6 John White 



impossible to efifect the synthesis of the double compound by 

 causing the molecules to unite directly. 



The Sodium Salt. — Attempts were first .made to prepare 

 a sodium double salt by heating together lead iodid and 

 sodium acetate in sealed tubes, using as solvent various sub- 

 stances, such as ethyl acetate, glacial acetic acid, water, etc. 

 A reaction took place in each case, and when sufficient sodium 

 acetate was used the lead iodid was completely dissolved. Gen- 

 erally a thin, straw-colored or white liquid was obtained, but it 

 was found impossible to remove the contents from the tubes, 

 for, immediately on pouring the liquid out, there ensued, 

 in nearly every case, first a crystallization and then a rapid 

 decomposition of the crystals ; this decomposition was accom- 

 panied by a marked evolution of heat. 



After considerable experimentation, the method of prep- 

 aration finally adopted was as follows : A rather concentrated 

 solution of sodium acetate in (about 80 per cent) alcohol was 

 prepared by the aid of heat ; this was used as the solvent, and 

 to the boiling solution recently precipitated lead iodid was 

 added in small quantities at a time, until the solution became 

 noticeably yellow. It was observed that the previous addition 

 of a small quantity of glacial acetic acid, usually 2 to 5 cc, 

 very materially increases both the amount of iodid which dis- 

 solves and the readiness with which it passes into solution. 

 If the yellow solution, after filtering, be allowed to cool in the 

 air, marked decomposition ensues, with deposition of lead 

 iodid, but if instead the air be excluded, this does not happen. 

 To this end, the vessel containing the solution was cooled by 

 placing in a vacuum desiccator over concentrated sulfuric 

 acid, the desiccator partially exhausted by means of a good 

 pump, and set aside to crystallize. Under these conditions 

 nearly white — often very pale sulfur yellow — cr3^stals were 

 obtained. 



Attempts were made to determine with certainty the 

 proper proportions to be used in preparing these crystals, but 

 no satisfactory results could be obtained. It was found that the 

 satisfactoriness of the preparation depended to a very con- 



312 



