MARINE BIOLOGICAL STATION AT PORT ERIN. 43 
times free in the air and water in the form of the gas carbon 
dioxide, sometimes in the bodies of living things, all of which 
are largely formed of carbon, and sometimes stored up in 
solid form as coal and other materials from which it can again 
he set free and pass into circulation. 
The same phenomenon of periodic change is seen in the 
case of the calcium in beds of chalk or limestone. When dis- 
solved from that rock it is set free, and may next appear in 
solution in fresh or salt water, and then in the form of a tropical 
coral reef, the shell of an oyster, or the bone of an animal’s 
skeleton. These corals and shells may again be consolidated 
on the sea-bottom to form beds of chalk and limestone. Like 
carbon, the calcium is a necessary part of our food—and we 
take it in, for example, sometimes in too large quantities, in 
our milk—and then those of us who are no longer young begin 
to store it up in solid limy form in parts of our bodies that 
would be better without it. 
I need not give further examples in detail. I would only 
add that the same principle of periodic change or circulation 
of necessary substances in Nature is seen in many other cases. 
Nitrogen, a necessary part of all living things, is also found 
in the nitrates and other fertilisers of our soil, in the sewage 
poured in ever increasing quantity from our great cities into 
the sea, and again in the enormous harvest brought back 
annually from the sea to the land. When herrings and other 
fish are brought ashore for the markets, when the large brown 
sea-weeds cast ashore by storms are carted up to spread as 
manure on the fields, we are reclaiming from the sea some of 
the valuable nitrogen the land gave up in the form of sewage 
and other matters carried down by streams and storms. 
One more case, put briefly :—The silica of our island 
may be in the form of a white quartz vein running through the 
metamorphic Manx slates of Foxdale or Bradda Head, or 
united with alumina in the clays and shales, or may 
