Joseph Black 67 



spending phenomenon was observed when water was 

 converted into steam, but, owing to the greater experimental 

 difficulties, the numerical value obtained was not so accurate. 

 Black also had clear ideas on the differences in the amounts 

 of heat required to raise different substances through the 

 same range of temperature; but handed over this part of 

 the subject to his pupil Irvine. 



An interesting paper by Black on " The supposed effect 

 of boiling on water in disposing it to freeze more readily, 

 ascertained by experiments " (Phil. Trans. 1775) is worth 

 reading as an example of clear thinking, lucid description, 

 and good experimenting. It is still to-day the common 

 belief of plumbers, and those who derive their knowledge 

 of science from plumbers, that hot-water pipes freeze more 

 readily in winter than cold ones. This belief seems to have 

 had its origin in the report, made on good authority, that 

 when water is exposed at night in the dry atmosphere of 

 the Indian winter, in order to convert it into ice through 

 the loss of heat by radiation, it is essential to boil it 

 previously. In order to find the reason for this, Black exposed 

 two similar cups, one filled with boiled and the other with 

 unboiled water, to a temperature below the freezing point, 

 and saw, indeed, ice crystals appearing on the surface of 

 the former, while the latter remained clear. But on intro- 

 ducing thermometers, he discovered that the temperature 

 of the unboiled water had fallen below the freezing point, 

 without being converted into ice, which, however, formed 

 as soon as the water was stirred. Black was aware of 

 Fahrenheit's observation that water, when kept perfectly 

 quiescent, could be cooled considerably below the normal 

 temperature of freezing. The question that remained to 

 be solved was, therefore, this : why should the unboiled 

 water be more easily undercooled than that which had been 

 boiled? The only effect that boiling can have on the water 

 is to expel the absorbed air, and one might be tempted to 

 reason from the above experiment that the absorbed air 

 favours the undercooling. But this explanation is negatived 

 by the circumstance that Fahrenheit's experiments were 

 conducted in a vessel from which the air had been removed 

 by the air pump. Black, realizing, therefore, that water 



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