70 EEPORT— 1869. 



behold the coloured bands which form the outline of the cones ; and then even 

 the violet rays (scarcely perceptible under ordinary circumstances) can be easily 

 seen in the interior of the first cone. One may thus easily follow and account 

 for the varied hues which the whitish vapour assumes when the variations of 

 tempera tui-e waft it about the cones, intermingled with the black streaks 

 arising from such portions of the gas as have not yet been acted on. 



But to return to the sulphurous acid : What was the substance, blue at 

 first, then whitish, thus obtained '? The two hues so different were merely 

 the consequence of a difference in diameter — the first hue belonging to 

 the bodies (perhaps atoms) precipitated in the minutest state of division. The 

 white and pearly colours appear when the diameter has sufficiently increased, 

 and goes on stiU increasing. These circumstances induced me to endea- 

 vour to measure this diameter, a thing feasible by different means, but 

 not with ease and faciUty by microscopic inspection, however ; for the small 

 spark-like bodies move too rapidly through the field of the microscope. But 

 there were other methods. It may be first observed that if you expose sul- 

 phurous acid for a sufficient length of time to solar action, the precipitated 

 substance becomes sufiiciently abundant for a sort of haze to become per- 

 ceptible in the tube. A part of the gas has been acted on ; and the two 

 lumiuous cones, when moved about in the tube, find everywhere reflecting 

 precipitated molecules. If the tube is left long enough in repose and dark- 

 ness, the cloud collapses, forms a deposit, and the gas is restored to its primi- 

 tive transparency. If at this jDoint the tube be again placed under solar 

 action, the same successive phenomena of haze and return to transparency 

 may be repeated indefinitely, tUl there remains no more gas to be decomposed, 

 which is a very long process, for the very formation of the precipitate inter- 

 cej^ts itself the chemical rays which form it. 



But since transparency has been produced, a deposit of molecules must 

 have taken place ; and in this case, if the tube remains in a horizontal posi- 

 tion and a broad slip of glass be placed inside it, it is on this slip that the 

 molecules will be found. Their diameter might then be measured, either di- 

 rectly or by the rings of diffraction ; but unfortunately it became impossible to 

 use this expedient : the sulphur was dissolved by sulphuric acid ; and instead 

 of the molecules which I expected to find, very small drops only were per- 

 ceived. But nevertheless it was possible to obtain the molecules of sul- 

 phur in large quantities, and to submit it to all reqiiisite reactions so as to 

 recognize it completely. It is sufficient to place in the tube after the 

 sulphurous acid three or five cubic centimetres of distilled water ; and after 

 the solar rays have sufficiently acted on the gas, the tube is then shaken ; 

 the water dissolves the sulphuric acid and renders it powerless for dissolving 

 the precipitated sulphur ; the water then becomes milky and the sulphur 

 is collected. It is then boiled in a small glass vessel (to di'ive out the sul- 

 phurous gas) and filtered in order to collect the sulphur ; the water then 

 gives abundant proof of the presence of sulphimc acid. 



The atoms of the molecules of sulphuj'ous acid are therefore not able to 

 support the shock of the undulations of the chemical rays ; they divide into 



sulphur and sulphuric acid, 3 S0^=S + 2S0^ (and from the form 



they change to C&kmD> '^^ '"'^ich state they are able to resist the shock of 

 the chemical rays) ; but they exhibit other phenomena, to which I shall return 



