The H ion concentration is ordinarily _indicated_ by the conventional symbol P{H}, _e.g._, the concentration in pure water which is regarded as neutral is expressed as P{H} 7; of normal HCl, P{H} 0; of normal NaOH, P{H} 14. The figure after P{H} does not in reality represent the concentration of H ions in the solution. This, like the concentration of acids, is expressed on the basis of normality, _i.e._, as compared with the concentration of a normal solution (1 g. equivalent) of H ions. Concentration of H ions in pure water is N/10,000,000, _i.e._, is 1/10,000,000 of the concentration in a normal solution of H ions. Expressed in other words, it is the concentration in a normal solution of H ions diluted ten million times. 10,000,000 = 10 to the 7th power = 10⁷. Hence the figure after the P{H} indicates the _logarithm of the number of times the solution is diluted_. Therefore this number _increases with the dilution_, and the larger the figure after the P{H}, the _less acid the solution is_. Most saprophytic organisms and many parasitic ones grow within a wide range of H ion concentration so that titration with phenolphthalein gives sufficient accuracy for media for such organisms. On the other hand, many organisms grow within a very narrow range of H ion concentration, hence accurate standardization to a definite H ion concentration is necessary. It is also evident that for comparative work, such standardization is essential because this reaction can be reproduced in other media and by other workers.[21] Broth may be prepared from Liebig’s or Armour’s meat extract by adding 5 grams of either, 10 grams peptone and 5 grams NaCl to 1000 cc of water, boiling to dissolve, then titrating and filtering as above. The author after much experience finds _meat juice_ preferable to meat extract for broth and other media for pathogenic bacteria, and has abandoned the use of meat extracts for these organisms. =Glycerin Broth.=--Glycerin broth is made by adding 4 to 6 per cent. of glycerin to the broth just previous to the sterilization. The glycerin serves as a source of carbon to certain bacteria which will not grow on the ordinary broth--as _Mycobacterium tuberculosis_.