HEAT, 



41 



in this difficult research are not such as 

 to engage our confidence, or to admit of 

 being made the basis of other calcula- 

 tions. 



CHAPTER IX. 



Of Latent Heat, including Fluidity, 

 Steam, Evaporation, and Distillation. 



THE honour of having made the im- 

 portant discovery that large quantities 

 of heat must enter into bodies, and be 

 concealed to enable them to pass from 

 the solid to the fluid state, or from the 

 fluid state to that of vapour, is uni- 

 versally ascribed to Dr. Black. His 

 first decisive experiment was made in 

 December, 1761, at Glasgow, where he 

 was then professor of chemistry. This 

 experiment consisted in comparing the 

 length of time which a given weight of 

 water required to raise its temperature 

 one degree, with the length of time which 

 the same weight of ice required for its 

 liquefaction, an equal heat being ap- 

 plied in both cases ; and also reversing 

 the experiment, he compared the length 

 of time required to depress the tempera- 

 ture of a given weight of water one de- 

 gree with the length of time required to 

 freeze the same quantity : he was thus 

 enabled to determine that the quantity 

 of heat necessary to enable a given 

 weight of ice to assume the fluid form, 

 is equal to that which would raise the 

 temperature of the same weight of water 

 140 . He also found that an equal 

 quantity of heat is set free from water 

 when it assumes the solid form. Since 

 the increased quantity of heat, thus 

 proved to be essential to the fluid state, 

 is not capable of being detected by the 

 touch, or by the application of a ther- 

 mometer, Dr. Black called it concealed 

 or latent heat. 



He was led to this discovery by no- 

 ticing what takes place in some natural 

 operations, particularly the melting of 

 ice and snow. Portions of these being 

 brought into a warm room, gradually 

 attain the temperature of 32, if previ- 

 ously below that point ; they then begin 

 to melt, and continue at the same tem- 

 perature until the whole is melted ; all 

 the heat which enters into the melting 

 ice or snow being converted into latent 

 heat, to promote the liquefaction : when 

 the whole is liquefied the temperature 

 again rises, and continues to do so un- 

 til it becomes the same as that of the 

 room. The slow manner in which ice 



melts in ice-houses, and in which ice 

 and snow, where they are accumulated 

 in large quantities, assume the fluid 

 state, were observed by Dr. Black ; and 

 he described in his lectures the effects 

 which would happen if large quantities 

 of heat were not necessary to enable ice 

 and snow to liquefy. In that case, he 

 affirmed, that torrents and inundations 

 would be irresistible and dreadful, tear- 

 ing up and sweeping away every thing 

 so suddenly, as scarcely to permit the 

 human inhabitants of those districts to 

 escape from the ravages. 



Dr. Black put five ounces of pure 

 water into each of two thin globular 

 glass vessels, about the same size and 

 weight ; the water in one of the vessels 

 was completely frozen by immersion in 

 a mixture of snow and salt ; the vessel 

 was then set on a wire ring attached to 

 a reading-desk, in a large hall, where it 

 remained until it was entirely melted. 

 The other vessel containing the same 

 quantity of water cooled to 33, and, 

 having a delicate thermometer sus- 

 pended in it, was placed in a similar 

 situation. In about half a minute the 

 thermometer assumed the temperature 

 of the water, after which the increase of 

 temperature was observed every five or 

 ten minutes during half an hour, at the 

 end of which time the degree of heat in- 

 dicated was 40 of Fahrenheit. When 

 the glass containing the ice was taken 

 out of the freezing mixture, it was four 

 or five degrees colder than melting 

 snow; when it arrived at the freezing 

 point, and was just beginning to melt, 

 the time was noted, and the glass was 

 then left undisturbed ten hours and a 

 half. At that time a small spongy mass 

 of the ice remained unmelted in the 

 upper part of the water, although that 

 part of the water which was near the 

 sides of the vessel had attained the tem- 

 perature of 40. In a few minutes more 

 the whole of the ice had become liquid. 

 Thus it appears that the same quantity 

 of heat which was capable of raising 

 the temperature of the water-glass seven 

 degrees in half an hour, required ten 

 hours and a half, or twenty-one half 

 hours, to raise the ice-glass to the same 

 temperature; so that 21 multiplied by 

 7, will give the number of degrees of 

 heat. 



The temperature to which the two 

 glasses were exposed, under precisely 

 the same circumstances, was 47. The 

 water-glass attained the temperature of 

 40 in half an hour, being an increase 



