Dr. Woods on Chemical Combination. 371 



than those belonging to the ordinary changes in simple matter, 

 and extend the laws which regulate the latter to combining and 

 decomposing substances. 



If two similar bodies unequally heated be placed together, one 

 expands and the other contracts until an equal volume is attained 

 by each. 



If two bodies of equal volume be placed together, and then 

 pressure be applied to one, the other becomes heated or expanded 

 in proportion to the pressure or diminution of volume of the 

 former. 



If any substance expand, it deprives some other of heat or 

 volume, as in the solution of salts, &c. 



In the foregoing molecular changes of matter, it is evident, 

 that, to whatever cause they may be attributed, there are equal 

 movements simultaneously occurring in opposite directions (No. 1). 

 Every substance is compressible, and has a certain specific gra- 

 vity, proving that it is composed of particles, whether these be 

 what are called atoms or otherwise ; and that, because it has a 

 specific gravity, these particles must be separated by a definite 

 space or distance, which is always the same for the same "body 

 at like temperatm-e and pressm'e ; and at any distance from the 

 zero of temperature, or absolute contact of particles, these par- 

 ticles must be at difi'erent distances in difi'erent bodies, for bodies 

 expand differently when heated. It follows, then, that tlic space 

 and matter which compose every substance must be related ; or 

 in other words, that the distance between the particles of a body 

 must be some function of, or have some dependence on, the nature 

 of the matter composing it. (No. 2.) 



To these propositions may be added a third, that particles form 

 one body when they are at insensible distances. (No. 3.) 



Now before chemical action can take place, the bodies about 

 to act must be brought together at insensible distances, that is, 

 form one body ; but if two substances of different kind form 

 one body, and the distance between particles be some function 

 of the nature of the matter (No. 2), it follows that the distance 

 between the particles of the mixture of the two bodies cannot be 

 the same as that of either. Hence the distances must change; 

 and as (No. 1) every molecular change must be accompanied by 

 an equal and opposite one, when the distances diminish, the 

 distances between the particles of some other body increase or 

 heat is jjroduccd. Chemical action may therefore be said to 

 consist in an alteration of the distances between the particles of 

 matter consequent on the change that is produced in the nature 

 of the matter itself by the substitution of a mixture of two bodies 

 for one; and as, if two substances be brought together from 

 sensible to insensible distances, any alteration iu the distance 

 2 B2 



