35 6 CHEMICAL ACTIVITY OF ASPERGILLACE^E. 



the presence of free acid. The acidity gradually increases to a 

 maximum, declining once more during the next few weeks, to 

 gradually fall to zero when the experiment is prolonged, the 

 reaction being even alkaline finally. The capacity of this fungus 

 for destroying free acid can be demonstrated by placing the mature 

 vegetable growths on dilute solutions of oxalic acid (containing 

 0.5 per cent, of crystalline acid). The limit of the accumulation 

 of acid averages about 0.2 per cent, of the volume of the liquid. 

 The amount of sugar is immaterial, but the general conditions of 

 the environment are important. 



The acidification is primarily dependent on the organic nutri- 

 ment presented, sugars, or chemically allied substances being 

 essential, whereas no free acid is produced when salts of organic 

 acids, amides or peptone are used, though an abundance of oxa- 

 lates is formed. A decisive influence is also exerted by the inor- 

 ganic bodies present, especially the source of nitrogen for the 

 growing fungus, the liberation of acid being absent when ammo- 

 nium chloride or sulphate is substituted for potassium, calcium, 

 or ammonium nitrate (even in presence of sugar') ; and, in fact, 

 these additions will prevent the formation of acid in cultures that 

 would otherwise acidify at once. Temperature also plays an 

 important role from the outset, and has a determining effect 

 on success, lower temperatures favouring the accumulation of 

 acid, whilst high temperatures have an adverse effect, so that at 

 the optimum temperature for the growth of the fungus (about 

 37 C.), acidification ceases to occur, the highest production (up 

 to about i per cent.) being attained at a few degrees above the 

 minimum growth temperature (about 7 C.). The fungus being 

 actually capable of far more readily decomposing free oxalic acid 

 at higher temperatures (and even when 04 per cent, is present), 

 the accumulation at lower temperatures is therefore solely the 

 result of retarded oxidation, that is to say, enfeebled oxidising 

 action. The whole shows clearly that, contrary to preconceived 

 ideas, a relative scarcity of oxygen is not the cause of the produc- 

 tion of oxalic acid, since all the growths in these experiments had 

 an equal supply of oxygen, and the acid must therefore be 

 regarded as the product of incomplete oxidation only in the sense 

 that this oxidation has been prevented by some adverse influence 

 or other. 



A remarkable influence is exercised on the process by the 

 addition of salts able to combine with the acid. In this case the 

 accumulation of the combined acid is progressive, and finally 

 attains extraordinary dimensions. The resulting calcium oxalate 

 may amount to more than 100 per cent, of the sugar originally 

 present, so that 15 grms. of sugar furnish about 10 grms. of 

 (anhydrous) oxalic acid (corresponding to about 7 grms. of sugar), 

 without the crop of the fungus being affected. In this way the 

 fungus produced the following quantities of calcium oxalate 



