CALORIC. 19 



for alcohol, 172°; for ether, 96°; for sulphuric acid, 620°; for 

 mercury, 662°. The boiling point is chiefly influenced by the degree 

 of pressure to which the liquid is subjected. On the earth's surface 

 the atmosphere exerts a pressure upon every object of 15 pounds 

 to every square inch ; this force must be overcome by the elasticity 

 of the vapour of a liquid before it can boil ; hence, as the atmo- 

 spheric pressure varies, the boiling point must equally vary ; hence 

 water will boil at a much lower point on a mountain, or in a par- 

 tially exhausted receiver, than at 212°. So constant is the ratio 

 between the depression of the boiling point and the diminution of the 

 atmospheric pressure, that it forms a good method for ascertaining 

 the height of mountains, — a depression of one degree being equi- 

 valent to an elevation of about 548 feet. Liquids boil, in vacuo, at 

 a temperature 140° lower than in the open air. The influence of 

 diminished pressure in depressing the boiling point of a liquid, is 

 shown in the experiment denominated the culinary 'paradox. A 

 small flask of water is made to boil for a few minutes until the steam 

 freely issues from the orifice; when it is firmly corked. On re- 

 moving it from the heat, the ebullition of course ceases ; but it may 

 be made to commence again by simply applying cold water to the 

 upper part of the flask ; the cold condensing the vapour, and thereby 

 diminishing the pressure. 



On the other hand, the boiling point may be much elevated by 

 increasing the pressure. In this way, water may be prevented from 

 boiling by the pressure of its own vapour — being confined in a strong 

 metallic vessel. There is no limit to the degree to which water may 

 be thus heated, provided the vessel be strong enough to bear the 

 enormous pressure of the vapour thus generated. It is on this prin- 

 ciple that the high pressure steam engine acts. 



As already mentioned, a large amount of heat is rendered latent, 

 by the production of steam. 



A cubic inch of water, in becoming steam, under the ordinary 

 pressure of the atmosphere, expands nearly into a cubic foot. 



Evaporation differs from ebullition only in being a slower pro- 

 cess, and not attended with the appearance of boiling ; it occurs at 

 common temperatures ; — takes place in all fluids, and some solids, as 

 camphor ; — is much more rapid in such as have a low boiling point, 

 as ether and alcohol. Evaporation is influenced by 1, extent of 

 surface; 2, temperature: — heat is favourable to it; 3, hygrometric 

 state of the atmosphere : — dryness favours it ; 4, by a current of air ; 

 5, by amount of pressure. The eflect of removing the pressure is well 

 seen by putting ether under a receiver, and removing the air ; the 

 evaporation is so rapid as to produce ebullition. 



Cold is always produced by evaporation, in consequence of the 

 amount of heat which is rendered latent. This is shown by placing 

 a few drops of ether on the hand and exposing it to the air ; or 



