ACIDITY AND ALKALINITY OF MEDIA. gl 
(6) Upon or in media of different composition: 
(c) Upon or in media of different degrees: of 
acidity and alkalinity. 5 
(2) In cultures of different ages. 
(e) As well as to the variations in the size and 
shape of different individual bacteria obtained from 
one culture and appearing often in the same field 
of view—z. e., subjected to exactly the same condi- 
tions of growth. 
(e) Determination of the Thermal Death- 
point.—In determining the thermal death-point, 
the facts required to be known are (1) the time of 
exposure to heat, (2) the presence or absence of 
moisture, (3) the presence or absence of spores, (4) 
the age of the culture, (5) the amount: of the culture 
used for the tests, and (6) the character of the con- 
taining vessel. 
The temperature required to destroy the species 
under consideration is to be determined within 2 
degrees C.; thus, if samples are exposed to temper- 
atures of 50°, 52°, 54°, 56°, 58°, and 60° C., and it 
is found that development in a suitable medium 
occurs after exposure to 56° C., but not after expos- 
ure to 58° and 60° C., the thermal death- -point | is 
to be given as 58° ., although further study 
might show it to be domewhat less than this. 
(f). Relation to Free Oxygen.—For the 
methods of determining the aérobic properties of 
organisms, see under Anaérobic Cultures, on page 
3. 
: (g) Relation to Acidity and Alkalinity of 
Media.—In-determining the relation of growth to 
