106 BACTERIA IN RELATION TO PLANT DISEASES. 
Plates, tubes, and flasks containing pure cultures or designed for inoculation 
should zever be opened in the general laboratory on a windy day. or in air currents. 
Pour two uninoculated agar or gelatin plates in the proper way. Keep one covered 
and uncover the other fora few moments in a current of air, z. e., as long as the time 
required to make a plate culture. Then keep the two plates together and com- 
pare from time to time. A few experiments of this sort will convince the most 
skeptical of the necessity of avoiding drafts. 
The person and clothing of the experimenter should be as clean and free from 
dust as possible. - White duck coats are very convenient. They show at once when 
they are soiled and need washing and ironing. 
Organisms which for some reason may be difficult to obtain in ordinary plate 
cultures. and which differ. markedly from their associates in some particular way, 
e. g., by more rapid growth, by indifference to heat, to acids, to thymol, to chloro- 
form, to absence of air, etc., or which can use, as food, substances which will not 
support the growth of most bacteria, may sometimes be isolated very readily by 
providing conditions suited to their growth and unsuited to that of the bacteria with 
which they are mixed. This is Winogradsky’s principle of elective culture. As he 
defines it, this is a culture “which presents conditions favorable only to a single 
definite function or, more exactly, to a function as strictly limited as possible.” Such 
Fig. 91.* 
media or methods are exactly the opposite of universal. Nutrient starch jelly and 
nutrient silica jelly are good examples or such media. Nutrient fluids rich in acid 
potassium phosphate or destitute of nitrogen are additional examples. 
Heat is often an excellent means of separation. Winogradsky separated his 
Clostridium Pasteprianim from all but two of the contaminating species by heating 
ten minutes at 75°C. (Archives des Sci. Biol., Vol. III, p. 310). The isolation 
of Streptococcus (Leuconostoc) mesenteroides by Liesenberg & Zopf and of Bacillus 
hortulanus by Sturgis are other examples of separation by heat. Omélianski’s 
separation of his hydrogen-cellulose ferment from his methane-cellulose ferment by 
exposure of the recently established methane ferment to 75° C. for fifteen minutes 
is another good example. ; 
THE FINAL DISPOSAL OF INFECTIOUS MATERIAL. 
Diseased material should not be left around the laboratory any longer shox is 
necessary. When it has served its immediate purpose that which is not to’ be pre- 
served permanently should be thrown into-the furnace. Small amounts may be 
sterilized by putting into beakers or jars and covering with cleaning mixture or 
equal parts of ‘crude sulphuric acid and water. Crude vegetable and animal sub- 
*Fic. 91.—Instrument ‘for making puncture-inoculations. It consists of a bone handle with a 
metal-screw socket, into which a sewing needle is thrust. ‘The needle is usually of small size—a 
No, 8 or 10. 
