E. HORTON AND E. S. SALMON 271 



As will be seen from the above, the statements as to the fungicidal 

 value of arsenic rest on observations made in the field, except in two cases 

 where experiments were made with germinating spores. We have not 

 been able to find in the literature of the subject a single case in which the 

 experimenter has used arsenical solutions of varying strengths on a 

 fungus kept under close observation and determined the necessary 

 strength for complete fungicidal action. 



Materials used. Sodium arsenates. In the first instance the sodium 

 hydrogen arsenate Na.2HAs04 and trisodium arsenate solutions were 

 prepared by neutralising a solution of a known weight of pure arsenic 

 acid with the theoretically necessary weights of sodium hydroxide 

 (Exjjeriments 2.3, 25, 27). Afterwards pure Na2HAs047H.,0 was pre- 

 pared by cr3^stallisiug the commercial dodecahydrate above 20°, and the 

 trisodium arsenate was prepared from it by treating the solution with 

 the calculated weight of sodium hydroxide solution and crystallising. 

 The amount of arsenic in these salts was estimated by distiUing with 

 cuprous chloride and hydrochloric acid and titrating the distillate with 

 iodine (Experiment 41). 



Calcium Arsenates. In the first instance sprays of the required con- 

 centration of these substances were prepared simply by taking a solution 

 of the corresponding sodium salt of double the required strength and 

 diluting to double the volume with a solution of the necessary amount 

 of calcium chloride (E.xperiments 19, 20, 22, 28). The spray solution of 

 course contained sodium chloride (which at so great a dilution was prob- 

 ably quite without eft'ect on the hop leaf) as well as the calcium arsenate. 

 In this method of preparation (with dilute solutions) calcium chloride 

 produces a precipitate with sodium hydrogen arsenate but the precipitate 

 dissolves on dilution; with trisodium arsenate the precipitate is insoluble. 



For later experiments two calcium arsenates were prepared, the first 

 by treating a solution of 11-5 grams of calcium chloride in 25 c.cm. of 

 water with a solution of 32 grams of crystallised disodium hydrogen 

 arsenate (Na2HAs047H20), the second by adding a solution of calcium 

 chloride of the same strength to a solution of 31 grams of crystallised 

 disodium hydrogen arsenate which had been previously treated with a 

 solution of 4 grams of pure sodium hydro.xide. In each case the precipitate 

 was filtered on a Buchner funnel, thoroughly washed with cold water and 

 dried in the air at room temperature. For the purpose of spraying the 

 calculated quantities of these arsenates were weighed out, triturated with 

 water and the suspension diluted (partly with 10 per cent, calcium 

 caseinate solution) to the required strength (Experiments 31. 33, 34). 



Jcum. of Agric. Soi. xn 19 



