12 



HEAT. 



There are a few partial exceptions to 

 the expansion of bodies by heat, and 

 their contraction by cold, of which water 

 presents the most remarkable. This 

 fluid contracts in cooling until it arrives 

 at a certain point, and then expands as 

 if heat were applied. This property of 

 water was first observed by the Florentine 

 Academicians, in cooling a thermometer 

 glass, filled with water, by immersion in 

 a mixture of ice and salt ; but De Luc 

 afterwards investigated the subject, and 

 considered the greatest density of the 

 water to be attained when it arrived at 

 40, as it ceased to contract from that 

 point, although the cooling process con- 

 tinued ; on the contrary, expansion was 

 found to result both from the addition 

 and the abstraction of heat, after the 

 water arrived at the above-mentioned 

 temperature. Sir Charles Blagden and 

 Mr. Gilpin discovered that De Luc had 

 omitted to make the proper correction 

 for the expansion of the glass ; which, 

 when they had done, they found that 

 water arrives at its greatest density on 

 being cooled to 39 of Fahrenheit. Dr. 

 Hope considers the greatest density to 

 be at 39 5', and the French chemists at 

 40. Taking the temperature at which 

 the greatest density occurs to be 40, the 

 density of water at 48 and 32 will be 

 the same. 



It was ascertained by the experiments 

 of Sir Charles Blagden, Mr. Dalton, 

 and M. Gay-Lussac, that the expansion 

 of water continues below 32, when it 

 is cooled lower than that temperature 

 without freezing ; and it was expanded as 

 much in some of the experiments al- 

 luded to, as it would have been if heated 

 to 75. 



The enlargement of bulk occurring in 

 water as it cools, from 40 to the freez- 

 ing point, was ascribed by Hooke, and 

 afterwards by Dalton, to the contrac- 

 tion of the glass vessels which contain 

 the water used in the experiments. That 

 the bulb of a thermometer used in such 

 experiments does contract by being 

 cooled, contributing in some measure to 

 raise the liquid in the tube, cannot be 

 doubted, but from other experiments 

 and considerations yet to be stated, it ap- 

 pears certain that water expands on cool- 

 ing, independently of the contraction of 

 the vessel in which it is contained. 



Dr. Hope filled a glass jar, 8k inches 

 deep and 4| inches wide, with water at 

 32; one thermometer was suspended 

 nearly in the axis, with its bulb about 

 half an inch from the bottom, and ano- 



ther at the same distance from the sur- 

 face of the fluid. The jar being ex- 

 posed to a temperature of 60, gradually 

 rose to 38, during which increase of 

 temperature the lower thermometer was 

 at least one degree higher than the ther- 

 mometer at the surface, which indicated 

 that the density of the water must have 

 increased along with its temperature ; 

 for if the water had expanded with the 

 increase of heat, it would have ascended 

 to the surface, and the uppermost ther- 

 mometer would have had its temperature 

 the most raised. In cooling water to 

 32, the temperature of the thermometer 

 near the bottom of the jar was si ill 

 found to be higher than the upper one. 

 While water is cooling to 40, its cold- 

 est particles are always at the bot- 

 tom ; but when it arrives at 40, it re- 

 mains there until the whole of the fluid 

 arrives at the same point ; and then in 

 cooling lower, the colder particles col- 

 lect at the surface, those at the bottom 

 being four degrees \varmer. 



These experiments of Dr. Hope 

 prove that water is most dense at a 

 considerable number of degrees above 

 the freezing point, and that it expands 

 as it cools^to that point. 



The expansive force of water on freez- 

 ing is well known, as water-pipes and 

 vessels filled with it are often burst 

 by its force. Even in our bed-rooms, 

 in very cold weather, water-bottles are 

 frequently broken, if they be quite full 

 of water, and have such narrow T necks 

 as to prevent the free expansion of the 

 fluid in freezing. The flat stones of 

 pavements are frequently raised out of 

 their places by the freezing of the water 

 beneath them, trunks of trees are split, 

 and rocks are rent asunder by the same 

 force. The agency of frost is very bene- 

 ficial in occasioning the substance of 

 rocks and of soils to moulder to powder, 

 thereby fitting them better for the pur- 

 poses of vegetation. A strong brass 

 globe, the cavity of which was only one 

 inch in diameter, was used by the Flo- 

 rentine Academicians, for the purpose 

 of trying the expansive force of conge- 

 lation, by which it was burst, although 

 the force required was calculated to ex- 

 ceed 27,720lbs. Experiments were tried 

 by Major Williams, at Quebec, in one 

 of which an iron plug, nearly 3lbs. in 

 weight, was projected from a bomb-shell 

 to the distance of 415 feet; and in 

 another, the shell was burst by the freez- 

 ing of the water which it contained. 



Several attempts have been made to 



