
MEMOIR OF THE LATE DR THOMAS CHARLES HOPE. 425 
upper; a proof that, as its temperature rose from 32° to 38°, the water had 
become more dense. On reversing this experiment, by placing water at 53° ina 
medium cooled to 32°, he found, while the temperature of the water descended to 
40°, that the water at the bottom was always the coldest, and that this difference 
between upper and lower thermometers was sometimes as much as 7° or 8°; but 
that in cooling from 40° to the freezing point, the thermometer at the bottom 
remained higher than that near the surface of the liquid. | 
The experimentsof Dr Hore, which were varied in different modes, led him to 
fix the point of greatest density of water at the temperature of 39°°5 Fahrenheit. 
These well-devised though simple experiments are perfectly conclusive on 
the question of the greatest density of water being several degrees above its 
freezing point ; and Mr Darron, the most able advocate of the opposite doctrine, 
afterwards admitted the general correctness of the observation, though he con- 
sidered that the greatest density was not at so high a point as Dr Hore supposed. 
There are, however, many facts which would lead us to infer, that the greatest 
density of water cannot be far from the point assigned by HopE—as, for instance, 
the remarkable uniformity of temperature in deep alpine lakes, which is about 
40°, according to the observations of Picrrr and others. 
From a long note attached to this paper of Dr Hopr*® we also learn, that at 
an early period he had experimentally proved the fallacy of Count Rumrorp’s 
assertion, that liquids were absolute non-conductors of heat. This philosopher 
had alleged, that when heat was applied to the upper surface of a fluid, the heat 
could only affect a thermometer placed below the surface of the liquid, by trans- 
mission downwards through the medium of the sides of the containing vessel ; 
because, according to him, the particles of fluids communicate none of the caloric 
they receive to the contiguous particles (as takes place in solids), and that when 
heat is applied below, they become heated only by currents set in motion by the 
diminished gravity of the heated particles. 
In these experiments, Dr Hop employed a wide glass jar to contain the 
liquid to be the subject of trial, and applied heat to the surface of the liquid in a 
vessel 11 inches in diameter. The bulb of a delicate thermometer was placed 
half an inch below the surface of the liquid ; and all conduction by the sides of 
the vessel was prevented, by keeping it immersed in water equally cold as high 
as the surface of the liquid within the vessel. Notwithstanding these precautions, 
the thermometer, in several experiments, slowly rose. The liquids subjected to 
such trials were water, olive-oil, and mercury. 
Other experimerfts were conducted in a different manner. Equal portions 
of liquids, such as alcohol, were rapidly mixed together at different tempera- 
tures ; and the mixture immediately indicated a mean temperature—which Hors 
* Trans. R. Soc. Edin. V., p. 394. 
