170 
PEOEESSOE TYNDALL ON THE DEPOETMENT AND VITAL 
while the wire was raised to the highest possible degree of incandescence. The infusions 
employed were turnip and cucumber, a group of three tubes being charged with each. 
After the air had been calcined, the infusions were boiled for five minutes in an oil-bath. 
With this mode of treatment not a single failure occurred last year, turnip-infusion being 
among the number of liquids thus treated. This year two days sufficed to render every 
one of the six tubes turbid with organisms and to cover the infusion with a heavy scum. 
I, however, had occasion to doubt the closeness of these shades. The wax intended to 
seal the junction of the tin collar with the plate of wood had cracked and yielded here 
and there, and the entry of contamination through such cracks was possible. Six new 
shades were therefore mounted and surrounded by collars which were imbedded in 
white lead and firmly screwed down to the plate of wood. The height of the collar, 
which measured the depth of the filtering layer of wool, was much greater than it had 
ever been last year. As before, the period of incandescence was ten minutes, during 
which the platinum spiral was brought as close as possible to its point of fusion. 
Each of these six shades covered a group of three test-tubes. Two such groups were 
charged with turnip, two with cucumber, and two with artichoke-infusion. The infu- 
sions, as usual, were boiled for five minutes after calcination. They were all brilliant 
when prepared ; but in two days every one of them had become turbid, and had covered 
itself with a fatty scum. This gradually augmented until it reached in some of the 
tubes a thickness of half an inch. The weight of the scum caused it in some cases to 
bag downwards, forming a kind of inverted cone, the apex of which was more than an 
inch from its base. These bags finally broke and scattered their organisms in the sub- 
jacent liquid. 
§ 13. Further precautions against Infection. 
At the beginning of December, my attention being keenly aroused by those successive 
failures, I watched more closely than I had previously done the filling of the test-tubes 
through the pipette. Now and then I noticed minute bubbles of air carried down with 
the descending infusion. On escaping from the end of the pipette, these small bubbles 
I concluded would break, and scatter such germs as they contained in the air of the 
chamber. Last year I should have found it difficult to believe that a cause so small 
could lie at the root of the observed anomalies ; but this year I had learned to respect 
small causes, and accordingly took measures to effectually exclude the air. 
On December 4th three chambers, which had been previously left quiet for 
several days, were charged with carefully prepared cucumber-infusion, and two other 
chambers with turnip-infusion prepared with equal care. The following precautions 
were taken : — The funnel of the pipette formerly employed was broken off from its 
shank, and for it was substituted a “ separation-funnel ” with a glass stopcock. This 
was connected by closely fitting india-rubber tubing with the shank of the pipette. 
But before the connexion was made, the funnel was filled with the infusion, and the stop- 
cock turned on for a moment, until the liquid issued from the orifice below. The 
stopcock being then turned off, the flow of the liquid ceased, and the column in the 
