44, TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
be a flint arrow-head dug up from a neolithic tumulus and now 
preserved in the Museum at Castle Rushen, or it may be in 
the form of the delicately-sculptured shells of microscopic 
diatoms swarming in the seas round our Island at certain 
times of the year. Silica, quartz, or flint is a most insoluble 
substance, but all the innumerable millions of diatoms which 
make their appearance in the sea every spring, must obtain 
the necessary material for their shells from dissolved silica in 
the sea-water, and the sea must obtain its supply from the 
land. Here again, we see the circulation or periodic change 
that I am demonstrating. All these important materials are 
kept in circulation. Gold is not the only element upon the 
circulation of which the prosperity of man depends. We could 
get on very well without the gold, but we could not exist 
without the constant circulation of the carbon, and calcium, 
and nitrogen of which I have been speakine—or, to mention 
another case, without the iron in our blood, and again in living 
plants, or even without the silica upon which it is scarcely 
too much to say that all life in the sea depends. 
I have no doubt that these and other examples of periodic 
change are known in more or less detail to many. Of course, 
the general notion that the material making up the bodies of 
living organisms may at another time be lifeless, inorganic 
stuff, is centuries old. In Elizabethan times the genius who 
wrote Shakespeare’s plays makes Hamlet say :— 
‘ Tmperial Cesar, dead, and turned to clay, 
Might stop a hole to keep the wind away.” 
And Lucretius, if I am not mistaken, sixteen centuries before 
that expressed his belief in a general circulation of matter. 
But there is the greatest difference between a somewhat 
vague belief and the scientific demonstration that definite 
chemical elements, such as I have been speaking of, do circulate 
periodically through earth, and air, and water around us. 
Let me take as my last example those periodic changes in 
