108 



THIRTY-FOURTH V FRUIT-GROWERS' CONVEXTIOX. 



The con^ealinp' of pots is commonly spoken of by fumitrators as 

 freezin^i. This freezing- is. I believe, usually attributed by them to be 

 due to the presence of an insufficient amount of acid to transform all 

 the cyanide of potassium which was placed in the generator. Such, 

 however, is not the case. 



I have just explained how excess of acid in the residue will precip- 

 itate potassium sulphate. Now, let me take up another and very im- 

 portant consideration, the reason for using- water in the generation of 

 hydrocyanic acid gas. I previously mentioned that Avater did not 

 enter in the chemical reaction. Then, why is it used ? In the first place, 

 Avater is used to dissolve the potassium cyanide, as the reaction is more 

 complete when the cyanide is in solution than when it is in the solid 

 condition. A piece of cyanide thrown into a mixture of acid and water 

 directly gives up some of its mass in solution. Scarcely has the cyanide 

 passed into solution when it is transformed partly into gas. The heat 

 liberated during this process assists in causing more to hastily pass into 

 solution Avhich is also immediately transformed. This continues until 

 the reaction stops. In addition to this, water dissolves the potassium 

 sulphate formed and so prevents it from coating the potassium cyanide. 

 In the presence of an insufficient amount of water the potassium sul- 

 phate forms a coating around the pieces of cyanide, which retards, or 

 even in part prevents, the reaction. In such cases this undissolved sul- 

 phate usually congeals and causes the freezing of the pots. Hen<;e it 

 is desirable to add water enough to dissolve the sulphate of potassium. 

 Experiment has demonstrated to us that the chemistry of the reaction 

 will not allow us to use less than two parts of water if we wish to avoid 

 frozen generators within the time of exposure in common practice. 

 The water also acts with the acid to furnish the degree of heat neces- 

 sary to bxing about a rapid liberation of the hydrocyanic acid gas. 



During the month of November experiments were undertaken at 

 Orange, California, to determine the dosage required for the destruction 

 of the purple scale in all its stages, as well as to determine the effect 

 of different lengths of exposure on results secured. In the first experi- 

 ment the length of exposure was 30 minutes. . In this experiment a 

 series of tests was employed to determine the effect of different strengths 

 of dosage. These tests were accomplished in the following manner: 

 One series of trees were dosed at the rate of % of an ounce of cyanide 

 per hundred cul^ic feet of space enclosed by the tent : a second series 

 ^A as dosed at the rate of 1 ounce per hundred cubic feet : a third series 

 at the rate of I14 ounces per hundred cubic feet, and so on. increas- 

 ing the dosao-e of each succeeding series at the rate of i/4 of an ounce 

 per hundred cubic feet. The highest dosage used was 2i4 ounces per 

 hundred cubic feet. A second experiment was performed which was 

 the exact counterpart of the lirst in all respects, except that the time 

 of exposure was 60 minutes instead of 30. A third experiment was 

 perfoiHied agreeing in all respects with the first two. except in length 

 of exposure, which was l^o hours. 



From the data secured from each of these experiments one would 

 expect to be able to determine the killing dosage of the purple scale. 

 proA^ded the killing dosage ranged between the different strengths of 

 dosage employed. To insure that the dosage sought would fall within 

 the scope of our schedule the limits were made very broad. From the 



