: often form mountain chainsand wliole t hickn----e- of 

 -T rat a. t he-e consi.-t in n" of calcium earhonate, ( 'a< '* > . 



I is \\ . have -een o\v;_ f eii in a free -tate and in various compounds 

 of dill'crent decree- of stahilitv, from, the un-tahle salts, like I'.ert hollet '- 

 -dt and nitre, to the most -laMe silicon compound-, -udi as exist in 

 granite. \\"e -,aw an entirely -imilar gradation of -lability in the com 



Is of \\ater and of h vdro^en. In all it s aspect s oxygon, as an 

 element, a- a -uK-tance. remain- the same in it-elf in the nio-t varied 

 cliemical -tate-. just a- a -uhstance mav appear in dill'erent physical 

 1 a_;'u i'ei;at e ) -tales, lut our notion of the immense varietv of the 

 chemical >tates in which oxygen can occtii 1 would nor lie completely 



underst 1 if we did not make ourselves acquainted with it in the 



Toi-m in \\hich it occur- in ozone and peroxide ot hydrri^en. In the-e 

 it i- nio-t active, its eiier^v seem- to ha\c inci'ea-ed. 'Then the fre-h 

 a-jiect- of chemical correlation-, ,-md the \ariet\- of the form- in which 

 in uter can appear, -land out (dearly. \Ve will therefore consider these 

 ' wo -uhstances some\\-]iat in detail. 



