 

 had the power of producing themselves containing tens of millions 

 of molecules, each composed of 94 atoms, grouped in admirable har- 

 mony. The motions of these chemical atoms take place under the 

 influence of the same forces that guide the motions of those enor- 

 mous agglomerations of atoms called stars. The revolution of one 

 sun around another takes a thousand years, while these atoms in 

 course of combination perform hundreds of millions of such revolu- 

 tions in the millionth part of a second ! 



By varied and delicate calculations, Thomson has succeeded in 

 establishing the fact that, in liquids and transparent or translucent 

 solids, the mean distance between the centres of two contiguous atoms 

 is comprised between the ten-millionth and the two-hundred-millionth 

 part of -^g- of an inch. It is not easy to form an exact conception of 

 dimensions so small, of which nothing, among the objects that affect 

 our senses, can convey any idea. Thomson judges that the following 

 comparison may aid us to appreciate them : If we imagine a sphere as 

 large as a pea magnified, so as almost to equal the earth's volume, and 

 the atoms of that sphere enlarged in the same proportion, they will 

 then have a diameter greater than that of a shot, and less than that 

 of an orange. In other words, an atom is to a globe the size of a pea 

 what an apple is to the terrestrial globe. By arguments of quite an- 

 other kind, drawn in part from the study of chemical molecules, in 

 part from the phenomena of capillarity, Gaudin has ascertained, for the 

 dimension of the smallest particles of matter, figures very nearly the 

 same as Thomson's. The maximum distance apart of the chemical 



