496 Johnston, Memo in, "anal Williamson — 



Table 16. Summary of precipitation experiments carried out as 

 specified below. 



In presence of solid calcium sulphate 



(24) Solid CaS0 4 .2H 2 shaken with 0-1 ilfK 2 C0 3 at 25° for 



24 hours ; the resulting ppte. proved to be calcite. 



(25) 1-5 g. CaS0 4 .2H 2 + 1-5 g. K 2 C0 3 + 100 C0 H 2 O shaken 



for l-£ hours at 25° ; ppte. calcite. 



(26) 1-5 g. CaS0 4 .2 H 2 + 800 cc H 2 to which 100 cc 0-1 M 



K 2 C0 3 was dropped in \\ hours at 

 (a) 25° ; ppte. calcite 

 (lb) 90° ; ppte. aragonite with admixture of sulphate. 



In presence of ammonium bicarbonate at room temperature 

 (approx. 22° C.) 



In each case C0 2 was passed through the solution while the car- 

 bonate was added, so that no immediate precipitation of cal- 

 cium carbonate took place. The precipitate was examined 

 next day. 



(27) 35 g. NaCl + 2 g. Am. Carb. + 1-4 g. CaCl 2 .2 H s O + 



1000 g. H a O ; ppte. calcite + //. 



(28) As (27) + 2 g. CaS0 4 .2 H 2 ; ppte. ■- calcite + n 



(29) Artificial sea water as used by Linck ; ppte. calcite + ^ 



Each of these three precipitates contained enough 

 of the /x-form to be mistaken for aragonite if the 

 coloration test with Co(N0 3 ) 2 had been the only 

 test made use of. 



Miscellaneous 



(30) 2 g. MgCO s + 10 g. CaCl,.2 H 2 4- 100 cc H 2 G shaken for 



1-J hours at 



(a) 24° ; ppte. calcite 



(b) 85° ; ppte. aragonite 



(31) 25 cc 0-1 J/K 2 C0 3 + 25 cc 0-1 M CaCl 2 + 100 oc H 2 after 



2 hrs. at 20° gave a visible ppte. which, examined after 

 24 hrs., proved to be calcite. 



(32) On mixing equal volumes of 0-1 M K 2 C0 3 and 0-1 M 



CaCl 2 directly a very fine indeterminable ppte. formed ; 

 the filtrate, after standing for a week at 20° deposited 

 aggregates and a little /j, 



(33) Saturated solutions of CaCl 2 and of K 2 C0 3 when mixed 



gave a jelly of CaC0 3 . 



(34) 0-0lJ/CaCl 2 + 0-02ilfMgSO 4 mixed with 0-01 JlfNaHCO, 



while C0 2 was passed ; solution stirred continuously. 

 Precipitate formed after stream of C0 2 was stopped ; 

 crystals of calcite, combination of (1011) and another 

 rhombohedron close to or exactly (7071) — plane angle 

 between edges = 15° ± 3°. 



