ON THE CHEMICAL REACTIONS OF LIGHT. 67 



passes tlirough it without the couca beiug in the least perceived ; the tube 

 is optically vacuous ; but wheu the gas or vapour is iutroduced, a blue cloud, 

 or blue precipitate, of au incomparable delicacy appears after a varying 

 lapse of time, first at the summit of the cones, then in the converging cone. 

 As long as the precipitate is of that beautiful blue colour, the light which it 

 sends to the eye is perfectly polarized as Mr. TyndaU has described it. 

 Then by slow degrees the precipitate increases, and slowly becomes white, 

 the light emitted stUl remaining polarized; but as the sides of the tube 

 lighted by the cones send to the eye light wliich is also polarized, and as it were 

 the product of a polarizer, the whitish vapour of the cones then behaves like 

 a thin polarizing lamina under inspection through the Nicol held by the 

 observer ; and when the two polarization planes are perpendicular to one 

 another, the whitish cones assume a magnificent blue colour, exactly as a thin 

 lamina of selenite of the required thickness would do ; but iu proportion 

 as the white precipitate increases, all polarization disappears. The precipi- 

 tated molecules become heated, both at the summits of the cones and against 

 the entrance glass plate, and there arises a motion, slow at first, which 

 brings to the cones other particles of the body under experimentation, not 

 yet acted on by the light ; and as these particles, owing to transparency, 

 appear dark, their iutermiugling with the white particles produces a series 

 of the most beautiful, varying, and often most regular images, such as 

 have been described with expressions of genuine admiration by Mr. Tyndall. 



The precipitated molecules seem to increase in diameter, and the two 

 cones are then resplendent with reflected light. But there are, for the eye, 

 certain points where the cone takes a fine rosy colour ; and the position of 

 these points varies in the course of the same experiment. They are some- 

 times on a line which forms an angle of 45° with the axis of the converging 

 cone, sometimes on a line forming an angle of 90° + 45° with the same axis. 

 There is, as in the rainbow, a line of position, and for the precipitated 

 molecules a zone of cfiicacious rays, which demand special and ulterior 

 inquiries. 



Let me resume the exposition of facts already so well described by the 

 English physicist. 



It will be easily seen that either a synthesis or a decomposition of a body 

 has taken place, i. e. a new grouping of the atoms, when under the influence 

 of concentrated solar light the blue cloud invades the cones. 



The first body of which I have attempted the synthesis is that which I 

 have so often and so easily obtained by electricitj% by causing the induction 

 spark to pass through a gaseous mixture, formed of one of oxygen, two of 

 nitrogen, and three of sulphurous acid *. It is the compound which is 

 formed in the leaden chambers in the preparation of sulphuric acid. 



I submitted the gaseous mixture to the solar action immediately the 

 compound was produced. The same gaseoiis mixture enabled me to recognize, 

 like Mr. Tyndall, that the special rays which produce these reactions are 

 neither the less refrangible cjdorific rays, nor the red rays even when highly 

 concentrated. I have for this object made xiae of the greatest variety of 

 screens — smoky quartz, iodine dissolved in bisulphide of carbon, CS", then 

 coloured glasses, red, orange, yellow, green ; biit I dislike and distrust the 

 glass screens ; I prefer by far liquid ones. The action produced was insigni- 

 ficant. It became, on the contrary, extremely energetic with blue and 

 violet glasses. 



It is therefore under the shock of the more rapid oscillations of the chcmi- 

 * Jlde Annales cle Chimie et do Physique, vol. vi. series 4, 



f2 



