DR HOPE ON THE MAXIMUM DENSITY OF SEA-WATER. 245 



heat to the apparatus, to see whether the liquid contracted as water does when 

 its temperature is elevated to 39°^. 



The second circumstance creating surprise is, that the scientific world were 

 satisfied with so imperfect an experiment, and based a general law upon so nar- 

 row a foundation. 



In investigating the question I prosecuted the inquiry in two very different 

 methods. In the first series of experiments I employed an apparatus similar to 

 what I conceive Blagden's to have been ; it was a very large thermometer glass. 



Experiment, No 1. The apparatus containing a saturated solution of chloride of 

 sodium i. e. common salt, was immersed for some time in a mixture of snow and 

 water, and then plunged into a mixture of salt and snow, whose temperature was 

 _ 2. The liquor descended in the tube regularly, tiU it became stationary upon 

 reaching the temperature of the mixture. In this progress there was not the small- 

 est indication of the existence of any peculiarity in the salt-brine in regard to the 

 usual effect of cold in causing contraction. I next transferred the apparatus firom 

 the freezing mixture into melting snow, and after some time into water at 40"^. 

 The solution immediately began to rise in the tube, and continued to do so regu- 

 larly, without the smallest interruption or retrocession, and obeying the usual 

 law of expansion by heat. 



Experiment, No. 2. was performed with the same apparatus and fluid as the 

 preceding, and in all respects in the same manner, with the exception of suspend- 

 ing the apparatus in the air, which at the moment had a temperature of 45° or 

 thereby, when in the commencement of the second stage of the experiment, it was 

 withdrawn from the frigorific mixture. The object of doing so, was to prevent the 

 saltus immersionis which accompanies the removal of any similarly constructed 

 apparatus from a cold medium into one considerably warmer, by securing the 

 slow and gradual ingress of heat from the atmosphere. 



It is indispensably necessary to attend to the saltus immersionis, when very 

 small alterations of volume are to be estimated by slight risings or fallings of the 

 fluid in the tube, else there will be a chance that we deceive ourselves. For example, 

 when the apparatus having the saline fluid cooled to zero is plunged into water 

 of 40°, the liquor instantly falls somewhat in the stem, and so assumes the ap- 

 pearance of its being contracted by heat. This, however, proceeds from the sud- 

 den expansion of the glass, and consequent enlargement of the capacity of the 



vessel. 



The result of this experiment in both its stages agreed Avith the preceding, and 

 both led to the conclusion, that a saturated solution of common salt obeys the 

 o-eneral law of expansion by heat, and contraction by cold, in the temperatures 

 near its congelation, as it does at all higher temperatures, and that it does not 



