78 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



to prevent contamination at the time the blood is drawn. 

 It is then allowed to stand until it clots when the clear 

 serum is pipetted off or it is shaken with glass beads or 

 whipped with a glass rod until defibrinated. 



Beer-wort is probably the most satisfactory medium 

 for cultivation of molds and yeasts. The unhopped wort 

 may be secured from breweries or it may be prepared by 

 soaking malt in warm water. The starch of the malt is 

 rapidly changed into the sugar maltose and certain of the 

 protein constituents likewise pass into solution. 



Solid media Solid media may be divided into three 

 types, media which are permanently firm or solid, media 

 which are at first liquid and later solidified and in which 

 the condition is irreversible, and media which may be liquid 

 or solid at will, that is, solid media which are liquefiable. 



To the first group of natural solid media belong materials 

 such as potatoes and other vegetables and the meats. Pota- 

 toes when sliced and sterilized prove to be a suitable medium 

 for the growth of many kinds of bacteria. 



Blood sera, or mixtures of blood serum with broth, may 

 be solidified by exposure to heat, and constitute satisfactory 

 media for the growth of many of the pathogenic bacteria. 

 Silicic acid in pure colloidal solutions may be caused to 

 form a gel by the addition of certain salts. This consti- - 

 tutes, when mixed with these salts, a satisfactory solid 

 medium for the growth of certain of the soil bacteria which 

 develop best in the absence of organic compounds. 



The liquefiable solid media are prepared usually by the 

 addition either of gelatin or of agar-agar to a liquid 

 medium. 



Gelatin is manufactured by boiling bones, joints, tendons, 

 and similar animal tissues, concentrating the extract, and 

 cooling and drying. When dissolved in water it has the 

 property of solidifying or gelatinizing when cool, and of 

 being liquid when heated. Chemically gelatin is closely 



