162 OF THE PRESSURE OF FLUIDS OF VARIABLE DENSITY 



separated by the prism ; mercury, when squeezed through the pores of leather, or 

 raised in fume and received upon clean glass, which exhibits globules similar and 

 undistinguishable. In short, every mass of matter is divisible into particles, which 

 we designate by the Greek term atom, or that which is so exceedingly minute that 

 it cannot be further cut or divided, and which therefore, as far as sense is concerned, 

 is the ultimate resisting particle. It must be obvious, that the density or quantity 

 of atoms which exist in a given space is very different in different substances. 

 Hence, if it be asked why bodies are called dense ? the answer is, Because they 

 contain more atoms than others of the same size. There are more atoms in a cubic 

 inch of lead than in a cubic inch of cork : the former is forty times heavier than the 

 latter. A cubic foot of rain water weighs 62 Ibs. ; but an equal volume of mercury, 

 which is fourteen times heavier than water, weighs (62^x14) =875 Ibs. 



Density must depend on three circumstances, to which we should carefully attend 

 in all our disquisitions : first, the size or weight of the individual atom ; secondly, on 

 porosity, or the arrangement of the atoms by cohesion, or mechanical and physical 

 arrangement ; thirdly, the proximity of the atoms determined by the substance of 

 which they are constituent particles, possessing tenacity and incompressibility. 

 Thus, heat dilates some bodies and contracts others. A pound of tin and a pound of 

 copper melted together form bronze ; but this new mass occupies less space by one 

 fifteenth than the two masses did when separate j proving that the atoms of the one 

 are partially received into what were empty spaces of the other. In other words, 

 the affinity of cohesion is one fifteenth greater in the bronze than in the tin and 

 copper separately. Two pounds of brine are made out of a pound of salt and a 

 pound of water ; but the mass is of less bulk than the aggregate of the ingredients 

 apart. 



Water, we have seen, resists compression very powerfully, but at the depth 

 of 1000 fathoms yielding a very small part of its bulk at the surface, shows the 

 particles not to be in contact, and that the fluid may acquire density in propor- 

 tion to its depth. Wood swims in water, because the water has more atoms in the 

 same bulk than the wood, and therefore more weight or central force than the wood ; 

 consequently, the water falls first and leaves the wood behind; in other words, the 

 wood floats upon the water the wood is borne on the surface of the water with a force 

 exactly proportional to the difference between its weight and that of an equal bulk 

 of water. The pressures which the fluid exerts in supporting the wood are together 

 equivalent to a force directed upwards through the centre of gravity of the fluid 

 displaced, and equal to the weight of a quantity of the fluid so displaced by the 

 immersed part of the body. But it is not necessary here to dwell further on this 

 topic, the density of water. We therefore pass on to another character it possesses, 

 viz. gravity or weight : and it is, in fact, by comparing the weight of a body with 

 the force which holds it up in the fluid, that the comparative weights or specific 

 gravities are found, as of metals compared with water, and of admixtures of metals 

 for the purpose of ascertaining at once the proportion of each in the compound 

 mass. 



Water is the common standard with which all other substances are compared, 

 whose weight we would fix and record in tables of specific gravities. When 

 we say, therefore, that gold is of the specific gravity of 19, and copper of 9, and 

 cork of one seventh, we mean that these substances are just so much heavier or 

 lighter than their bulk of pure water in its densest form, viz. at the temperature of 

 40 degrees of Fahrenheit's thermometer. It appears, therefore, that the terms 



