Mar. 2, 1920.] Agricultural Gazette of N,S.W. 209 



Perhaps the nature of these two compounds will be better understood if the 



analogy existing between phosphates and arsenates is referred to. In the former 



the calcium phosphates are widely known. There are three of them, viz. : — 



(a) 1 jpart calcium oxide with 1 part pJiosphoric acid, forming mono-calciura 



dihydrogcn phosphate, acid calcium phosphate, or superphosphate. 

 (6) 2 parts calcium oxide ivith 1 part phosphoric acid, forming dicalcic 

 phosphate or dicalcium monohydrogen phosphate, or basic calcium 

 phosphate . 

 (c) 3 parts calcium oxide with 1 part phosphoric acid, forming tricalcic 

 phosphate, neutral phosphate, or rock phosphate. 

 • Referring now to arsenates, and with special reference to lead arsenates^ 

 the compound analogous to (a) is not known to exist. 



Diplumbic arsenate, however, exists, and corresponds to (&), while tri- 

 plumbic arsenate corresponds to (c). 



The amounts of di- and triplumbic arsenates present in the various samples 

 aro given in the attached table. It was considered that the fineness of division 

 of the particles in the samples of lead arsenate purchased might have a 

 bearing on the rate of settling out when suspended, as in applying them for 

 spraying. An attempt has been made to separate the particles into two sizes 

 — " impalpable," or those of about •00-5 mm. diameter, approximating 

 " clay" particles in size; and " not impalpable," or particles over -005 mm. 

 diameter and under -01 mm. On referring to the table under the heading 

 " mechanical analysis," it will be noted that although in three cases out of 

 four the arsenates in paste form contain the largest proportion of their 

 particles in an impalpable form, this is not so in all cases, because we 

 have one case (No. 5, dry powder) where the percentage of impalpable 

 matter is higher than it is in one of the paste forms (No. 3). Arranged in 

 the order of greatest amount of impalpable matter, or smallest amount of 

 coarser particles, we would have :— No. 6 (paste). No. 9 (paste), No. 4 

 (paste). No. 5 (dry). No. 3 (paste), No. 1 (dry) No. 8 (dry). No. 2 (dry), 

 No. 7 (dry). 



It will be noted that those brands of lead arsenate examined naturally 

 group tlvemselves under three classes, viz. : — 



(a) Those in which the diplumbic salt predominates, viz., Nos. 9, 2, 1, 3, 



arranged in the order of largest content of diplumbic arsenate. 

 The ratios of diplumbic arsenate to triplumbic arsenate in the above 

 are respectively 7-9 to 1, 5-7 to 1, 4-6 to 1, and 3-5 to 1. 



(b) Those in which there is nearly an equal quantity of di- and tri- 

 plumbic arsenate, viz., Nos. 7 and 8. The ratio of diplumbic 

 arsenate to triplumbic arsenate is 1-0 to 1 in No. 7, and 1-3 to 1 in 



No. 8. 



(c) Those in which the triplumbic arsenate present is greater than the 



diplumbic arsenate, viz., Nos. 6, 5, and 4. The ratio of diplumbic 

 arsenate to triplumbic arsenate in Nos. 6, 5, and 4 is 0-9 to 1, 0-8 

 to 1, and 0-5 to 1 respectively. 



