22 Control of the Greenhouse White Fiji 



Later, when the packet breaks down, the acid will no longer act on the 

 charred mass. The cyanide should be dropped free into the acid so that 

 the reaction may be brisk and unimpeded. A so-called "Safety Cyanide 

 Package" is on the market and this consists of a metal container the 

 side of which is made of thin zinc foil. An additional amount of sulphuric 

 acid is used and the zinc dissolves away completely so that the acid has 

 free access to the cyanide. There is no scientific objection to this container. 

 There is, however, ample time for the operator to drop loose cyanide into 

 the acid and to move on to the next jar at a slow walking pace without 

 detecting the slightest odour from the gas. 



A series of eighty fumigations with this gas was carried out in an 

 experimental greenhouse 18 ft. long by 20 ft. wide, height Hi ft. to 

 the ridge and 4 ft. to the gutter, capacity 2500 eft. The recommenda- 

 tions finally made were checked in blocks of tomato houses in trade 

 nurseries and in greenhouses of mixed plants. Temperature, humidity, 

 duration, time of day, size of dose, and the condition of the plants were 

 all varied. Tomato plants in pots were always included, and some of 

 these were of large growth growing in 12-inch pots, to study the effect of 

 the gas on normal plants. Others, tomatoes or beans, were heavily 

 infested with white fly in all stages and were enclosed in muslin sleeves 

 at the end of the fumigation and were examined every day or two for 

 two or three weeks in order to estimate the effect of the gas on the insect. 

 In general three infested plants were included and placed in different 

 positions and at different heights, but it was found that position made 

 no appreciable difference. A fumigation chamber with a capacity of 

 350 eft. was also constructed, but it was found that results obtained in 

 a chamber are useless when applied to a greenhouse owing to the diffe- 

 rence in leakage and its use was abandoned except for a few special 

 experiments. 



Temperature. The results confirmed the work of other investigators 

 that a somewhat low temperature (below 60° F.) renders the plants less 

 liable to damage, but in practice as the fumigation of tomato houses is 

 most often done in summer and early autumn the operation has generally 

 to be carried out at a somewhat higher temperature than this. On 

 30 evenings from May to July fumigations were commenced at dusk and 

 on only two occasions was the temperature of the house below 60° 

 though artificial heat was cut off in nearly every case, and on 1 1 occasions 

 it was above 65°. As will be shown presently it is possible to counteract 

 the harmful tendency of high temperature by withholding water from 

 the plants. It is exceedingly difficult to make any definite recommenda- 



