In chemical causation, on the other hand, cause and effect (at least, as they present themselves to us) differ in almost every way:  in the act of combination the properties of elements (except weight) disappear, and are superseded by others in the compound.  If, for example, mercury (a heavy, silvery liquid) be heated in contact with oxygen (a colourless gas), oxide of mercury is formed (red precipitate, which is a powder).  This compound presents very different phenomena from those of its elements; and hence Mill called this class of cases “the heteropathic intermixture of effects.”  Still, in chemical action, the effect is not (in Nature) heterogeneous with the cause:  for the weight of a compound is equal to the sum of the weights of the elements that are merged in it; and an equivalence has been ascertained between the energy of chemical combination and the heat, light, etc., produced in the act of combination.

The heteropathic intermixture of effects is also found in organic processes (which, indeed, are partly chemical):  as when a man eats bread and milk, and by digestion and assimilation converts them into nerve, muscle and bone.  Such phenomena may make us wonder that people should ever have believed that ‘effects resemble their causes,’ or that ’like produces like.’  A dim recognition of the equivalence of cause and effect in respect of matter and motion may have aided the belief; and the resemblance of offspring to parents may have helped:  but it is probably a residuum of magical rites; in which to whistle may be regarded as a means of raising the wind, because the wind whistles; and rain-wizards may make a victim shed tears that the clouds also may weep.

Sec. 7.  Another consideration arises out of the complex character of causes and effects.  When a cause consists of two or more conditions or forces, we may consider what effect any one of them would have if it operated alone, that is to say, its Tendency.  This is best illustrated by the Parallelogram of Forces:  if two forces acting upon a point, but not in the same direction, be represented by straight lines drawn in the direction of the forces, and in length proportional to their magnitudes, these lines, meeting in an angle, represent severally the tendencies of the forces; whilst if the parallelogram be completed on these lines, the diagonal drawn from the point in which they meet represents their Resultant or effect.

Again, considering the tendency of any force if it operated alone, we may say that, when combined with another force (not in the same direction) in any resultant, its tendency is counteracted:  either partially, when the direction of the resultant is different; or wholly when, the other force being equal and opposite, the resultant is equilibrium.  If the two forces be in the same direction, they are merely added together.  Counteraction is only one mode of combination; in no case is any force destroyed.