The Several Forms of Calcium Carbonate. 481 



stirrer, in such a way that liquids dropped from the burettes 

 will enter the beaker on opposite sides, and so mingle only in 

 the body of the liquid ; these burettes contain equivalent 

 (usually 0*1 or 0*2 N) solutions of calcium chloride and potas- 

 sium carbonate respectively, and the rate of dropping is so 

 adjusted that a gram of carbonate is precipitated in two or 

 three hours. It is essential that the two solutions should 

 mingle only in the body of the liquid contained in the beaker — 

 in other words, that the precipitation be the result of mixing 

 effectively very dilute solutions ;* for otherwise the precipitate 

 will be so fine-grained that its definite identification will be 

 difficult, if not impossible. Moreover for many other purposes, 

 e. g., for determinations of solubility, it is very desirable to 

 have fairly coarse-grained material in order that it may be 

 adequately washed and dried, and easily handled. 



Aragonite is precipitated, as is well known, even at ordinary' 

 temperature in a solution of a salt from which a carbonate iso- 

 morphous with aragonite may be precipitated ; in this case it 

 is necessary either that the salt be in sufficient concentration to 

 insure that the foreign carbonate is precipitated first, or that 

 it come down in solid solution with the aragonite. Aragonite, 

 precipitated in presence of a lead salt, formed around nuclei of 

 cerussite (PbC0 3 ) in parallel orientation ; with the carbonates 

 of strontium and barium the effect is not visible either by 

 reason of their similarity to aragonite in refringence or because 

 homogeneous mix-crystals are formed. The isomorphous car- 

 bonate is in all probability ineffective in inducing the forma- 

 tion of aragonite unless it is actually being precipitated when 

 the aragonite begins to separate. Therefore in order to insure 

 the appearance of aragonite by seeding with PbC0 3 it is best 

 to wait until the solution in the beaker is nearing the point 

 where precipitation would begin and then to add enough lead 

 nitrate solution to cause PbC0 3 to form ; since the solution is 

 already supersaturated with respect to aragonite, its precipita- 

 tion by this means is induced at once. We have found calcite 

 deposited on cerussite (see Table I, No. 17) in precipitations in 

 which these conditions probably did not obtain ; it follows 

 therefore that the addition of a few previously prepared crys- 

 tals (of PbC0 3 or of aragonite itself) will not always produce 

 the desired result. These details are given because Tetterf 

 casts doubt on the efficacy of seeding as a means of insuring 

 the appearance of aragonite. 



When slow precipitations, carried out as described above, 

 are made in a medium at a temperature of 80-90°, no isomor- 



* For the rationale of this process, see Johnston, J. Ani. Cheni. Soc, 

 xxxvi, 16, 1914. 



fVetter, Z. Kryst., xlviii, 45, 1911. 



