ATMOSPHERE IN RELATION TO PUTREFACTION AND INFECTION. 
63 
atmosphere mapped, so to speak, in the infusion. But in such a tray the organisms 
would intermingle and thus mar the revelation of their distribution. Valuable informa- 
tion I thought might be gained by breaking up the infusion into isolated conterminous 
patches, and exposing them to the air. 
A square wooden tray was accordingly pierced with one hundred circular apertures ; 
into each of which was dropped a test-tube 3 inches long and 1 inch wide, with its rim 
resting in each case upon the rim of the aperture. There were ten rows of tubes, with 
ten tubes in each row. On the 23rd of October, 1875, thirty of these tubes were filled 
with an infusion of hay, thirty-five with an infusion of turnip, and thirty-five with an 
infusion of beef. The tubes with their infusions had been previously boiled ten at a 
time in an oil-bath. 
One hundred circles were marked upon paper so as to form a plan of the tray, and 
every day the state of each tube was registered upon the corresponding circle. Seven 
such maps or records were executed. 
I will use the term “ cloudy ” to denote the early stage of turbidity, distinct but not 
strong. The term “ muddy ” will be used to denote thick turbidity. 
§ 26. Tray of one hundred tubes. 
On the 25th of October one or two of the tubes exposed on the 23rd showed signs of 
yielding; but the progress of putrefaction was first registered on the 26th. Map I., 
embracing the first record, is annexed (p. 64) ; it may be thus described. 
Hay . — Of the thirty specimens exposed, one had become ‘ muddy ’ — the seventh in 
the middle row reckoning from the side of the tray nearest a stove. Six tubes 
remained perfectly clear between this muddy one and the stove, proving that differences 
of warmth may be overridden by other causes. Every one of the other tubes containing 
the hay-infusion showed spots of mould upon the clear liquid. 
Turnip . — Four of the thirty-five tubes were very muddy, two of them being in the 
row next the stove, one four rows distant, and the remaining one nine rows away. 
Besides these, seven tubes had become clouded. There was no mould on any of the tubes. 
Beef . — One tube of the thirty-five was quite muddy, in the seventh row from the 
stove. There were three cloudy tubes, while seven of them bore spots of mould. 
As a general rule organic infusions exposed to the air during the autumn remained 
for two days or more perfectly clear. Doubtless from the first germs fell into them, but 
they required time to be hatched. This period of clearness may be called the “ period 
of latency ; ” and, indeed, it exactly corresponds with what is understood by this term in 
medicine. Towards the end of the period of latency the fall into a state of disease, if 
I may use the term, is comparatively sudden ; the infusion passing from perfect clearness 
to cloudiness more or less dense in a few hours. 
Thus the tube placed in Mr. Darwin’s possession was clear at 8.33 a.m. on the 19th 
of October, and cloudy at 4.30 p.m. Seven hours, moreover, after the first record of 
our tray of tubes, a marked change had occurred. For the purpose of comparison the 
