278 
OUR KNOWLEDGE OF 
which are as 3 to 2. From its high density and from the fact 
that when heated it returns to the condition of oxygen again, 
there is strong reason for supposing that in the formation of 
ozone heat is evolved/ and that the same amount of heat is taken 
up again on its reconversion to oxygen. 
Similarly phosphorus, when heated for a long while to 240° C., 
or if heated to about 200° C. with the addition of a little iodine, 
changes from a soft, waxy, poisonous solid, which may be ignited 
by the heat of the hand or by slight friction so easily that it must 
be kept under water, which melts at 44° C., and undergoes con- 
stant combustion in the air and so is beautifully luminous in the 
dark ; to a purple-red non-phosphorescent powder harder than 
limestone, which is not poisonous and is of higher specific gravity 
than common phosphorus, not easy to fuse, but which changes 
back to common phosphorus at about 270° C. and takes fire at a 
rather higher temperature ; and which is so little apt to burn 
that it may be kept in a bottle with a common cork for years, 
and may be handled wdth impunity ; changes, in fact, to a 
distinctly different body ; an altogether more stable body. This 
stability and its higher density strongly indicate that in its 
formation, as in the formation of diamond, there has been an 
evolution of heat. 
Now these various facts seem to show that the transforma- 
tion of the common forms of elementary bodies into their 
allotropic forms are analogous to the formation of chemical 
compounds. In each case we see a change of property, often a 
^ In which case ozone is theoretically the most stable form of 
oxygen, being, however, at ordinary temperature, under conditions 
relatively similar to those which exist for water at a white heat.. 
In a portion of space where the temperature was considerably 
lower than on our earth, ozone would, perhaps, be the usual form 
in which oxygen would exist. 
