348 
PROFESSOR TYNDALL ON THE ACTION OF RAYS 
2°. As long as the colour of the cloud remains distinctly blue, the light discharged 
from it normally is perfectly polarized ; this light may be utterly quenched by a Nicol’s 
prism, the cloud from which it issues being caused to disappear. Any deviation of the 
line of vision from the normal enables a portion of the light to reach the eye in all. posi- 
tions of the prism. 
3°. The plane of polarization of the perfectly polarized light is parallel to the direc- 
tion of the illuminating beam. Hence a plate of tourmaline with its axis parallel to 
the beam stops the light, and with its axis perpendicular to the beam transmits it. 
4°. A plate of selenite placed between the Nicol and the cloud shows the colours of 
polarized light, and as long as the cloud continues blue these colours are most vivid in 
the direction of the normal. 
5°. The particles of the incipient cloud are immeasurably small, but they gradually 
grow in size, and at a certain period of their growth cease to discharge perfectly polar- 
ized light. For some time afterwards the light that reaches the eye, when the Nicoi is 
in its position of minimum transmission, is of a magnificent blue colour. It is called 
in the following pages the residual blue. 
6°. Thus the waves that first feel the influence of size , both at the minor and major po- 
larizing limits of the growing particles, are the smallest waves of the spectrum. These 
waves are the first to accept polarization and the first to escape from it. 
7°. As the actinic cloud grows coarser in texture the direction of maximum polar- 
ization changes from the normal, enclosing an angle more or less acute with the axis of 
the illuminating beam. 
8°. In passing from section to section of the same cloud the plane of polarization 
often undergoes a rotation of 90°. In the following pages this is designated as a change 
from positive to negative polarization, or the reverse. 
§X. 
The experiments on benzol vapour and hydrochloric acid now to be described are of 
interest on optical rather than on chemical grounds. They were preceded by other expe- 
riments in which the vapour was mixed with nitric acid, and a minute residue of the 
latter lingering in the experimental tube may have influenced the results. The hydro- 
chloric acid employed, moreover, was the commercial acid, and could not be regarded 
as pure. Thus though the decomposition of a vapour was certain, that it was not the 
pure vapour of benzol mixed with pure hydrochloric acid gas may be taken for granted. 
Indeed other experiments executed with the pure acid reduced the action to nil. 
Dry air charged with the benzol vapour was permitted to enter the tube till a depres- 
sion of one inch of the mercurial column was obtained ; half an atmosphere of air charged 
with hydrochloric acid was then added. The action of light on this mixture was very 
powerful. The tube was for a moment optically empty, but its transparent contents 
were immediately shaken into a dense and luminous cloud. The normal polarization 
was here feeble, the oblique strong ; the selenite colours in the former case were weak, 
