LESSONS IN CIIKMISTUY. 



it again obeys the law ; 4 < f man- 



.'uro a given 



nuil lent volume. If this exception 



c\v did ii"! < evident that ice would be heavier 



than \\utt'!-, itiul therefore would sink ; the ioe of winter would 

 thru r> -t in the beds of the rivers and lakes, and tho summer 

 sun would not be able to melt it. Thus in a few years our 

 island would be as uninhabitable as an Arctic region ! 



Throughout tho wide domain of science there is no fact 

 which inon- distinctly indicates that an all-wise God overrules 

 and directs tho laws which are the offspring of his own groat 

 mind. If tin- deviation carried with it all other consequences of 

 an altered density, we might have thought it was only tho 

 ! result of some undiscovered law ; but here it stands 

 isolated, and the effects which in other cases would follow as 

 natural consequences are arrested. 



For instance, when light passes through a transparent medium, 

 it is diverted from its path, or " refracted." If the density of 

 this body be increased, its power of refraction increases. We 

 should expect, therefore, that as water increases in its density 

 from to 4, its refractive power would likewise increase, but 

 Arago and Fresnel have proved that such is not the case. 



When water freezes a sudden expansion takes place ; 1 volume 

 of water becomes 1-099 of ice. This is the reason why water- 

 pipes burst during a frost. If the pipes will not stretch they 

 crack, and when the thaw melts the ice the damage appears. 

 Thia is tho great geological agent in disintegrating rocks. 



When water is heated the thermometer steadily rises until it 

 reaches 100 Cent., when the liquid enters into ebullition. The 

 heat still passes into the vessel, but it does not affect the ther- 

 mometer ; as in the cose of the melting ice, it becomes " latent " 

 in turning the water into steam. The latent heat of steam is 

 said to be 536 units of heat; that is, in condensing a kilo- 

 gramme of steam at 100 into water at 100, as much heat is 

 given out as will raise 536 kilogrammes of water 1. 



"ration is the turning of water into steam at the surface 

 only, and it is carried on at all temperatures. Even ice and 

 enow evaporate. 



Ebullition, on the other hand, is the turning of water into 

 steam throughout the mass of the liquid. 



The surface of the water sustains the weight of the atmosphere 

 760 mm. : if, therefore, a particle of water be converted into 

 steam within the mass of the liquid, it must have a force suffi- 

 cient to lift up the superincumbent pressure of tho atmosphere ; 

 therefore ebullition is said to take place when the " tension " of 

 the steam is 760 mm., or when the steam has a force capable of 

 lifting 14 - 67 pounds on every square inch of the surface of the 

 vessel in which it is confined. 



At all temperatures steam has some " tension." 



At its tension is 4'GOO mm. 



50 ,. ,. 91-982 mm. 



100 780-000 mm. 



150 3581-23 mm. 



200 11688-96 mm. 



If at any temperature the tension of the steam equal the 

 pressure to which the water is subjected,ebullition will commence, 



Make some water in a Florence flask boil ; while the steam is 

 escaping cork it tightly. Now the flask contains no air, only 

 water and its vapour. Invert tho flask, so that the hot water 

 fills the neck ; pour cold water on the flask. Tho steam inside 

 is condensed, and therefore the pressure on the water greatly 

 diminished, whereupon it will again boil. It will be found that 

 when the flask is suddenly jerked the water strikes the glass 

 like a solid body, because there is no air to cushion the blow. 

 This is the " philosopher's hammer." 



Whenever a liquid becomes a vapour it can only do so by 

 absorbing a large amount of heat. By exhausting the air from 

 a vessel containing water or ether, the liquid is made to evapo- 

 rate very rapidly. To effect its evaporation it abstracts heat 

 from all the bodies in its neighbourhood. By this means, as we 

 shall afterwards find, we can produce great cold. 



Water may be made to freeze by its own evaporation. A 

 shallow dish of water, A (Fig. 30), is supported on a wire 

 triangle over a vessel containing a little sulphuric acid, and 

 both placed under the receiver of an air-pump. Upon exhaust- 

 ing the air, the water evaporates, and the acid absorbs the 

 rapour as soon aa it ia formed, and thus the rate of evaporation 



Fig. 30. 



in increased. The water, rorrendering all its own heat to 

 produce tho vapour, begins to freeze. 



A limited space can only contain a certain quantity of vapour. 

 When it cannot hold any more it is naid to be w 

 \\ '.. thcr there be air or not in the space doe* not affect the 

 / of vapour ; bat if the space be a vacuum at Tom- 

 oelli's vacuum it is immediately filled ; whereas if there be air 

 in it, tho evaporation goes on 

 slowly; and just as hot water 

 can contain more calt than cold 

 water, so hot air can contain 

 more steam than cold. Hence 

 when the cold east wind meets 

 a warmer current, the moisture 

 which this latter contains is 

 condensed, and we have rain. 

 The less moisture the air con- 

 tains that is, the " dryer " the 

 air the more rapid will be the 

 evaporation. Around the body 

 there is an atmosphere contain- 

 ing much moisture, which is con- 

 stantly evaporating from the 

 skin. If we blow upon any 

 part of the body we remove this 



atmosphere loaded with moisture, and replace it by a much 

 dryer one ; therefore we increase the rate of evaporation, and 

 consequently more heat is demanded from that part, and we feel 

 "cold." 



Bodies have different " capacities for heat," or their " specific 

 heats " vary. If, for example, we place at the same distance 

 from the same fire, for the same time, a gloss of mercury and a 

 glass of water, we shall find that although each of the liquids 

 has received the same quantity of heat, yet the mercury baa a 

 very much higher temperature than the water. The water, 

 having absorbed the heat, gives very little out ; whereas the 

 mercury, being capable of containing only ^ of the heat taken 

 in by the water, gives off a much larger quantity, and therefore 

 has a much higher temperature. 



Water has the highest specific heat of any solid or liquid, 

 hence the " specific heat of water " is taken as the standard, 1. 



This fact is of great service in the economy of Nature. The 

 seas are great reservoirs of heat, and during the hot times they 

 cool the countries whose shores they wash, and during the 

 nights and the winters they give out their heat, and thus equalise 

 the temperature. 



In perusing the following examples the subject of "latent 

 heat" will be rendered clearer. 



If I place 5 kilogrammes of ice at in 60 kilogrammes oj 

 water at 100 Cent., what wiU be the resulting temperature t 



Let x = the required temperature ; the ice in melting into 

 water at would require 



79 x 5 kilogs. = 395 units of heat. 



This water would now be raised from to 2, and would require 

 for this 



5 kilogs. x x = 5z units of heat. 



That is, to turn ice at into water at , 395 + c units of 

 heat are required. This heat is supplied from the cooling of 

 the 60 kilogrammes of water from 100 to x, that is, through 

 100 - x. 

 This heat would be 



60 kilogs. x (100 -*)=6000 60r units of heat 



395 + 5* = 6000 60* 

 65* = 5605 



Therefore 



that is, the temperature of the water after the ice is melted and 

 the mixture has become of uniform temperature, is 86*2. 



How much steam at 100 must be passed into a tank contain- 

 ing 400 kilogrammes of water at 15 Cent, in order to make the 

 water boil ? 



Let x = the kilogrammes of steam required. In condensing 

 this steam will give off 



536 x x units of heat. 



