26 NEW METHOD FOR DETERMINING COMPRESSIBILITY 



As before, it was important to know accurately the specific gravity 

 of the substance at 20. Gay Lussac ' found the value 4 948 (i7/i7) 

 and this reduced to (i7/4) gives a value 4.942. Billet 2 found the 

 following values : 



Temperature 40.3 6o 79.6 89.8 



Specific gravity 4-9 I 73 4.886 4.858 4.841 



but unfortunately it is not evident whether these values are based on 

 water at 4 , or at some other temperature. We can, however, obtain 

 the inclination of the curve from his results and plot a parallel curve 

 passing through the value obtained by Gay Lussac. The density thus 

 obtained is 4.938 (20/4). The value used in the calculations 

 was 4.94. 



The method employed for determining the compressibility of iodine 

 was essentially that used in the bromine experiments. The question 

 of the isolation of the iodine from contact with the surrounding mer- 

 cury was, however, even more perplexing than with bromine. The 

 problem was finally solved according to the following method. Two 

 small bulbs, similar to those used with bromine, were prepared (A 

 and B) . Into A was put a saturated solution of iodine in water, a 

 few wisps of glass wool, and a small quantity of solid iodine. Into 

 B was put a saturated solution of iodine in water and a large quan- 

 tity of iodine. Both were sealed and subjected to quantitative com- 

 pression. The glass wool served to hold the small bits of iodine at 

 various positions throughout the solution, thus facilitating the speed 

 of any possible shift in the solubility-equilibrium when the pressure 

 changed. 



If there are a grams of water and b grams of iodine in A, and c 

 grams of water and d grams of iodine in B, and if volume change of 

 A under pressure is m and of B for same pressure is n we have the 

 following relationship : 



a a + b m 

 c ~ >c+ <5 ~ " *' 



where d is a quantity of iodine which bears the same relation to c 

 that b bears to a (that is, ajb = cjS)^ and x is the volume change 

 of the water c in tube B plus the volume change of a small quantity 

 of iodine, 5. Evidently, then, {n x) represents the volume change 

 of the quantity of iodine (d 8). This method is, of course, appli- 

 cable in all cases where the solid under consideration is attacked by 



1 Ann. de Chim., 91, 5 (1814). 

 2 Jahresbericht, (1855) 46.^ 



