174 PHYSIOLOGY OF MICROORGANISMS 



be true if the water in the former case were substituted by a weaker 

 solution of gelatin or, in the latter case, by a weaker solution of salt. 

 There is a tendency to equalize or become uniform and homogeneous. 



Microbiologists are also familiar with certain special phenomena. 

 Litmus agar becomes reduced by the growth of microorganisms. Oxy- 

 gen has been consumed. When the culture is allowed to remain ex- 

 posed to the air for a time, the microorganisms cease to grow and 

 multiply; the litmus, beginning at the top, gradually resumes its color 

 as the air works its way down through the culture. There has been a 

 gradual diffusion of the air throughout the litmus agar. Many cul- 

 tural phenomena could be recalled in this connection. One will suffice. 

 The heating of culture media to drive off the air for anaerobic cultivation 

 is of frequent occurrence, for it is well known how the air soon penetrates 

 when media are allowed to stand. 



Apparently there are encountered in the first two paragraphs dis- 

 tinct phenomena or a single phenomenon modified in the one or the 

 other instance. The usual explanation, however, is covered by the 

 word "diffusion." 



The recent developments in the understanding of diffusion attribute 

 to diffusion the same forces operating in gases. It is the drive possessed 

 by the molecules to expand or press out until equalization or equilibrium 

 is established. This movement is from the more concentrated solution 

 toward the less concentrated or toward the pure solvent. The nature 

 of a substance, difference in concentration and temperature materially 

 influence this movement. 



This accords with the forces of osmosis as well: The pressure upon 

 the obstructing membrane through which the particles, molecules or 

 ions of a substance are attempting to make their way is called osmotic 

 pressure; the particles are held back or restrained in their movements 

 outward. It has been found, however, that " the osmotic pressure of a 

 dissolved substance is exactly the same as the gas-pressure which 

 would be exerted if the solvent were removed and the dissolved sub- 

 stance in gaseous form were left behind to occupy the same volume 

 at the same temperature." It is also known that "where two liquids 

 which will mix are separated only by a porous membrane there is a 

 movement of the liquid in both directions through the, membrane. 

 The greater movement is usually from the less dense to the more dense 

 so as to cause the line of the more dense liquid to rise above that of the 



