OF HIGH REFRAN GIBILIT Y UPON GASEOUS MATTER. 
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acid, may either greatly intensify or greatly diminish the visible action of the light on 
many vapours decomposable alone or when mixed with air ; Avliile the presence of the one 
or the other of the same acids may provoke energetic actions in substances which are 
wholly inactive when left to themselves. 
We need not go beyond the nitrite of amyl for an example of this kind. For, prompt 
and copious as the decomposition of this substance is when mixed with air, the energy 
and brilliancy of the action are materially augmented by the presence of hydrochloric 
acid. Let a quantity of the nitrite vapour mixed with air be sent into the experimental 
tube till the mercury column sinks, say, 8 inches. Then let the flask containing the 
nitrite be removed and one containing strong hydrochloric acid be put in its place. Let 
purified air which has bubbled through the acid be carried into the experimental tube 
until a further depression of 8 inches is obtained. On allowing the convergent beam 
to play upon this mixture a cloud of extraordinary density and brilliance is precipitated. 
The beam appears to pierce like a shining sword the nebulous mass of its own creation, 
tossing the precipitated particles in heaps right and left of it. This experiment is very 
easily made, and nothing could more finely or forcibly illustrate the phenomena here 
under consideration. 
By varying the proportions of the vapour to the acid we vary the effects. For exam- 
ple, the proportion of 1 inch of the nitrite vapour to 15 inches of the hydrochloric acid 
did not produce so brilliant an effect as the proportion 8 : 8. The same is true of the 
proportion 15 inches of nitrite vapour to 1 inch of hydrochloric acid. But in this latter 
case, though the general action was less intense than in the case of 8 : 8, the iridescences 
due to diffraction were much finer. No doubt for each particular substance a deter- 
minable proportion exists which corresponds to the maximum of actinic action *. 
The nitrite of butyl affords another striking example of the influence of a second body 
in the experimental tube. With air, or alone, it was not visibly affected by the light ; 
there was no cloud formed by its exposure. It was also mixed with nitric acid in various 
proportions, but no visible effect was produced by the beam. 
It was then tried with air which had been permitted to bubble through pure hydro- 
chloric acid in the following proportions : — 
1. 1 inch of air and vapour to 15 inches of air and acid. 
2. 8 inches „ „ 8 
3. 15 inches ,, „ 1 inch „ „ 
In the first case a dense and brilliant cloud was immediately precipitated. In the 
second case the precipitation of the fine white cloud was confined to the convergent 
luminous cone, coarser particles being scattered through the rest of the tube. In the 
third case the cloud was very coarse and very scanty. The experiment indicates that 
the best effect is obtained when a small quantity of the vapour is mixed with a consider- 
able quantity of the acid. 
Benzol is also a good example of a substance which, when alone, defies the power of 
* This might form the subject of an interesting inquiry. 
