358 
PEOFESSOE TYNDALL ON THE ACTION OF EATS 
I looked through the Nicol towards the cloud. For a minute it was absolutely extin- 
guished. Continuing to look in the same direction the residual colour appeared, and 
passed from a rich deep violet to a hard whitish blue. It was exceedingly interesting 
to watch the growth and change of the residual colour. At a certain period of its exist- 
ence it rivalled the richest blue of the spectrum. 
In two or three minutes the anterior portion of the tube was filled by a thick cloud 
generated by the beam. The cloud rapidly diminished in density as the more distant 
end ot the tube was approached. It was composed of two longitudinal lobes, which, 
looked at obliquely in a vertical plane, discharged light polarized in planes at right 
angles to each other. 
When the cloud was looked at normally, the line of vision being horizontal, on one 
-side of the centre the polarization was positive, on the other side negative. Moved to 
and fro across the neutral section, the sudden expansion and contraction of the selenite 
bands was very curious. 
After twenty minutes’ action the neutral section was abolished, and the normal polar- 
ization (now feeble) became the same throughout the entire length of the cloud. 
II. Air and benzol vapour ..... 8 inches ; then 
Air and aqueous nitric acid . , . S inches. 
On starting the light the tube was not optically empty, but crowded with particles. 
Through them the beam appeared to force its way like a spear, bringing down upon 
itself a finer cloud, which soon swathed and masked the coarser spherules. 
This experiment was many times repeated, but it was found impossible to bring the 
benzol and nitric acid together in the quantities here employed without the formation 
of a crowd (cloud would hardly be the word) of coarse particles. Chemical action had 
manifestly set in without the intervention of the light. 
The chemical action without light appearing to depend on the quantity of benzol 
vapour and nitric acid present, I varied that quantity. When 2 inches of each were 
admitted into the experimental tube, no particles were seen when the lamp was ignited. 
A quarter of a minute after the starting of the lamp the track of the beam became blue. 
This light remained perfectly polarized for a minute. In three minutes a dense cloud 
had filled the tube. In the two ’rectangular' positions of the Nicol the cloud exhibited 
a salmon-colour and a hard bluish greenish white. 
When the quantities of the two vapours were 4 inches each, there were no particles in 
the tube when the lamp was ignited. No doubt the substances were ready to attack 
each other, and in less than a quarter of a minute the beam precipitated the attack. 
The action was exceedingly vigorous. For a moment, and only for a moment, the 
polarization was perfect. In less than a minute the rapid thickening of the cloud and 
the quick growth of its particles abolished almost all traces of polarization. 
When the quantities were 5 inches to 5, particles were found in the experimental tube 
on starting ; and the same occurred with all greater quantities. When, for example, the 
