137 
He then proceeded to perform an experiment shewing new 
actions effected under new conditions, rays of light producing 
chemical action. Into a long glass tube carefully exhausted, a 
small quantity of air had been allowed to pass through tubes 
containing (1) cotton wool, (2) caustic potash and marble, (3) 
sulphuric acid and glass, (4) nitrite of amyl, with the vapour of 
which last substance the air in the experimenting tube was 
loaded. 
A beam of electric light was then passed along the tube, and 
a beautiful cloud was at once formed, gradually extending from 
the end nearest the lamp towards the other. The tube was then 
reversed, and the same phenomenon produced at the other end. 
What, asked the lecturer, is the vapour in which this change 
has been produced? A collection of molecules, each molecule 
being built up of nineteen atoms; the waves of light beating 
against these invisible molecules have broken them up and re- 
arranged them in such a form that they become visible. 
In this case the vapour of the substance itself was directly 
exposed to the action of the light; other substances require to 
be introduced into the experimenting tube in connexion with 
vapours which aid as it were the process of decomposition. Ben- 
zole vapour for example is unaffected, but if mixed with air 
passed through aqueous nitric acid, with which it has a tendency 
to combine, it is precipitated at once on the beam of light. This 
is an illustration of one of the commonest of nature’s operations, 
vegetation : the carbonic acid and the chlorophyll are side by 
side in leaves, ready to unite but unable to do go of themselves ; 
the ray of sunshine falling on the leaves consummates the union, 
gives the green colour to the leaves, and sets oxygen free into 
the air. 
The action of which an example has been given might be 
made very slow if the vapour tested were sufficient] y attenuated ; 
the growth of the visible particles might be very slow. What 
should we expect under these circumstances ? 
