NATURAL PniLOSOPIIT. 



169 



colder liquid may affect the needle, cand mask tlie true electrical 

 relation between the two, so far as it depends upon then." chemi- 

 cal nature. 



FRIGORIFIC MIXTURES. 



The degree to which the temperature can be reduced by dis- 

 solving a salt in water, will, in general, be the greater in the 

 proportion to the dissolved quantity of the salt. Since this 

 quantity depends upon a certain temperature, it will be necessarj^ 

 in order to obtain the greatest effect, to bring the salt and men- 

 struum together precisely in the proportion in which they yield a 

 saturated solution at the desired low temperature. Any excess 

 of water beyond that needs also to be cooled down, and there- 

 fore consumes part of the effect. The fact that this point has 

 generally been left out of view is the cause that the data as to 

 frigorific effect and lowest temperature vary considerably with 

 different observers. The best and safest way is to bring together 

 the ingredients in such conditions that an instantaneous solution 

 must immediately take place, that is, the salt in as fine a state of 

 division as possible, and very slightly in excess of the calculated 

 proportion, after both salt and water have remained for 12 to 18 

 hours in thin glasses in the same room together, to allow them to 

 equalize their temperature with that of the room. In the follow- 

 ing comparative table of averages the results were obtained by 

 adding the water to the salt and stirring with the thermometer. 

 The maximum lowering is attained within a minute at most. 



Alum, 



Common Salt, . . 

 Sulphate Potash, . 

 Phosphate of Soda, 

 Sulphate Ammonia, 

 Sulphate Soda, . . 

 Sujphato Magnesia, 

 Carbonate Soda, 

 Nitrate Potassa, 

 Chloride Potassium, 

 Carb. Ammonia, 

 Acetate Soda, . 

 Chloride Ammonium, 

 Nitrate Soda, . . 

 Hyposulphite Soda, 

 Iodide Potassium, 

 Chloride Calc'm cryst., 

 Nitrate Ammonia, . 

 Sulphocyanide Ammonia, 

 Sulphocyanide Potash, 



Temperature sinks 

 from to 



by 



52 



54 



58 



51 



56 



54.5 



52.5 



51 



56 



56 . 



60 



51 



56 



56 



51 



51.5 



51.5 



60.6 



55.7 



51.5 



49 



50 



53 



45 



44 



42 



38 



35 



37.4 



33 



37.5 



24 



23 



22 



17 



11 



9 



7 



— 1 



-19 



3° 



4 



6 



6.5 

 12 

 12.5 

 14 

 16 

 18.6 

 23 

 22.5 

 27 

 33 

 33.5 

 33.4 

 40.5 

 42 

 49.6 

 56.7 

 70.0 



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15 



