20 CHEMISTRY. 



putting ether on the bulb of a thermometer, and noting the depres- 

 sion on the scale. By means of the cold produced by evaporation 

 water may be frozen, by placing it over sulphuric acid in a receiver, 

 and quickly exhausting the air. The same thing is also shown by 

 Wollaston's Cryophorus or frost-carrier, which consists of a glass 

 tube of the figure represented in the cut. The bulb contains water, 



Fig. 5. 



the rest of the space being filled with its vapour. The empty extre- 

 mity being plunged into a mixture of snow and salt, the solidifica- 

 tion of the vapour gives rise to such a quick evaporation from the 

 surface of the water, that the latter freezes. 



Specific heat^ or capacity for heat. — By this is meant the ratio of 

 the heat a body may contain, with the bulk or weight of the body. 

 A simple experiment will prove that different bodies, though exhi- 

 biting the same apparent temperatures, contain, in reality, very 

 different amounts of heat. Thus, in mixing a pound of mercury at 

 162° with a pound of water at 100°, the temperature of the mixture 

 will be 102° ; here, the mercury, by losing 60°, raises the water 2°. 

 But if the water be at 162°, and the mercury at 100°, the tempera- 

 ture of the mixture will be 160° ; in this case, the water, by losing 

 2°, raises the mercury 60°. From this it appears that the same 

 heat which would raise water 2°, will raise an equal weight of mer- 

 cury 60°, being in the ratio of 1 to 30. By a similar experiment 

 in oil and water, it is found that the ratio between them is that of 

 2 to 1. 



There are three different modes of ascertaining the specific heat 

 of various substances. The first is by observing the quantity of ice 

 melted by a given weight of the substance heated to a particular 

 temperature ; the second, by noting the time which the heated body 

 requires to cool down through a certain number of degrees ; the 

 third is the method of mixture, just pointed out, and is the one 

 usually preferred. 



Means of producing heat. — It will suffice here merely to mention 

 them ; viz., lenses and mirrors, percussion, friction, condensation, 

 combination, electricity, galvanism, fermentation, and vitality. 



SECTION II. 

 LIGHT. 

 There are two theories of Light : one — the Newtonian — is that 



