A Sketch of the Laws of Chemical Combination. 317 



jilum affords an interesting example of these properties. The alumina may 

 be replaced by the sesquioxides of iron^ chromium, or manganese j ammonia 

 may be substituted for the potash; and selenic, chromic, or manganic acids 

 for the sulphuric ; so that the composition of this salt may be partly or 

 entirely changed, without its external form being affected. Isomorphous 

 substances have often very close points of resemblance independent of 

 form : thus arsenic and phosphorus have the same odour, they both form 

 gaseous compounds with hydrogen, they difier from nearly all other bodies 

 in their mode of combining with oxygen, but agree with one another, and 

 their salts are disposed to combine with the same quantity of water of 

 crystallization. A similar analogy exists between sulphur and selenium, 

 baryta and strontia, lime and magnesia, cobalt and nickel. 



Another interesting discovery of recent date is, that the same elements 

 uniting in the same ratio, may give rise to compounds differing in external 

 form and in chemical properties : to such the term isomeric has been ap- 

 plied. , Interesting examples of this fact are afforded by the two cyanic 

 acids, one of which has been termed fulminic , from its giving rise to power- 

 fully detonating compounds ; also by the tartaric and paratartaric acids 5 

 and it has been shown that the animal product urea, and the cyanide of 

 ammonia, produced by artificial means, are, in like manner, isomeric. 

 "Unexpected as was the discovery of isomerism," says Dr. Turner (p. 238), 

 " it is quite consistent with our theories of chemical union, insomuch as 

 the same elements may be grouped or combined in different ways, and 

 thereby give rise to compounds essentially distinct." 



The compounds oi water have, until lately, been very little investigated ; 

 for it is only since the establishment of the laws of definite proportion, 

 that chemists have been led to perceive that it has a share in the atomic 

 constitution of various substances, as important as that of any other binary 

 combination. In some instances, it appears to unite with other bodies in 

 a variable ratio, as in solutions of salts, or combinations with acids, alco- 

 hol, &c. ; but, as Dr. Turner observes, " in all such compounds, the ele- 

 ments are united by a feeble affinity, and preserve, when combined, more 

 or less of the properties which they possess in a separate state." In other 

 cases, water unites with various substances in certain fixed proportions ; 

 and this class of compounds may be divided into two; — in the first, water 

 is essential to the constitution of the body, acting as a direct chemical 

 agent ; whilst in the second, it appears merely requisite to enable certain 

 compounds to assume their regular crystalline forms, and is therefore 

 termed water of crystallization. 



Respecting the indefinite combinations of water, there is little more 

 known than what has been already stated, with regard to the class to 

 which it belongs ; but, with regard to its definite combinations, several re- 

 markable facts have been recently ascertained, which deserve notice. 



Water bears a very striking analogy to some of the metallic oxides, 



No. 5.— Vol. I. 2 T 



