Chemical ^Affinities. 73 



orbital distances, and the centre of each semi-orbit is the 

 point from which the planet may be said to start, as the centre 

 of the, perihelion, whence it runs down at an ever-increasing 

 ratio of acceleration till it reaches the centre of the aphe- 

 lion, from which point it moves within an ever-decreasing 

 ratio of acceleration, till it reaches the same point in the 

 perihelion from whence it started (and during the whole of 

 the time the axoidal motion is not identical to the second 

 in its duration, but is always suffering some small increase 

 or decrease of time). The cause of this motion in an 

 inclined plane is not very apparent. (Airy's " Lectures," pp. 

 83, 84.) But, probably owing to some peculiar internal 

 construction of each individual planet as yet not determined, 

 so in like manner the cycle of the year, unless artificially 

 interfered with, is the usual time in which the circuit of 

 morphological changes in plants undergoes every order of 

 form, from the point of comparative rest and quiescence to 

 the same condition again, saving in that order of growth in 

 the vegetable kingdom known as cryptogams, whose order 

 of growth and^decay observe a fewer order of changes. 



The more simple forms observe a much shorter period 

 of vitality, but these appear, in many instances, to obey the 

 influence of the moon rather than the sun, and are more of 

 nocturnal and lunar periodic order than those of a higher 

 and more complex organization. How far the lower forms of 

 animal life, as madrepores, millepores, etc., or some of the 

 porifera, etc., etc., are influenced by the moon does not 

 appear very evident, but in the vegetable world lunar 

 influence has been long observed, especially by water 

 engineers, in the amount of vegetable growth in reservoirs as 

 the moon gets towards the full. Though these phenomena 

 are explicable upon principles of simple light and heat, 

 yet they retain so close an analogy in the history of periodicity 

 in some diseases, in relation to diurnal motion, as is 



