128 A MANUAL OP BACTERIOLOGY 



Milk may be steamed at 100° C. for fifteen minutes on four or five 

 consecutive days. It should then be incubated at 30° C. for three 

 days, and all tubes -which show signs of microbial activity can be 

 thrown away. It is better to discard a few tubes in which growth 

 occurs than to lose all on account of overheating. 



7. Blood serum. If the slaughter of the animal can be supervised, 

 it is well to have the region of the wound and the knife previously 

 sterilized. Collect the blood from the sheep or ox in a sterile glass 

 jar in a sterile cylinder. Allow the vessel to stand long enough to 

 form a clot before leaving the slaughterhouse. At the laboratory 

 detach the clot from the sides of the jar by means of a sterile glass 

 rod and place the cylinder in the ice chest for forty-eight hours. The 

 clear serum is drawn out with sterile pipettes and placed in sterile, 

 cotton-plugged tubes. 



The tubes of sermn are sterilized by heating them to 56° C. in a 

 water bath for half an hour on each of three consecutive days. On 

 the fourth day the tubes are placed in a sloping position in a serum 

 inspissator and heated to 72° C. until coagulation occurs. The tubes 

 are then transferred to an incubator at 37° C. for forty-eight hours. 

 Those which show indications of infections are then discarded. 



8. Soil. For many experiments with bacteria and phytopathologi- 

 cal fungi sterilized soil is required. Unfortunately, successful sterili- 

 zation of soil is diificult to accomplish. If all germ life is destroyed, 

 so high a degree of heat is required that profound changes in the 

 constitution of the soil are brought about. The air held in the soil is 

 very difficult to remove and acts as a heat insulator. This condition 

 may prevent the soil in the center of a large pot from receiving 

 enough heat to kill all germ life. 



Attention may be called to three methods of soil sterilization 

 which have been used by investigators : dry heat, moist heat, and 

 volatile antiseptics. 



Richter ^ found that exposure to a dry heat of 100° C. for six hours 

 on three consecutive days increased its absorptive power for water, 

 the amount of nitrogenous material soluble in water or dilute hydro- 

 chloric acid, and the amount of water-soluble organic matter. 



Seaver,^ and Seaver and Clark,* have studied the effect of dry 

 heat upon soil with relation to the growth of pyrophilous. fungi and 

 green plants. They have shown that heat alters the soil beneficially 



1 Landw. Versuchsst. 47 : 269. 1896. 2 Mycologia 1 : 131. 1909. 



8 Mycologia 2 : 109. 1910. Biochemical Bulletin No. 1: 413. 1912. 



