Hopsox.—On Spontaneous Generation. 188 
belief in spontaneous generation is pretty general; when organisms appear 
in situations where they are not expected, and under ciruumstances which, 
to a superficial observer, would appear calculated to exclude them, the 
easiest mode of accounting for their presence is to assume that “ they came 
of themselves"; and, by way of parenthesis, I will say here that, when we 
wish to account for or explain observed facts, we are always right to take 
the simplest theory, provided such theory does not clash with any other 
known facts. 
The possibility of organisms having a spontaneous origin has been 
narrowed and narrowed by successive observers, until it is only the Bacteria 
which at the present day are presumed, under certain conditions, to arise 
spontaneously. These Bacteria are exceedingly minute rods and spheres 
that invariably appear wherever decomposition of animal or vegetable 
matter is going on. For instance, suppose we take any organic infusion 
such as can easily be made by soaking a piece of meat, or hay, or turnip in 
water for an hour or so; we strain off the clear liquid and set it on one side. 
If we examine it after a few days (a week or ten days in winter, a couple of 
days in summer), we shall find that the clear liquid has become turbid, 
and that it begins to smell offensive,—in common parlance it has turned 
bad, or decomposed! Why has it decomposed? Modern science tells us 
that organic matter cannot decompose without the presence and help of 
Bacteria of some kind or other; that a complex organic infusion, provided 
we rigidly exclude germs, will remain as stable as a solution of sulphate of 
copper. We have a practical application of the principle in the various 
tinned meats. The germs in or on the meats or fish are first killed by 
exposure to a high temperature, and then the tins are hermetically sealed so 
as to exclude air, or rather germs; for it is not the air that does the harm. 
How can we prove this? 
I have here three glass tubes, which I will call Nos. 1, 2, and 8. Some 
months ago I put into each of these tubes some chopped hay and water. 
No. 1 I left untouched in a sheltered place but exposed to air and light; 
No. 2 I boiled ; into No. 8 I inserted a cotton wool cork, quite permeable to 
air, but which has been found to act as a sieve to Bacteria and their germs, 
' and then I boiled it for about five minutes. The steam issued freely from 
the cotton wool, demonstrating its perfect permeability to vapour and air. 
After four days I looked at the tubes. No. 1 appeared clear, No. 2 distinctly 
turbid, No. 8 clear. A drop of No. 2 placed under the microscope showed 
numerous rod-like Bacteria. The earlier appearance of turbidity in No. 2 
is easily explained by the boiling having made the infusion quicker (and so 
prepared the fluid for the reception and growth of germs) than the 
water did. At the end of a week I examined the tubes a second time. Nos, 
