220 SCIENCE AND FRUIT GROWING 



acid when applied in strong doses 1 (namely, i oz. of sodium 

 cyanide, or 1-3 oz. of potassium cyanide, to every 100 cubic feet 

 of air space) ; but such results cannot always be counted upon, 

 and the effect of the gas, which is soluble in water, would 

 probably vary considerably with the degree of dryness of the 

 trees at the time of fumigation. 



Certainly the use of such a deadly poison is one which should 

 be avoided, if possible, 2 and the Woburn investigations showed 

 that more satisfactory results could be obtained by dipping the 

 trees in a low-boiling paraffin oil. The trees to which this treat- 

 ment was applied were similar to those used in the fumigation 

 trials, and consisted of paradise and crab stocks which had 

 been worked in the previous season. The immersion lasted 

 for 5 minutes, and the trees were allowed to drain for another 

 30 minutes before being planted : 40 trees were used in each set 

 of tests. With a high-boiling oil, such as solar distillate, all the 

 trees were killed, and with an ordinary lighting oil there was 

 considerable mortality ; but with petrol, the mortality was only 

 10 per cent, when the scions were considered, and 5 per cent, 

 when the roots were considered values probably in excess of 

 those which might usually be expected, as the trees were far 

 from being in a robust condition and the surviving trees re- 

 mained quite free from aphis during the ensuing season, showing 

 as much vigour of growth as others which had not been treated. 

 Equally good results were obtained by dipping the trees into 

 the alkaline winter wash containing 6 per cent, of solar distillate 

 (p. 161). 



A form of treatment which proved preferable to any of those 

 depending on the use of paraffin consisted in immersing the 

 trees bodily in water at 115 F. for 10 minutes; the mortality 

 amongst them was only half of that observed in the case of those 

 treated with paraffin, and was probably due entirely to their 

 feeble condition, and not to the treatment itself; whilst the 

 absence of aphis in the following season was complete, and the trees 

 seemed otherwise to have actually been benefited by the treat- 

 ment. In selecting the temperature and duration of the immer- 



1 Report on Economic Zoology, 1908, 52. 



2 A hitherto unnoticed source of danger was found in cases where the 

 cyanide was wrapped up in paper, and lowered into the sulphuric acid 

 from a distance by means of a string; for the acid sometimes acted on 

 the paper, converting it into parchment paper, and preventing the access 

 of it to the cyanide. An improved method of making the gas is to 

 enclose the cyanide in 'small zinc boxes, which may be placed directly 

 into the acid, giving the operator time to leave the house before the zinc 

 is dissolved. 



