234 hifestation of Fungus Cultures hy Mites 



each lot of medium should be treated in the same way during steriUsation 

 so that no variations in dilution should take place. The flasks were as 

 far as could be judged of the same size and the cotton- wool plugs rolled 

 in the same way and fitting as nearly as possible the necks of the flasks 

 with an equal tightness. Pyridine was added from a capillary pipette, 

 and the flasks inoculated and incubated as in previous series at 18-5°- 

 19-5° C. for a period of 21 days. 



At the end of this period they were filtered and after careful washing 

 and drying the yields obtained were weighed. A portion of the filtrate 

 was set aside and the Pyridine estimated by the method of Harvey and 

 Sparks (7) which we had found to give results of fair accuracy. As the 

 medium itself gave a precipitate with Iodine solution in Potassium 



005 



0-35 04 



01 015 2 25 03 

 Pyridine in gms. per 100 c.c. 



Fig. 3. Toxicity of Pyridine to Aspergillus niger. 



Iodide, a blank estimation was done on the controls and the amount 

 deducted from that found in the tests. We were thus able to ascertain 

 whether the concentration of Pyridine remained the same throughout 

 the experiment. The means of the amounts of Pyridine added initially 

 and found at the end were taken and plotted against the yields, the 

 curve being drawn freehand through the points. 



Table IX and Fig. 3 show that the toxic effect of Pyridine is at first 

 very gradual, the growth yields diminishiiig very slowly up to a con- 

 centration of '225 per cent. The inhibitive effect of the base from that 

 point onward is, however, increasingly marked and the yields diminish 

 rapidly with small increases in the concentration of Pyridine until at a 

 strength of -325 per cent, they are almost negfigible after which the curve 

 of growth yields tails off very gradually, thus taking a sigmoid shape. 



