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° in a 

 medium cooled to 32', he found, Avhile 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 experiments of Dr Hope, which were vai-ied 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 simi^le experiments are perfectly conclusive on 

 the question of the greatest density of water being several degrees ahyve its 

 freezing point ; and Mr Dalton, 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 Hope supposed. 

 There are, however, many facts which would lead us to iafer, 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 Pictet and others. 



From a long note attached to this paper of Dr Hope* we also learn, that at 

 an early period he had experimentally proved the fallacy of Count Rumfokd's 

 assertion, that liquids were absolute non-conductors of heat. This philosopher 

 had alleged, that when heat Avas applied to the upper surface of a fluid, the heat 

 could only affect a thermometer placed helow the surface of the liquid, by trans- 

 mission doMTiwards 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 pai'ticles (as takes place in solids), and that \\-hen 

 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 Hope 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 Hope 



* Trans. R. Soc. Edin. V., p. 394. 



