SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 59 



changed from 19'50° C. at the commencement of the experiment to 19"41° C. when the 

 observations were completed. The value of the specific gravity as calculated from this 

 experiment was r004542. This value was not included in the accepted results, and 

 the second series of experiments with the same hydrometer, where no change of tempera- 

 ture occurred in the solution during the experiment, gave a specific gravity value of 

 1'004570, while the mean of the whole series was 1 004579. 



This variation in the solution temperature could only be due to the temperature of 

 the hydrometer itself being considerably lower than that of the solution, and this factor 

 operates in the twofold manner of lowering the solution temperature, and by virtue of 

 the fact that there is a contraction in the volume of the hydrometer at a lower tempera- 

 ture, the added weight to sink it to a given scale division is less than at the higher 

 temperature, so that the specific gravity value is lower than that obtained when the 

 hydrometer is at the standard temperature. This is indubitably the explanation of the 

 change in the solution temperature during the experiment quoted above, and may not 

 improbably account for the observation that the first reading of the day is sometimes 

 not comparable with the later results obtained in observations made on the same 

 solution. 



It is not possible to directly ascertain the temperature of the hydrometer, and since 

 it is necessary that it should be acclimatised to the experimental temperature, the 

 precaution of immersing the hydrometers in distilled water at the expey'imental 

 temperature for some tim,e before commencing hydrometric observation is im^portant. 

 It ensures that the hydrometer shall be at the experimental temperature and that its 

 volume shall be normal for the given temperature. 



The water value of one of the hydrometers is 11 gram-degrees centigrade. It is 

 possible to calculate the temperature of the hydrometer which would reduce the 

 solution temperature from 19'50°C. to 19*41° C. in the instance mentioned above, 

 assuming no loss of heat due to radiation (this loss is negligible in any case under 

 the conditions of experiment). The weight of the water in solution is about 600 grams 

 (specific heat= l). Hence, applying the principle of the determination of specific heat 

 by the method of mixtures, the temperature of the hydrometer to produce this must 

 be 14-59° C. 



The effect of immersing the hydrometer at that temperature was to make the earlier 

 readings lower than they would have been if the hydrometer were at normal temperature ; 

 but the hydrometer is expanding, because it is in a warmer medium, so that the later 

 readings in the same series of observations would approach the values that would have 

 been obtained if the hydrometer had been at normal temperature. 



It is not difficult to show that the hydrometer was at this temperature (14'59° C.) 

 at the beginning of the experiment, since we have its coefficient of expansion, 

 namely^, 0*003 c.c. per degree difference of temperature. 



If we assume a mean hydrometer temperature of 17'00° C. — which is the mean 

 between the initial and final hydrometer temperatures — then the volume of solution 



