FUMIGATION OF CITRUS TREES. 47 



It has been a common practice among fumigators to increase the 

 dosage when fumigating a tree severely infested with scale. It also 

 has been a common practice — in fact so common as to be almost 

 universal — to increase the proportion of water when using such heavy 

 dosages. It was believed that this extra water reduced the tempera- 

 ture of the gas, thereby preventing the burning of the foliage. Very 

 naturally, the use of extra water might produce less injury, but this 

 would not be due to the reduction of temperature, as has been 

 believed, but to the decrease of the amount of gas given off. This 

 practice has caused a great waste of cyanid and wide disparity in 

 results. Indeed, the writer believes that no one factor has had more 

 to do with the wide variation in results secured in fumigating citrus 

 trees than has this erratic use of water. 



THE CORRECT PROPORTION OF WATER. 



It has been shown that 2 parts of water to 1 part each of cyanid 

 azid sulphuric acid will produce the maximum amount of available gas. 

 It is impractical, however, to use 2 parts of water in field work 

 because with this proportion the residue, especially when small 

 dosages are used, will frequently solidify within one hour's time, 

 which is the usual period for leaving tents on the trees. Although 

 this proportion of water apparently is sufficient to dissolve the sul- 

 phate at first so that a complete reaction takes place, it appears 

 unable to hold the sulphate in solution long enough afterwards to 

 prevent "freezing." This phenomenon is an impediment to rapid 

 field work, for some little time is required to remove this congealed 

 residue from the constricted-neck generating pots in common use. 

 It is evident in this instance that a "frozen" generator does not 

 imply an incomplete generation, although in some other cases the resi- 

 due left may be congealed and the generation incomplete. "With 3 

 parts of water the residue seldom congeals and this is the proportion 

 recommended by and used in all the field work of the writer. With 

 dosages of 12 ounces of cyanid or above, a 2^-ounce ratio of water can 

 be used without danger of k 'freezing." The water should be measured 

 carefully with a glass or dipper graduated to ounces. 



THE MOST ECONOMICAL PROPORTION OF CHEMICALS TO USE. 



In the preceding discussion it has been shown that for various 

 reasons 1 fluid ounce of commercial sulphuric acid and 1 ounce 

 (avoirdupois) of 96 to 100 per cent potassium cyanid in combination 

 with 3 fluid ounces of water give a complete reaction. Thus the 

 1-1-3 formula, hitherto recommended by the Bureau of Entomology, 

 is fully indorsed for fumigation work in the field. 



A review of the use of hydrocyanic-acid gas for fumigation, both in 

 California and elsewhere, shows frequent divergence from the more 



