HEAT. 



Steam . 



Although Dr. Black did not find op- 

 portunities for the performance of such 

 experiments as satisfied him completely 

 respecting the quantity of heat which 

 becomes latent in the conversion of 

 water into steam before 1762, he had 

 made many observations as to the fact 

 a considerable time before. He had 

 observed that every addition of heat 

 applied to a fluid produces an elevation 

 of temperature until it arrives at the 

 boiling point , but however violently the 

 fluid may boil, it does not become hot- 

 ter, nor does the steam that arises from 

 it indicate a greater degree of heat than 

 the water: a large proportion of the 

 heat, therefore, he had no doubt, en- 

 ters into the steam, and becomes latent. 

 He also observed the great heat im- 

 parted to the worm-tub of a still, and 

 was convinced that when vapour is 

 condensed into a liquid, its latent heat 

 is again set free. 



The experiments by which these opi- 

 nions were proved to be correct were 

 simple and convincing. He applied 

 heat to a small quantity of water at 50, 

 and having noticed what length of time 

 was required to make the water boil, 

 he continued to apply the heat until all 

 the water was converted into vapour, 

 and found that the vaporization of the 

 water required five times as much heat 

 as was requisite to make it boil, or 

 810; from which he inferred that 810 

 of heat had been carried off by the 

 steam. Reversing the experiment, he 

 converted a pound of water into steam, 

 and made it pass through the worm 

 of a still, by which he found that 

 20 of heat were imparted to 40lbs. of 

 water in the worm- tub, and inferred 

 from the effect produced that 800 of 

 heat must have been given out by the 

 steam. 



A strong phial was half filled with 

 water, close corked, and heated in a 

 vessel of sand; after the water had 

 boiled, and there was a strong pressure 

 within the phial, Dr. Black suddenly 

 withdrew the cork, and was delighted 

 to find, as he had been led to anticipate, 

 that a portion of the water only was 

 converted into vapour, and that the 

 temperature of the remaining water was 

 reduced to the boiling point, and thus 

 ascertained that all the excess of heat 

 was expended in the formation of va- 

 pour. 



A similar experiment was soon after- 

 wards performed by Mr. Watt, in a 

 more satisfactory manner. *' He put 

 three inches of water into a small cop- 

 per digester, and screwing on the lid, 

 he left the safety-valve open ; he then 

 set it on a clear fire of coke, and after 

 it began to boil and produce steam, he 

 allowed it to remain on the fire half an 

 hour with the valve open ; then taking 

 it off the fire, he found that an inch of 

 water had boiled away. In the" next 

 place, he restored that inch of water, 

 screwed on the lid, and set it on the 

 fire ; and as soon as it began to boil, 

 he shut the safety-valve, and allowed it 

 to remain on the fire half an hour, as 

 before. The temperature of the whole 

 was many degrees above the boiling 

 point. He took it off the fire, and set 

 it upon ashes, and opened the valve a 

 very small matter: the steam rushed 

 out with great violence, making a 

 shrieking noise for about two minutes. 

 When this had ceased, he shut the 

 valve, and allowed all to cool. When 

 he opened, it he found that an inch of 

 water was consumed. Black's Lec- 

 tures, vol. 1, page 160. 



The inferences drawn by Dr. Black 

 from these experiments were, that the 

 same quantity of heat entered into the 

 water in the second as in the first, and 

 that as much escaped with the steam 

 which rushed out, as was carried off by 

 the vaporization of the^ water when the 

 vessel was open. 



Under the usual pressure' of the air, 

 water cannot be heated above the boil- 

 ing point ; but when exposed to greater 

 pressure, by being confined in the ves- 

 sel, it may be raised to a much higher 

 degree of heat, and if the excess of 

 heat should be insufficient to convert 

 the whole of the water into vapour, a 

 portion of it would rush out in steam on 

 opening the vessel, and the remaining 

 water would be at the boiling tempera- 

 ture. 



The low temperature at which water 

 boils in vacuo, was observed by Dr. 

 Black, and explained upon the same 

 theory. The evaporation of alcohol 

 and ether affords still more striking 

 illustrations. Dr. Cullen found that on 

 wetting the bulb of a thermometer with 

 either of these, and suspending it in the 

 air, its temperature was rapidly reduced. 

 The greater the tendency of the liquid to 

 assume the state of vapour, and the 

 more its evoporation is hastened by 



