246 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 
imparts heat more slowly than air. On the other hand, lar<i;e incu- 
bators cannot be surrounded with water jackets because of mechanical 
difficulties. The regulation of temperature within incubators is con- 
trolled by bimetallic regulators or thin-walled metal capsules which 
actuate valves or electromagnets controlling the supply of gas or 
electricity which heats the chamber, or by mercurial thermoregulators 
working upon the principle of the mercury thermometer. Bimetallic 
regulators, in which the movement imparted to the regulator of the 
source of supply of heat is due to the differential expansion or contrac- 
tion of two dissimilar metals, are more sensitive to slight Aariations in 
heat and they possess the additional advantage of being less fragile than 
mercurial regulators. Various patterns of thermoregulators of tried 
efficiency are on the market and a selection between them is largely 
a matter of mechanical adaptability to local needs. 
VI. The Study of Bacterial Cultures. — 1. Growth in Solid Media.— 
(a) (^olonies.— The macroscopic appearance of bacterial colonies upon 
solid media is of considerable value for the dift'erentiation and recog- 
nition of the various types; in a similar manner their microscopic 
appearance, stained or unstained, permits of some difl'erentiation. 
The aspect of a colony is influenced : 
1. By the kind of organism— Streptococcus colonies, for example, 
are habitually small and nearly transparent; anthrax colonies are 
habitually larger and opaque. 
2. By the consistency of the medium — in firm, dense media the 
growth of bacteria is limited and relatively dry; in moist, semi-solid 
media the growth of the same organism is usually luxuriant, moist and 
spreading. 
3. By the composition and reaction of the medium— the addition 
of specifically nutritive substances, as of fresh sterile tissue, to media for 
the cultivation of anaerobes; of utilizable carbohydrates to media for 
the cultivation of carbohydrophilic bacteria; of fresh, defibrinated blood 
to media for the cultivation of hemoglobinophilic organisms; these may 
improve conditions otherwise unfavorable for bacterial development. 
The reaction of the medium, furthermore, is important; many 
bacteria are extremely sensitive to slightly acid media; the aciduric 
bacteria thrive in media too acid for the existence of other organisms. 
Even the ordinary laboratory media, made according to a definite 
formula, vary sufficiently in chemical and physical properties to 
influence materially the appearance of bacterial colonies. The degree 
of influence is more pronounced in the feebly growing forms, but it 
may aft'ect the appearance of colonies of the more hardy types as well.^ 
4. The rate of growth of bacteria also aft'ects the appearance of 
colonies. 
It is useless, as a scientific procedure, to attempt to recognize 
' Generally speaking, cultural media intended for the pathogenic bacteria require 
an initial reaction of practically exact neutrality, measured in terms of hydrogen-ion 
concentration. 
