30 
PKOEESSOE TYNDALL ON THE OPTICAL DEPOETMENT OE THE 
The arrangement is represented in fig. 2, where w w Fig. 2 * 
are the side windows through which the searching beam 
passes from the lamp L across the case C ; p is the 
pipette, and a b are the bent tubes connecting the inner 
and outer air. The test-tubes passing through the 
bottom of the case are seen below. 
On the 10th of September this case was closed. The 
passage of a concentrated beam across it through its 
two side windows then showed the air within it to be 
laden with floating matter. On the 13 th it was again 
examined. Before the beam entered, and after it quitted 
the case, its track was vivid in the air, but within the 
case it vanished. Three days of quiet sufficed to cause 
all the floating matter to be deposited on the interior 
surfaces, where it was retained by a coating of glycerine, with which these surfaces had 
been purposely varnished. 
§ 3. Deportment of Urine. 
The pipette being dipped into the tubes, fresh urine was poured into eight of them 
in succession on the 13th of last September. Each tube was about half-filled with the 
liquid. The tubes were then immersed in a bath of brine, raised to ebullition, and per- 
mitted to boil for five minutes. Aqueous vapour rose from the liquid into the chamber, 
where it was for the most part condensed, the uncondensed portion escaping, at a low 
temperature, through the bent tubes at the top. Before the brine was removed little 
stoppers of cotton-wool were inserted in the bent tubes, lest the entrance of the air into 
the cooling-chamber should at first be forcible enough to carry motes along with it. 
As soon, however, as the ambient temperature was assumed by the air within the case 
the cotton-wool stoppers were removed. 
The front and back of this chamber were squares of 14 inches the side, the depth of 
the chamber being 8*5 inches. It contained, therefore, 1666 cubic inches of air, which 
had unimpeded access to the liquid in the tubes. No stoppers were employed. The 
air was unaffected by calcination, or even by filtering. Neither cotton-wool nor 
hermetic sealing was resorted to. Self-subsidence was the only means employed to rid 
A second series of eight tubes were filled at the same time with the same liquid, and 
subjected to the same boiling process. The only difference between the two series was, 
that these latter tubes were placed in a stand beside the former and exposed to the 
common air of the laboratory. 
For the sake of distinction I will call the tubes opening into the case the protected 
tubes, and those opening into the common air the exposed tubes. 
On the 17th of September all the protected tubes were bright and clear, while all 
the exposed tubes were distinctly turbid. Specks of mould, moreover, were in every 
