424 MEMOIR OF THE LATE DK THOMAS CHARLES HOPE. 



experimentally, that the dhninution of bulk in atmospheric air is always propor- 

 tional to the compressing force ; or its volume is inversely as the pressm-e which 

 it sustains ; and philosophers had generally, from analogy, inferred the same of 

 other gases. 



I hud, from some notes of Dr Hope, that in 1803 he instituted a series of 

 experiments to ascertain " whether the principal permanently elastic fluids, viz., 

 oxygen, nitrogen, hydrogen, and carbonic acid, observe the same law of compres- 

 sibility from pressure which air does." 



In these experiments the compression was obtained by means of a column of 

 mercury in a siphon tube, in the same manner as in the experiments of Boyle, 

 and of later experimentalists. The result was, that they all follow the same 

 law of compression. 



(3n the 9th of January 1804, Dr Hope read a memoir to the Royal Society of 

 Edinburgh, " On the contraction of water by heat, at low temperatures,''^ which 

 appeared in the bth voiume of the Transactions, in 1805, j). 379. 



The Florentine Academicians had published, in 1667, the singular fact, that 

 water expands as it cools towards its freezing point ; and in 1683, the same was 

 stated to the Royal Society of London by Dr Croune, the Gresham lecturer. 

 His experiments shewed that water, when cooling, begins to expand as its 

 temperature sinks, from several degrees above the freezing point, until it begins 

 to congeal. Several subsequent writers endeavoured to confirm these observations, 

 but differed as to the point at which water attains its maximum density ; some 

 contending for the 40 or 41 of Fahrenheit ; others for the 42' or 43°. All those 

 experiments were made iti tubes with large bulbs at one extremity, resembling 

 in form the glass of a thermometer, but on a larger scale. 



On the reading of Croune's paper, it Avas contended by Dr Hooke, one of the 

 most acute but most disputaceous philosophers of his age, that this expansion 

 was apparent, not real ; arising from the sudden contraction of the material of 

 the bulb, on the application of cold. This opinion has since been maintained by 

 several very eminent men ; among whom we may mention Dalton, whose experi- 

 ments on this subject are most ingenious, and who, in a private letter, drew Dr 

 Hope's attention to this curious phenomenon. It occurred to Dr Hope, that this 

 point might be decided by experiments, in which a change in the capacity of the 

 containing vessels could have no influence on the result. 



He took a cylindrical glass vessel, 8^ inches deep and 4^ inches wide, which 

 was filled with water at the freezing point, 32°. Two delicate thermometers were 

 suspended in the axis of the jar, so that the bulb of one was half an inch below 

 the top of the liquid, and that of the otlier as far from its bottom. This apparatus 

 was placed in a room at a temperature 60", and the progressive temperature of the 

 water was carefully noted, as indicated by both thermometers. The result was, 

 that up to 38', the lower thermometer was invariably one degi-ee higher than the 



