D 



Dr. Woods on the Heat of Chemical Combination. 303 



The following explanation of expansion taking place in an in- 

 creasing ratio does not require the supposition of a force gra- 

 dually decreasing between the atoms. Instead of an action 

 being exerted from particle to particle, let the operating cause 

 of expansion and contraction be from one body to the other, and 



let the bodies be represented by the j^ g 



lines AJ3 and CD. Now if a point 

 P be taken in the centre of the figure, 

 and that in the opposite movements • P 



of expansion and contraction the 

 operating cause be represented by a 

 line passing from one body to the 

 other, and that this line be always at right angles to a line drawn 

 from P, which point P, like the centre of gravity in a system of 

 bodies moving equally in opposite directions does not change its 

 place, the amount of expansion and contraction will be shown 

 to be in an increasing ratio. Let the figure be completed. Join 

 B and D. Then BD represents the 

 operating cause which preserves the 

 bodies relatively equal. It is at 

 right angles to PT. Now let AB 

 expand and CD contract. If the. 

 operating cause pass still from one to 

 the other at right angles to an equal 

 line drawn from P, suppose PT', then 

 BF the increase is larger than GD the decrease ; and by drawing 

 in the same way other lines of force from one body to another, 

 if tiny always pass at right angles to a line drawn from P, it can 

 be shown that the amount of expansion will be a constantly in- 

 creasing quantity. 



It is evident that if a number of lines are thus drawn, the 

 locus of their intersections (PT with FG, &c.) will be the circum- 

 ference of a circle. Hence, as in the action of masses on each 

 other — attraction — the operating cause has reference to a centre ; 

 in the case of expansion and contraction, or the phenomena of 

 heat, it may be said to have reference to the circumference. 



If C and D represent two dissimilar particles which can che- 

 mically unite, brought together at an insensible distance, so as to 

 form one body, and that combination takes place between them 

 so as to bring them to the points and 0', by drawing lines 

 from these points at right angles to a line equal to PT, we. should 

 sic how greatly A..B, the body which compensates by its expan- 

 sion lor tin- coming together or contraction of CD, 'must be ex- 

 panded or heated; and this is the theory I advance to account 

 lor the cause of the heat of chemical combination. 



