OF HIGH REFRAN GIBILIT Y UPOA GASEOUS MATTER. 
347 
§IX. 
I have now to introduce, though only for partial treatment, a subject which might 
with advantage be kept isolated, but which is so mixed up with my notes of 1868 as to 
be inseparable from the descriptions of chemical action which they contain. I refer to 
the blue colour always exhibited at the birth of clouds obtained from small quantities 
of vapour in the case of active substances, and often from large quantities in the case of 
substances of slow decomposition. The first distinct record of this appearance occurs in 
my notes for the 10th of October, 1868. On the 9th I had been engaged upon the 
iodide of allylwith reference to its interaction with hydrochloric acid. Small quantities 
only of the vapour had been employed ; and it was found that when the acid was fresh 
and strong the action was vigorous, that it declined in energy as successive charges 
of dry air were sent through the acid, becoming vanishingly feeble on the fifth filling 
of the experimental tube. 
On the morning of the 10th the tube used on the preceding day was washed with 
distilled water, and swept out by a current of dry air. A mixture of air and hydrochloric 
acid was then sent into it, no vapour of any kind being employed. When the light first 
passed through it, and for some time afterwards, the experimental tube appeared per- 
fectly empty. Slowly and gradually, however, upon the condensed beam a cloud was 
formed which passed in colour from the deepest violet, through blue, to whiteness. To 
this record of my note-book the remark is added, “ connect this blue with the colour 
of the sky.” 
In fact it was impossible to avoid seeing the relationship of both. Previous to this 
entry the blue had attracted my attention. It was unfailing it its appearance when the 
action was slow. The blue colour was in all cases the herald of the denser actinic cloud. 
I took a pleasure in developing it in connexion with general actinic action, and in deter- 
mining whether in all its bearings and phenomena the blue light was not identical with 
the light of the sky. This to the most minute detail appears to be the case. The inci- 
pient actinic clouds are to all intents and purposes pieces of artificial sky, and they furnish 
an experimental demonstration of the constitution of the real one. 
Reserving the fuller discussion of the subject for a subsequent paper, it maybe stated 
in a general way that all the phenomena of polarization observed in the case of skylight 
are manifested by these blue actinic clouds ; and that they exhibit additional phenomena 
which it would be neither convenient to pursue, nor perhaps possible to detect, upon 
the actual firmament. They enable us, for example, to follow the growth and modifica- 
tion of the phenomena of polarization from their first appearance in the barely visible 
blue, to their final extinction when the cloud has become so coarsely granular as no 
longer to scatter polarized light. 
These changes, as far as it is now necessary to refer to them, may be thus described. 
1°. The incipient cloud, as long as it continues blue, discharges polarized light in all 
directions, but the direction of maximum polarization is at right angles to the direction 
of the illuminating beam. 
