6G REPORT — 1869. 



On the Chemical Reactions of Light discovered by Professor Tyndall. 

 By Professor Morren^ of Marseilles. 



[A communication ordered to be printed in exienso among the Reports.] 



SrtfCE the last session of the British Associatiou, Mr. Tyndall has published, 

 in several papers, highly interesting researches on a particular species of 

 luminous reactions, thus providing physicists and chemists with a new 

 instrument, both of synthesis and analysis, to which he invites the attention 

 and investigation of all whom it may concern. In obedience to this scientific 

 challenge, I have repeated, with the utmost care, all the learned gentle- 

 man's experiments. I have found them all as rigorously exact as they are 

 ably described. They refer to atomical evolutions in which wc almost seem 

 to detect Nature in her most mysterious operations. The molecules of 

 bodies, when powerfully lighted, the observer himself being in absolute 

 darkness, may be easily perceived in their infinitely minute motions ; in 

 following which, Mr. Tyndall, and every one who is passionately desirous 

 of penetrating the secrets of the constitution of bodies, cannot but feel the 

 most exciting curiosity. Mr. TjTidall has made rise principally of electric 

 light, and has caiiscd it to act mostly on organic bodies. I have followed in 

 this respect quite a different method and object. 



Most favourably situated with respect to solar light, in Provence, where 

 for months together we enjoy a cloudless sky, I proposed to limit myself to 

 the iise of solar rays only, and confine myself to those conditions which 

 the atmosphere afibrds us. I have carefully avoided organic bodies, the 

 molecules of which are higlily complicated and most difficult to follow in their 

 multifarious evolutions. I have even preferred the simpler bodies of mineral 

 chemistry, as offering an easier field of observation to the physicist desirous 

 of arriving at clear and precise results. 



In this exposition I shall follow, step by step, the order which, without 

 any preconceived or systematical ideas, directed my successive experiments. 

 It is, so to say, a journey in an unknown land. I shall thus the better 

 show the deceptions I met with, the incessantly supervening difficulties, 

 the necessities of modifying the apparatus, and the various incidents of the 

 route. I hope by this means the better to illustrate the object I had in view. 



At the outset I made use of an experimental ajiparatus entirely similar 

 to Mr. Tyndall's — a glass tube 8 to 9 centimetres in diameter and 1 metre 

 long, fitted at each extremity into a broad brass ring luted with cement, 

 and carefully ground in the anterior part ; two plain pieces of plate glass, 

 perfectly transparent, resting on the rings, and slightly lubricated on the 

 edges with a fatty substance, constituted an air-tight cavity when a vacuum 

 was formed. A glass cock, fitted on each extremity, and let into the 

 cylindrical rings, allowed me to make the vacuum at one end, and to introduce 

 at the other the gas or vapour on which the experiments were to be made. 

 Extreme, if not absolute cleanness and perfectly dry tubes arc indispen- 

 sable requisites. 



The light arrives in the tube after condensation by a lens which every- 

 thing induces me to believe was not achromatic in Mr. Tyndall's experi- 

 ments. In his apparatus, as in mine, two cones are formed joined at the apices, 

 the first, the converging cone, with an orange-red periphery, the other, the 

 diverging one, with a violet-blue peripherj^, circumstances wliich I notice 

 here, because the white molecules which are about to appear wiU often 

 assume, in passing through them, the hues of the luminous bands in which 

 they circulate. When a vacuum has been produced in the tube, the light 



