96 REPLIES TO CEITICISMS. 



masses; which may bo taken indifferently to be suns or 

 atoms. The relations between contraction and tho escape 

 of molecular motion, and between expansion and the ab 

 sorption of molecular motion, are expressed in their general 

 forms without reference to the kind of matter ; and, it 1 

 the degree of either that occurs in a particular kind of 

 matter is formulated, no note is taken of the quantity of 

 that matter, much less of its individuality. Similarly with 

 Chemistry. When it inquires into the atomic weight, the 

 molecular structure, the atomicity, the combining propor 

 tions, etc., of a substance, it is indifferent whether a grain 

 or a ton be thought of the conception of amount is abso 

 lutely irrelevant. And so with more special attributes. 

 Sulphur, considered chemically, is not sulphur under its 

 crystalline form, or under its allotropic viscid form, or as 

 a liquid, or as a gas ; but it is sulphur considered apart 

 from those attributes of quantity, and shape, and state, that 

 give individuality. 



Prof. Bain objects to the division I have drawn between the 

 Concrete Science of Astronomy and that Abstract- Concrete 

 Science which deals with the mutually-modified motions of 

 hypothetical masses in space, as &quot;not a little arbitrary.&quot; 

 lie says : 



&quot; &quot;We can suppose a science to confine itself solely to the factors, 

 or the separated elements, and never, on any occasion, to combine two 

 into a composite third. This position is intelligible, and possibly 

 defensible. For example, in Astronomy, the Law of Persistence of 

 Motion, in a straight line might be discussed in pure ideal separation ; 

 and so, the Law of Gravity might be discussed in equally pure sepa 

 ration both under the Abstract- Concrete department of Mechanics. 

 It might then be reserved to a concrete department to unite these in 

 the explanation of a projectile or of a planet. Such, however, is 

 not Mr. Spencer s boundary line. He allows Theoretical Mechanics 

 to make this particular combination, and to arrive at the laws of 

 planetary movement, in the case of a single planet. What he does 

 not allow is, to proceed to the case of two planets, mutually dis 

 turbing one another, or a planet and a satellite, commonly called 

 the problem of the Three Bodies. &quot; 



