FUMIGATION OF CITEUS TKEES. 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 
and 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 "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 
