156 Royal Society: — 



pared recent maps of the solar spectrum, one by Kirchhofr, the other 

 by Angstrom, made a few years apart and at different epochs with 

 regard to the sun-spot period. If you look at these maps you will 

 see a vast difference in the relative thicknesses of the C and F lines, 

 and great differences in the relative darkness and position of the 

 lines ; and if I had time I could show you that we now may be sup- 

 plied with a barometer, so to speak, to measure the varying pres- 

 sures in the solar and stellar chromospheres ; for, depend upon it, 

 every star has, has had, or will have a chromosphere, and there are 

 no such things as " worlds without hydrogen," any more than there 

 are stars without photospheres. I suggested in 1866 that possibly 

 a spectroscopic examination of the sun's limb might teach us some- 

 what of the outburst of the star in Corona ; and already we see that 

 all that is necessary to get just such an outburst in our own sun is to 

 increase the power of his convection-currents, which we know to be 

 ever at work. Here, then, is one cataclysm the less in astronomy 

 — one less " world on fire," and possibly also a bright light thrown 

 on the past history of our own planet. 



I might show you further that we now are beginning to have a 

 better hold on the strange phenomena presented by variable stars, 

 and that an application of the facts I have brought to your notice 

 this evening, taken in connexion with the various types of stars 

 which have been indicated by Father Secchi with admirable philo- 

 sophy, opens out generalizations of the highest interest and import- 

 ance, and that, having at length fairly grappled with some of the 

 phenomena of the nearest star, we may soon hope for more certain 

 knowledge of the distant ones. 



At present, however, we may well leave speculation for those who 

 prefer it to acquiring facts ; let us rather, emboldened by the work 

 which this new method of research has enabled us to accomplish in 

 this country, under the worst atmospheric conditions, in seven short 

 months, go on quietly deciphering one by one the letters of this 

 strange hieroglyphic language which the spectroscope has revealed 

 to us — a language written in fire on that grand orb which to us 

 earth-dwellers is the fountain of light and heat, and even of life 

 itself. 



ROYAL SOCIETY. 

 [Continued from p. 73.] 

 March 4, 1869. — Lieut. -General Sabine, President, in the Chair. 

 The following communication was read : — 



" Note on the Formation and Phenomena of Clouds/' By John 

 Tyndall, LL.D., F.R.S. 



It is well known that when a receiver filled with ordinary undried 

 air is exhausted, a cloudiness, due to the precipitation of the aqueous 

 vapour diffused in the air, is produced by the first few strokes of the 

 pump. It is, as might be expected, possible to produce clouds in 

 this way with the vapours of other liquids than water. 



In the course of the experiments on the chemical action of light 



