OP HIGH REFRAN GIBILIT Y UPON GASEOUS MATTER. 
tion of a faint cloud. It appeared first at the focus. In a couple of minutes more a 
faint blue, perfectly polarized along the normal, filled the anterior portion of the tube. 
The blue also extended from the place of most vigorous action down the tube. An 
amorphous cylinder of cloud soon filled the first 10 inches of the tube, and pushed gra- 
dually down it. It was followed by a complicated cloud-figure, and it again by a vase- 
shaped nebula fainter than either. At the end of fifteen minutes a body of light, which, 
considering the amount of matter involved, was simply astonishing, was discharged from 
the cloud. In one position of the Nicol this cloud was a salmon-colour, in the other a 
blue-green. When a plate of tourmaline, with its axis parallel to the beam, was passed 
along in front of the cloud, at some places it showed a particularly vivid blue-green. 
When placed perpendicular at these places, the field of the crystal was a yellow-green. 
I doubt whether spectrum analysis itself is competent to deal with more minute traces 
of matter than those revealed by actinic decomposition. I think it probable that if the 
weight of the cloud formed in this experiment were multiplied by trillions it would not 
amount to a single grain. Bodies placed behind it were seen undimmed through the 
cloud. The flame of a candle suffered no sensible diminution of its light. It was easy 
to read through the cloud a page which the cloud itself illuminated. In fact the cloud 
was a comet’s tail on a small scale. It proved that matter of almost infinite tenuity is 
competent to shed forth light of similar quality, and in far greater quantity than that 
discharged by the tails of comets*'. 
These facts render the statement intelligible that even when all reasonable precautions 
appear to have been taken it is not easy to escape every trace of chemical action on 
first charging the experimental tube even with an inert substance. In my earlier 
experiments, when distilled water only was employed to cleanse the tube, the first 
experiment with air alone was sure to develope an actinic cloud of a beautiful fern- 
leaf pattern. And even now, after the most careful employment of the soft soap and 
hot water, the first charge of pure nitric, or of pure hydrochloric acid often developes a 
blue and exceedingly delicate actinic cloud. As regards the optical question, these irre- 
gular clouds exhibit some of the finest effects. One additional Tact will illustrate a class 
of disturbances already touched upon. Pure nitric acid had been proved over and over 
again to exhibit no visible action ; but after having demonstrated its inertness, a case 
occurred where it produced rather dense actinic clouds five times in succession. Indeed 
there seemed to be no end to their possible development. The only thing to which this 
change from inertness to activity could be ascribed, was a change in the cerate used to 
render the ends of the tube air-tight. On examination it was found that the infinitesimal 
effluvia yielded by the new cerate to the nitric acid was the sole cause of the anomaly. 
Nitric acid, then, produces no actinic cloud ; hydrochloric acid produces no actinic cloud ; 
air passed throughout potash and sulphuric acid produces no actinic cloud, no matter 
how powerful or how long-continued the action of the light may be. 
* The action here referred to has been since developed into a formal hypothesis of cometary phenomena. I 
shall return to the subject. 
