1868.] Chemical Reactions produced by Light. 101 



the experimental tube, no diminution of its chemical energy was observed. 

 Permitting it to pass through a solution of hydrobromic acid, of the same 

 thickness, the chemical energy of the beam was wholly destroyed. This 

 shows that the vibrations of the dissolved acid are synchronous with those of 

 the gaseous acid, and is a new proof that the constituent atoms of the 

 molecule, and not the molecule itself, is the seat of the absorption. 



Hydrochloric Acid. — The aqueous solution of this acid was also employed 

 and treated like the solution of hydrobromic acid. I intend to invoke 

 the aid of an artistic friend in an effort to reproduce the effects ob- 

 served during the decomposition, if such it be, of hydrochloric acid by 

 light. But artistic skill must, I fear, fail to convey a notion of them. The 

 cloud was of slow growth, requiring 1 5 or 20 minutes for its full develop- 

 ment. It was then divided into four or five sections, every adjacent two 

 of which were united by a slender axial cord. Each of these sections pos- 

 sessed an exceedingly complex and ornate structure, exhibiting ribs, spears, 

 funnels, leaves, involved scrolls, and iridescent fleurs-de-lis. Still the struc- 

 ture of the cloud from beginning to end was perfectly symmetrical ; it was 

 a cloud of revolution, its corresponding points being at equal distances from 

 the axis of the beam. There are many points of resemblance between the 

 clouds of hydrochloric and hydrobromic acid, and both are perfectly distinct 

 from anything obtainable from the substances previously mentioned ; in 

 fact every liquid appears to have its own special cloud, varying only within 

 narrow limits from a normal type. The formation of the cloud depends 

 rather upon its own inherent forces than upon the environment. It is 

 true that, by warming or chilling the experimental tube at certain points, 

 extraordinary flexures and whirlwinds may be produced ; but with a per- 

 fectly constant condition of tube, specific differences of cloud- structure are 

 revealed, the peculiarity of each substance stamping itself apparently upon 

 the precipitated vapour derived from its decomposition. 



When the beam before entering the experimental tube was sent through a 

 layer of the aqueous acid, thirteen minutes' exposure produced no action. 

 A layer of water being substituted for the layer of acid, one minute's ex- 

 posure sufficed to set up the decomposition. 



Hydriodic Acid. — The aqueous solution of this acid was also employed. 

 On first subjecting it to the action of light no visible effect was produced ; 

 but subsequent trials developed a very extraordinary one. A family re- 

 semblance pervades the nebulse of hydriodic, hydrobromic, and hydro- 

 chloric acids. In all three cases, for example, the action commenced 

 by the formation of two small clouds united by a cord ; it was very 

 slow, and the growth of the cloud in density and beauty very gradual. 

 The most vivid green and crimson that I have yet observed were exhi- 

 bited by this substance in the earlier stages of the action. The de- 



