10 INTRODUCTION. 
exist. It is the cause of all fluidity : to it every pro- 
duction of the earth has been most essentially indebted, 
even for its form and structure; and in no respect do 
the power and goodness of the Almighty appear more 
conspicuous than in the creation, dispersion, and con- 
tinuance, of this most subtile and astonishing fluid. 
19. All the various substances with which we are 
acquainted must be considered either as solid or fluid. 
Every substance is defined to be a solid in which the 
parts are so united or connected that it requires an 
external force to separate them. A fluid, on the con- 
trary, is a body the parts of which are so loosely con- 
nected that they not only yield easily to any force im- 
pressed upon them, but also move freely amongst each 
other; and every fluid is a combination of caloric with 
some other substance. 
20. Fluids are of two kinds : one of these, called 
liquids, have, when at rest, a smooth and distinct sur- 
face, and are distinguishable both by the sight and 
touch ; the other, denominated gas, or gaseous fluids, 
have the appearance of air, and are not perceptible 
either to the sight or touch, except under certain cir- 
cumstances. The latter are principally oxygen (21), 
azote or nitrogen, and hydrogen (4-5). We shall at 
present have occasion to speak only of the first. 
GASEOUS FLUIDS. 
21. Oxygen, like caloric, is a fluid never found in 
an uncombined state. It forms one of the component 
parts of the air that we breathe, and of the water we 
drink ; but it approaches nearest to a state of purity in 
combination with caloric ^18), when it has the name of 
oxygen gas. It was formerly called vital air, because 
