322 : PROFESSOR LISTER ON THE GERM THEORY 
abounding particles of dust which a beam of sunlight reveals in an occupied 
apartment. 
A similar inference must be drawn from the circumstance before mentioned, 
that the sole result of forty minutes’ exposure of one of the glasses of this 
experiment to the air was the development of three plants of filamentous fungi, 
whereas the particles of dust which fell into it during that time must have been 
very much more numerous. 
If, then, the withdrawal of a drachm of liquid, or exposure for more than 
half-an-hour had so little effect, it was plain that the removal of one or two 
minims, executed nimbly so as to involve little more than momentary exposure, 
must be practically free from the risk of accidental contamination. 
I thus became possessed of a means of making observations upon these 
minute but highly important organisms, which promised to yield results of a 
more definite character than any which had been hitherto obtained. 
Various detailed accounts have been given of late years, not only of the 
spontaneous generation of animal and vegetable forms of more or less com- 
plexity, such as large ciliated infusoria from an infusion of hay, or torule and 
penicillia from milk globules, but also of the transition of one form of organism 
into another. But in the latter class, as in the former, the liability to decep- 
tion is so extremely great, in consequence of microscopic organisms acci- 
dentally present developing side by side with the minute objects investigated, 
and presenting the appearance of growing out of them, that, without the 
slightest doubt being thrown upon the good faith of the observers, the so-called 
facts are justly received with the gravest suspicion. But with the means now 
at our disposal the grand source of error in former similar inquiries might be 
eliminated, and results of a more satisfactory character might therefore be 
anticipated. I was thus led to prosecute the investigation far beyond what I 
had at first intended, and will now proceed to give a selection from the results. 
That which I will mention first has reference to the origin both of torule 
and of bacteria. 
On the evening of the 13th December 1871, during a drizzling rain which 
had been falling all afternoon, I took a “heated” wine-glass with its cover out 
into the street, and, raising the cover, allowed a few drops of rain to fall into 
the glass, and having covered it again and brought it back into the house, I 
charged it with unboiled urine from a “heated ” flask, the arrangements for 
obtaining the liquid being the same that have been before described. In the 
course of two days I noticed a tiny opaque streak proceeding vertically down- 
wards from a point on the inside of the glass; and on the following day the 
streak had increased, and the cloud of mucus was speckled with numerous 
white points. On the fourth day, while the speckling of the cloud had increased, 
and the streak had become coarsely granular, two little plants of filamentous 

