ON THE CHEMICAL REACTIONS OF LIGHT. 71 



a little further on. During sixteen days of uninterrupted sunshine, from the 

 14th to the 31st of July, I exposed to solar light 1900 cuhic centimetres of 

 sulphurous acid in a tuhc of 2000 cuhic centimetres capacity ; and the 

 decomposing action was, after that lapse of time, heing still carried on in an 

 always sensible and wonderful manner. It was CAadent that every day the 

 solar action was only partial ; it stopped as soon as the precipitated molecules 

 of sulphur in motion in the tube were abundant enough to intercept all the 

 chemical rays as an opaque screen. 



The action of light on sidphuric acid was interesting to study ; it is one of 

 the most beautiful and instructive experiments which can be executed. It 

 smokes abundantly on exposure to air ; and this effect is attributed to the 

 absorption of aqueous vapour by this substance. This is not a correct ex- 

 planation, since in a perfectly dry vacuum the same phenomenon takes place. 

 In the tube with a dry vacuum of i to -^ of a millimetre I had introduced a 

 very fine thin tube contaiaing anhydrous sulphuric acid. '\Mien I broke the 

 tube, the Uttle explosion, and certainly the great and sudden expansion of 

 the substance, scattered about the vapour of the sulphiu-ic acid, which, owing 

 to the cold generated, was condensed into a white cloud, and appeared with a 

 dazzling resplendency in the luminous cones. Here the chemical rays are 

 powerless, they cannot destroy what they have produced. There is no more 

 decomposition, but sulphuric acid is, if I may use the comparison, like water 

 in the vesicular state in a cooled medium through which heat is about to pene- 

 trate. Insensible to the chemical rays, the sulphuric acid absorbs the calorific 

 rays, on the contrary, with prodigious energy. This absorption is so perfect 

 that all molecular motion ceases instantly. The molecules remain motionless, 

 as if busy in absorbing the heat ; and, as aqueous vapoui' does when heated, 

 they pass into the state of a transparent gas, assuming previously to their 

 apparent annihilation aU the most magnificent hues. If during the operation 

 the cock is rapidly opened and immediately closed, the great movement 

 of molecules so rapidly and so energetically produced ceases at once, by 

 the absorption of the heat. 



This invisible vapour, when stiU further and sirfficiently heated, wiU have 

 its component atoms so shaken by the amplitude of their new osciUatory 

 vibrations that they will be removed beyond the radius of their sphere of ac- 

 tion, and the molecular edifice of sulphuric acid is (in its turn) destroyed. 



I am afraid of fatiguing the attention of my hearers if I dcvelope at a 

 greater length the details of various experiments made with a large number of 

 gases and vapours. We must stop here. Yet what residts are to be noted ! 



Thus, for instance, in the most natural, perhaps, of all the groups that 

 constitute the family of metalloids (that which comprises chlorine, bromine, 

 iodine, fluorine) strange anomalies arc observed. Chlorine and hj-drogen 

 unite under the action of chemical rays and form hj'drochloric acid. The 

 latter, either dry or humid, and prepared with pure crystals of mineral salt 

 of chloride of sodium, cannot be decomposed by chemical rays of solar light ; 

 whereas hydriodic acid, on the contrary, can be decomposed ; it is true (and 

 this circumstance must not be overlooked) it is very difficult to procure this 

 gas free from atmospheric air. A curious circumstance in the examination of 

 hydriodic acid is, that the first shock of light frees part of the iodine, which 

 appears with its peculiar violet hue at the summit of the cones, amidst the 

 movements which destroy the molecular edifices — movements, perhaps, pro- 

 duced by the calorific rays only. 



Bromine presents the peculiarity, that (as in the case of hydrogen with 

 chlorine) if piire and dry hydrogen is introduced with a small quantity of 



