110 THIRTY-FOURTH FRUIT-GROWERS ' CONVENTION. 



cubic feet will destroy the purple scale in all its stages on the leaves 

 and wood. If the tree to be fumigated contain fruit infested with 

 scale, it will be necessary to increase the dosage somewhat. The extent 

 of this increase can not be stated with definiteness from this one experi- 

 ment, due to the small amount of material available from which to 

 secure data. This point will be determined in future investigations. 

 From a comparison of the destroying dosage of the purple scale in all 

 its forms on the leaves of trees exposed 30 minutes with those of trees 

 exposed 60 minutes, we find that in using the longer length of exposure 

 the dosage can be decreased markedly. 



In the third experiment, the 90-minute exposure, a very small amount 

 of fruit was available in the majority of the tests, so we will confine 

 our attention entirely to the effect on the leaves and branches, which 

 were more or less infested on every tree. At the %-ounce dosage rate 

 we found a few live insects. Healthy eggs were present in all cases up 

 to the 2-ounce dosage rate. At this and above, all eggs were destroyed. 

 In experiment two we found that a li^-ounce dosage rate destroyed 

 all eggs on the leaves. How do we account for the fact that on these 

 trees which were exposed 30 minutes longer than those -of experiment 

 two it requires 14 of an ounce more cyanide per hundred cubic feet to 

 accomplish complete destruction of the insect in all its stages? The 

 conditions under which the trees were fumigated were practically the 

 same in both experiments. The chemicals used were the same. The 

 difference was this : The trees of the 90-minute experiment were much 

 smaller than those of the 60-minute one. There was more leakage sur- 

 face of tent per cubic foot space enclosed in the smaller trees of the 

 90-minute tests than in the larger trees of the 60-minute tests. We 

 know that a leakage of gas takes place through the tent. How much we 

 do not know, and this is one of the points we hope to work out in the 

 future. As the proportion of leakage must be greater in the small than 

 in the large tents, necessarily the gas will become weakened sooner in 

 the smaller tented space than in the larger. We attribute the difference 

 of results as secured in the 60 and 90-minute experiments due to the 

 factor leakage. 



Let me give a practical demonstration and I think you will agree with 

 me. A tree 5 feet high by 4 feet in diameter, when covered by a tent, 

 represents, approximately, 54 cubic feet of space. The exposed surface 

 of the tent is, approximately, 63 square feet. A tree 20 feet high by 16 

 feet in diameter represents practically 3,484 cubic ^eet. The exposed sur- 

 face area of a tent covering this 20-foot tree is practically 1,005 square 

 feet. In the 5-fopt tree there are 1.16 square feet of leakage surface to 

 each cubic foot of space enclosed. In the 20-foot tree there is but .20 of 

 a square foot of leakage surface to each cubic foot of space enclosed. 

 The layer of gas in contact with the tent is naturally the first to escape. 

 Suppose that after the hydrocyanic acid gas had become uniformly dis- 

 tributed throughout each of the tents covering these two trees that a 

 layer of this gas in contact with the tents an inch deep was to escape in 

 30 minutes. In the 5-foot tree this layer would represent 5.2 cubic feet; 

 in the 20-foot tree 83% cubic* feet. This 5.2 cubic feet of the smaller 

 tree would mean that 9 per cent of the total gas within had escaped, 

 whereas in the larger tree the 83% cubic feet would mean that only 

 2 per cent of its gas had been lost. In other words, this would signify 



