1877.] Properties of Homologues and Isomers. 



245 



I attempted to measure the vapour-tensions o£ this substance in 

 an apparatus similar to that now used for the analysis of gases, one of 

 the vertical tubes, however, being heated in a large quantity of water, 

 which was made to circulate by means of a screw. This apparatus 

 (which will be described on another occasion) answers perfectly in the 

 case of most substances, but both the iodides now under consideration 

 are decomposed by the mercury ; the normal iodide, however, being but 

 slowly acted upon in the cold, I was able to measure the vapour-tension 

 several times at about 7°. The mean result shows a tension of 15*4 

 millims. at 7 0, 5. 



The curves constructed by means of the above data diverge gradually 



until at a tension of 760 millims., the corresponding temperatures differing 



t 



by o, 5. The equation ~K = ab 1+mi (where H = the tension of the vapour, 

 t = the temperature in degrees Centigrade +7°*5) expresses the observa- 

 tions for Fraction III. with great exactitude when 



log a= 1-1875207, 

 log 6=0-0259870, 

 log m=3-6338070. 



Isopropyl Iodide. — This body was prepared by heating glycerin with 

 iodine and phosphorus, and purifying as in the case of the normal 

 iodide ; more than a kilogramme of the pure substance was obtained. 

 After fractional distillation three portions were obtained boiling as 

 follows : — 



I. 88-6 to 88-7 

 II. 88-7 to 88-8 

 III. 88-8 to 89-0 



The densities of these three fractions were as follows :— 



I. 1-70526 at 19°80 ; 1-74315 at 

 II. 1-70506 at 20-14 ; 1-74363 at 

 III. 1-70457 at 21-09 • 1-74504 at 0. 



The mean density at 0° is therefore 



D = 1-7440. 



The density of isopropyl iodide is given by Erlenmeyer (Ann. Chem. 

 Pharm. cxxvi. 309) as 1*714 at 16° ; by Linnemann (Ann. Chem. Pharm. 

 cxxxix. 229) as 1*735 at 0° ; by Buff (Ann. Chem. Pharm. Supp. iv. 129) 

 as 1-71732 at 17°. 



The observations on the expansion were as foHows : — 



Temperatures 

 \ uncorrected. 



