46 



HEAT. 



blowing a current of air upon it, the 

 greater the degree of cold produced. 

 A glass goblet, containing water, was 

 placed by him upon the plate of an air- 

 pump, a wide-mouthed phial containing 

 vitriolic ether being immersed in it'; a 

 thermometer was so suspended to a 

 wire passing through the top of the re- 

 ceiver, that its bulb could be dipped 

 into the ether and taken out at pleasure. 

 On exhausting the receiver, the ether 

 boiled violently, being rapidly converted 

 into vapour, and so great a degree of 

 cold was produced by this evaporation 

 that the water surrounding the ether 

 was frozen, although the temperature 

 of the air in the room was 54. 



By this experiment it was proved that 

 ether is incapable of existing as a fluid 

 when the pressure of the air is removed, 

 and that when it assumes the form of 

 vapour, heat is absorbed in large quan- 

 tity. 



Lavoisier proved by experiment that 

 in cases of combination, where any part 

 of the combining substances assumes 

 the gaseous state, less heat is evolved 

 than in other combinations where no 

 aeriform substance is produced, the heat 

 which would otherwise become sensible 

 being expended in enabling the disen- 

 gaged substance to assume its expanded 

 form. 



In order to prove the disengagement 

 of heat which had been absorbed by 

 water when it returned to the I state of 

 ice, Dr. Black exposed pure water and 

 water containing a little salt in solution, 

 in two similar vessels, to the influence 

 of an atmosphere colder than 32 ; the 

 pure water began to freeze, and its tem- 

 perature remained at 32; the heat set free 

 in the process of congelation being suffi- 

 cient to counterbalance the abstraction 

 of heat by the colder atmosphere. The 

 temperature of the water which had salt 

 dissolved in it, continued to descend 

 until it was cooled considerably below 

 the freezing point. 



But the evolution of heat from a 

 latent state is most strikingly shown 

 when vapour is condensed into water. 

 Dr. Black inferred from his experiments 

 on a still, that the quantity set free is 

 from 774 to 750 degrees. Mr. Watt re- 

 peated the experiments with a smaller 

 still, better adapted for the purpose, and 

 obtained as the medium result of many 

 trials 825 as the quantity of heat set 

 free. He adopted another mode of 

 pursuing this enquiry. A metallic 



cylinder, with a piston accurately 

 fitted to it, was filled with vapour; 

 the air having been previously expelled, 

 the piston was then suddenly forced 

 down, which compressed the steam 

 and occasioned the extrication of a 

 quantity of heat, which, according to 

 the calculation made at the time, would 

 have been sufficient to raise the tempe- 

 rature of an equal weight of a body 

 having the same capacity as water, and 

 which would not evaporate 943 de- 

 grees. 



In reviewing the progress of Dr. 

 Black in these important discoveries, a 

 predominant feeling of the mind is ad- 

 miration of the simplicity and clearness 

 with which he explained his opinions, 

 and demonstrated his assertions ; for 

 he, instead of affecting the repulsive 

 obscurity of scientific phraseology, still 

 too much in use, stated every thing in 

 such plain language that every pupil of 

 his class must have been able to under- 

 stand all that he advanced respecting 

 this new department of science ; which, 

 in other hands, might have been ren- 

 dered more than sufficiently difficult and 

 obscure. 



Another suggestion that arises in the 

 mind, and which challenges admiration 

 also, is the freedom of this distinguished 

 philosopher from suspicion and jealousy : 

 he had no sooner opened this delightful 

 field of enquiry, than it was eagerly 

 entered by other philosophers, several 

 of whom were encouraged and assisted 

 by Dr. Black himself. Mr. Watt was 

 one of these ; and, although he has 

 explicitly denied that any of his most 

 important inventions were suggested 

 by Dr. Black, yet it is obvious to all 

 who are acquainted with the circum- 

 stances under which these philoso- 

 phers were placed w T ith respect to each 

 other, that the very important inven- 

 tions alluded to were, in all probability, 

 facilitated by the previous discovery of 

 latent heat by Dr. Black. 



It is not intended in this essay to en- 

 large upon the application of these dis- 

 coveries to the improvement of the 

 steam-engine, but a brief sketch to that 

 effect may not be out of place. 



When Mr. Watt's attention was first 

 attracted to the steam-engine, motion 

 was given to it by the introduction of 

 steam below a piston moving/ steam- 

 tight, in a cylinder; when the piston 

 was raised by its elastic force, a jet of 

 cold water was made to play into the 



