350 
PROFESSOR TYNDALL ON THE ACTION OF RAYS 
I inch of the mixed air and vapour was now admitted into the experimental tube, and 
after it half an atmosphere of air which had bubbled through aqueous hydrochloric acid. 
The instant the beam passed through the experimental tube an intensely white cloud 
was precipitated. 
The tube being cleansed, one-tenth of an inch of the nitrite and air, followed by one- 
tenth of an atmosphere of air and hydrochloric acid, w r as sent into it. The blue of the 
incipient cloud was in this instance perfectly superb. The polarization at right angles 
to the beam was perfect, and the selenite colours exceedingly vivid. As the cloud 
thickened the polarization along the normal disappeared, but it became strong obliquely. 
Two neutral points were observed by oblique vision in the case of this cloud. This effect 
is not uncommon. 
The tube was withdrawn from the light for six minutes ; on reexamination the cloud 
was found to have lost its beauty of form ; and now the cloud-centre, by normal vision, 
polarized the light in a plane opposite to that of the two ends. 
Twelve bubbles of the air and nitrite vapour were then sent into the exhausted expe- 
rimental tube, and after them thirty-six bubbles of air and hydrochloric acid ; several 
minutes’ exposure produced no action. 3 inches of hydrochloric acid were then added, 
and the same superb blue as that noticed in the last experiment soon made itself manifest. 
It faded gradually as the cloud became more dense, and finally merged into whiteness. 
The mixture of nitrite of amyl and hydrochloric acid was also examined in small 
quantities; but though the blue was fine, it had not the splendid depth and purity of the 
colour obtained with the nitrite of butyl. 
§XI. 
The whole of the autumn of 1868 w 7 as devoted to the investigation from which I have 
taken the foregoing brief extracts. During this period 100 different substances must, I 
'think, have been subjected to examination, and in the case of many of them the expe- 
rimental tube must have been exhausted and refilled from 50 to 100 times. In some 
instances, indeed, the largest of these numbers falls considerably short of the truth. 
For a time I had no notion of the delicacy of the inquiry, nor of the caution required to 
prevent the action of infinitesimal residues and impurities from being mistaken for the 
decomposition of substances really inert. The necessity of thoroughly cleansing, or re- 
newing, every tube and every stopcock, on passing from one substance to another, be- 
came gradually apparent. Water, alcohol, caustic potash, and acids were successively 
employed to cleanse the experimental tubes ; but the method found most convenient, and 
that finally adopted, consists in the thorough lathering and sponging out of the tubes 
with soft soap and hot water, and the flooding of them with pure water afterwards. They 
are then dried with clean towels, and finally polished by the passing to and fro within 
them, by means of a ramrod, of a clean silk handkerchief. The stopcocks are cleansed 
by suitable brushes ; fresh cocks, a fresh tube, and a fresh plug of asbestos being employed 
for each fresh substance. 
