242 



Mr. F. D. Brown on the Physical [May 31, 



densities are referred to water at zero ; the error of the observations is 

 less than 0-00005. 



Rate of Expansion. — Dilatometers, similar to those used by Kopp, 

 Pierre, and others, were employed, the necessary data concerning them 

 being obtained in the usual way, slight modifications excepted. The in- 

 struments, when filled with the liquid to be examined, were compared 

 withthe thermometers A and B, in the large bath mentioned above, in 

 which they were completely immersed ; readings were taken both when 

 the temperature of the water was rising and when it was falling. When 

 both dilatometers were filled with the same liquid, the volumes obtained 

 at any given temperature differed only in the fifth decimal place. 



Vapour-tension. — The apparatus shown in fig. 2 was used in the 

 case of the iodides. The vapour of the liquid boiling in the bulb A 

 condenses in the tube BB and runs back by the small tube c ; any cooling 

 of the thermometer by the condensed vapour is thus avoided. R is a 

 reservoir of air with a tap T, with which an air-pump is connected, while 

 the tube E is connected with a pressure-gauge formed of two communi- 

 cating columns of mercury. The thermometer is placed in the tube d, 

 which is closed at the bottom and contains a little mercury. 



A liquid which is not decomposed by heat below its boiling-point will 

 boil in this apparatus at any temperature and for any length of time, 

 without the thermometer varying one hundredth of a degree. About 

 100 cub. centims. of the liquid were put in the bulb. 



Normal Propyl Iodide. — This substance was prepared by treating pure 

 propyl alcohol with hydriodic-acid gas ; the saturated liquid was distilled 

 with steam, the distillate shaken several times with a weak solution of 

 caustic soda, a little iodine added to destroy the phosphorus compounds, 

 again shaken with soda, then with water, and finally dried with phosphoric 



