424 MEMOIR OF THE LATE DR THOMAS CHARLES HOPE. 
experimentally, that the diminution of bulk in atmospheric air is always propor- 
tional to the compressing force ; or its volume is inversely as the pressure which 
it sustains; and philosophers had generally, from analogy, inferred the same of 
other gases. 
I find, from some notes of Dr Hops, 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. 
On the 9th of January 1804, Dr Hore read a memoir to the Royal Society of 
Edinburgh, “ On the contraction of water by heat, at low temperatures,” which 
appeared in the 5th volume of the Transactions, in 1805, p. 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, untilit 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 ih 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 was contended by Dr Hooks, 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 DaLron, whose experi- 
ments on this subject are most ingenious, and who, in a private letter, drew Dr 
Horr’s attention to this curious phenomenon. It occurred to Dr Hors, 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, 85 inches deep and 44 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 other as far from its bottom. This apparatus 
was placed inaroom 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 degree higher than the 

