248 DB HOPE ON THE MAXIMUM DENSITY OF SEA-WATER. 



sions and capacity of the instruments employed, as on a change of density of the 

 Huid, at different temperatures. 



He next made trial of what he calls the Hope method, being that which I had 

 employed in establishing the existence of a maximum density in water, but he 

 also put no great trust in his experiments conducted in this manner, obviously, I 

 conceive, ft'om want of care in executing them. Mr Erman then adopted a me- 

 thod which might at first sight appear to have no connection whatever with the 

 ol)ject of inquiry. This consists in observing the progi'ess of cooling of the fluid 

 wlien approaching its point of congelation. 



He observed that water as it cools from + 6° of R. (45°.5 Fahr.) to + 2° 

 (8t)°.5 Faiir.), exhibits a great retardation at that temperature, at which it at- 

 tains its maximum of density. Tlius, a small quantity of water contained in a 

 cylindrical vessel one and a half inch high and one inch in diameter, having a 

 thermometer suspended in it within one line of the bottom, exposed to a very 

 cold atmosphere, required to cool from the 6° of Reaum. (45°.5 Fahr.) to the 2° R. 

 (30^5 Fahr.), at an average, for each half degree, fifty-seven seconds, but required 

 one hundred and ninety-eight between the 4° and 3^° R. Now, the moment of 

 this retardation con-esponds with that of the maximum density. 



This retardation was still more remarkable in a second experiment, in which 

 the small cylinder containing the water was immersed in a very powerftil frigorific 

 mixtm-e of snow and muriate of lime, by which the whole period of refrigeration 

 was much shortened, and dirring which the thermometer fell from 7° to 1° R. 

 The average number of seconds required for each degree was 25.5 seconds, but 

 tlie cooUng from the 4° to the 3° requii-ed 208.2. 



Mr Erman, perceiving that the period of retardation coincided with that of 

 the maximum density, concluded that he could easily ascertain whether the so- 

 lutions of salt obeyed the same law as pure water, by observing whether they 

 exhibited any retardation during cooling. 



He found upon trial, that a solution of salt of specific gravity 1.027, the re- 

 presentative of sea- water, fell in its temperature from + G° to — 4° R. in a regular 

 progression, exhibiting no indication of a period of retardation. He next sub- 

 mitted a solution having a specific gravity of 1.020, and obtained a similar result. 

 Lastly, he tried a solution of the specific gi-avity of 1.010, and found that it did 

 display a retardation while cooling between the 6° and 1°.5 R. 



From these experiments, he concluded that such a quantity of sea-salt added 

 to water as increased the specific gravity to 1.010, had no other effect on the pe- 

 culiarity in the constitution of this fluid, than to lower the temperature of its 

 maximum density to 1°.5 R. That solutions of specific gravity 1.020 and 1.027 

 liave no maximum density at a temperature above their points of congelation. 



Mr Erman does not mention the circumstances which led him to investigate 

 the rates of cooling, nor does he attempt to explain the cause of the temporary 



