PROFESSOR THOMAS GRAHAm's SCIENTIFIC WORK. 205 



"It is couceivable that the various kinds of matter, now recognized as 

 different elementary substances, may possess one and the same ultimate 

 or atomic molecule existing iu different conditions of movement. The 

 essential unity of matter is a hypothesis in harmony with the equal 

 action of gravity upon all bodies. We know the anxiety with which 

 this point was investigated by Xewton, and the care he took to ascer- 

 tain that every kind of substance, 'metals, stones, woods, grain, salts, 

 animal substances,' &c., are similarly accelerated in falling, and are there- 

 fore erpially heavy. 



" In the condition of gas, matter is deprived of numerous and varying 

 properties, with which it appears invested when in the form of a liquid 

 or solid. The gas exhibits oidy a few grand and simple features. These 

 again may all be dependent upon atomic or molecular mobility. Let us 

 imagine one kind of substance only to exist — ponderable matter; and 

 further, that matter is divisible into ultimate atoms, uniform in size and 

 weight. We shall then have one substance and a common atom. With 

 the atom at rest the uniformity of matter would be perfect. But the 

 atom possesses always more or less motion, due, it must be assumed, to 

 a primordial imjiulse. This motion gives rise to volume. The more 

 rapid the movement the greater the space occupied by the atom, some- 

 what as the orbit of a planet widens with the degree of projectile velo- 

 city. Matter is thus made to differ only in being lighter or denser 

 matter. The specific motion of an atom being inalienable, light matter 

 is no longer convertible into heavy matter. In short, matter of different 

 density forms different substances — different inconvertible elements, as 

 they have been considered. 



" But further, these more and less mobile, or light and heavy forms 

 of matter, have a singular relation connected with equality of volume. 

 Equal volumes of two of them can coalesce together, unite their move- 

 ment, and form a new atomic group, retaining the whole, the half, or 

 some simple proportion of the original movement and consequent 

 volume. This is chemical combination. It is directly an affair of 

 volume, and only indirectly connected with weight. Combining weights 

 are different, because the densities, atomic and molecular, are different. 

 The volume of coml)ination is uniform, but the fluids measured vary in 

 density. This fixed combining measure — the mctron of simple sub- 

 stances — weighs 1 for hydrogen, 10 for oxygen, and so on with tlie other 

 ' elements.' 



"To the preceding statements respecting atomic and molecular mo- 

 bility, it remains to be added that the hypothesis admits of another 

 expression. As in the theory of light we have the alternative hy])otli- 

 eses of emission and undulation, so in molecular mobility the motion 

 may be assumed to reside either in separate atoms and molecules, or in 

 a fluid medium caused to undulate. A special rate of vibration or ]nilsa- 

 tion originally imparted to a jtortion of the fluid medium enlivens that 

 portion of matter with an individual existence, and constitutes it a dis- 

 tinct substance or element. 



