Sibyl T. Jewson and F. Tattersfield 2*27 



type, and showed that if the strength of the phenol be progressively- 

 raised the curve approximates to the logarithmic type, but that both 

 types of curve are exphcable by assuming variations of resistance 

 amongst the spores. 



In our case, working at the saturation point of poison in air, a similar 

 method was impossible, but even with so rapidly acting a poison as 

 Ammonia the curve obtained was distinctly sigmoid in type when sur- 

 vivors were plotted against time expressed in seconds instead of minutes. 

 Curves of this type would be expected in our case, where we have in an 

 inseparable mixture, adult mites of various ages, larvae and nymphs. 

 The distribution of the resistances varying in all three stages of develop- 

 ment would be complex, and the variations so great that with the most 

 highly toxic of materials the survival curve would be of the type obtained. 

 The toxic action of Ammonia proved so rapid that it is not expressible 

 with accuracy on the same scale as that of Pyridine and Aniline, and is 

 put therefore on a second scale in Fig. 1 b. 



In view of the above results it seemed of importance to ascertain 

 whether the considerable toxic action of Pyridine could be regarded as 

 specific. Tattersfield and Roberts (3) showed that molecule for molecule 

 Aniline in the vapour phase was about three times as toxic to wireworms 

 as Pyridine. For this purpose minute but progressively increasing 

 quantities of Pyridine or of Anihne were inserted by means of a gradu- 

 ated capillary pipette into cahbrated flasks (1100-1200 c.c.) fitted with 

 lead-hned rubber stoppers and glass hooks, and the cork inserted. When 

 the material had evaporated, mites (about the same number in each 

 case) were introduced in small test tubes closed by silk gauze and 

 attached by wire to the hook and allowed to stand for a period of three 

 hours. This time was convenient as permitting each set of tests to be 

 comfortably finished in one day and as giving nearly 100 per cent, of 

 deaths with a saturated concentration of Pyridine in air. The counts 

 were made in the usual way. The results are shown in Table VI and 

 percentages of survivors plotted against milHonths of gramme-molecular 

 concentrations of poison in a litre of air, in Fig. 2. 



The curve for Pyridine is distinctly sigmoid in character, indicating 

 that equal increases in concentration do not have a corresponding effect. 

 An increase of dose from 30 to 50 milhonths of a gramme-molecule shows 

 little or no increase in toxic action, but an increase of from 50-70 accounts 

 for 75 per cent, of the mites while further increases up to near the satura- 

 tion point produce effects only very gradually. The curve for Aniline is 

 not complete, as towards the lower end of the curve the flasks are 



15—2 



