HEAT. 



and when bodies that are usually solid 

 become fluid, in consequence of the 

 addition of caloric, it is said they are 

 melted, or fused. The transition of 

 water and of metals from a solid to a 

 fluid state being produced by the same 

 cause, we may consider water as melted 

 ice, with as much propriety as we do 

 the fluid state of metallic substances 

 melted metals. 



It is not possible, under usual cir- 

 cumstances, to melt numerous com- 

 pound substances, chiefly of animal or 

 vegetable origin, on account of the de- 

 composition which they suffer at com- 

 paratively low temperatures; but Sir 

 James Hall, by subjecting several sub- 

 stances of this kind, as coal and lime- 

 stone, to heat, under a great degree of 

 pressure, so as to prevent the escape of 

 their gaseous parts, succeeded in fusing 

 them. 



Ice cannot be raised higher than the 

 temperature of 32 without melting ; but 

 water may, under certain circumstances, 

 be cooled much lower without freezing. 

 Mr. Dalton succeeded in reducing it to 

 5 of Fahrenheit before it solidified. 

 Agitation is unfavourable to this expe- 

 riment, occasioning the w r ater to freeze 

 instantly, and its temperature to rise 

 to the freezing point. It was proved 

 by Dr. Black that water which has been 

 deprived of air by boiling freezes more 

 readily than unboiled water, on account, 

 as he supposed, of a slight agitation 

 upon its surface, occasioned by the 

 attraction of air. Whatever particles 

 impair the transparency of water, when 

 mixed with it, produce the same effect ; 

 but the most effectual method of deter- 

 mining the congelation of water which 

 is colder than the freezing point, is to 

 introduce a particle of ice or snow ; 

 crystallization instantly commences. 

 Sir Charles Blagden exposed to the 

 atmosphere two vessels containing dis- 

 tilled water, when the temperature was 

 about 20 and the day calm ; one of 

 the vessels he covered slightly with 

 paper, the other, being left uncovered, 

 the temperature of the water in the 

 covered vessel sunk many degrees be- 

 low 32 without freezing, while ice in- 

 variably formed upon the surface of the 

 water in the other vessel before a ther- 

 mometer immersed in it was cooled 

 quite to the freezing point. This diffe- 

 rence he accounted for on the supposi- 

 tion that the frozen particles which float 

 in^the air, at that temperature, being 



permitted to come into contact with the 

 water in the uncovered vessel, occa- 

 sioned the process of congelation to 

 commence. The effect of oil poured 

 upon the surface of water in preventing 

 it from freezing may be ascribed to the 

 same cause. 



The beautiful spiculae, which shoot in 

 different directions at certain angles 

 through water at the commencement of 

 freezing, and the enlargement of bulk 

 produced by the process, prove that it 

 is a species of crystallization, in which 

 the particles are united by certain points 

 or surfaces in preference to others ; and 

 the effect of agitation, in facilitating 

 the process, may be explained by sup- 

 posing that it assists the particles in 

 assuming that position which is most 

 favourable to their solidification; and 

 since this effect is more likely to be 

 produced by internal agitation among 

 the parts of the fluid than by the gene- 

 ral motion of the whole, it is found 

 that a sudden jerk of a vessel contain- 

 ing water cooled below 32 is the most 

 effectual kind of motion in promoting 

 congelation. The effect produced by 

 the introduction of a solid particle of 

 the same substance, is supposed to de- 

 pend upon the attractive power which it 

 exerts upon the contiguous particles, 

 occasioning them to arrange themselves 

 in the manner most favourable for their 

 union with other particles; and the same 

 influence being extended from particle 

 to particle, the solidification proceeds 

 with rapidity. 



Many of the circumstances alluded 

 to in relation to the freezing of water 

 may be illustrated by reference to other 

 fluids, the best of which for that pur- 

 pose is a solution of sulphate of soda. 



Ex. A flask of the shape of fig. 15, 

 being filled with a saturated solution of 

 sulphate of soda, near the boiling point, 



Fig. 15. 



