IODINE 27 



mercury. There are two distinct reasons for the use of mercury in- 

 stead of the direct use of the neutral liquid in the outer jacket first, 

 because the transmission of the heat of compression to the surrounding 

 water when mercury is used is quick and certain, and second be- 

 cause the compressibility of mercury is so small that any slight change 

 in volume due to a slightly different placing of the stopper cannot in- 

 fluence the results. With more compressible liquids the error thus 

 introduced might be considerable. 



The special advantage of the method in connection with iodine lies 

 in the fact that it obviates all possible error due to a change in the 

 solubility of iodine in water with change of pressure. There are, it is 

 true, two possible sources of error which it does not guard against : 

 first, the solubility of water in iodine ; and secondly, the change in that 

 solubility with change of pressure. When we consider, however, the 

 fact that liquids usually dissolve solids to a greater extent than solids dis- 

 solve liquids, and remember that iodine is soluble in water to the extent 

 of only about one part in three thousand, it is hard to believe that either 

 of these influences could have an appreciable effect upon the results ob- 

 tained below. Moreover, since the compressibilities of iodine and water 

 are of the same order of magnitude, the amount of water dissolved by the 

 iodine must be great before any difference would appear in the result. 

 In obtaining the weight of iodine and water, the method used was to 

 weigh the bulbs full of water and iodine ; then break them under a 

 solution of potassium iodide and titrate the iodine with recently stan- 

 dardized thiosulphate. The glass' particles were then collected, dried 

 and weighed, and the weight of the water was found by difference. 

 The small amount of iodine dissolved in the water was neglected as 

 too slight to cause appreciable effect. For the purpose of standard- 

 izing the sodic thiosulphate solution, 3.080 grams of dry, recently re- 

 sublimed iodine were dissolved in potassium iodide solution and diluted 

 to a volume of 0.2509 liter. This was titrated against the approxi- 

 mately decinormal thiosulphate solution. The results are given below, 

 burette corrections having been applied. As an average of three 

 closely agreeing determinations, 44.29 milliliters of the thiosulphate 

 solution required 25.92 milliliters of the iodine solution. Therefore 

 one milliliter of the thiosulphate solution was equivalent to 0.007 246 

 gram of iodine. 



The total quantity of iodine in bulb A required 22.67 milliliters of 

 the thiosulphate solution, therefore the weight of iodine present must 

 have been 0.1643 gram. 



The iodine in bulb B was dissolved in potassium iodide and diluted 



