20 Mr. T. S. Hunt on the Theory 



science." All chemical changes are reducible to union (iden- 

 tification), and division (differentiation). When in these changes 

 only one species is concerned, we designate the process as meta- 

 morphosis, which is either by condensation or by expansion (homo- 

 geneous differentiation). In metagenesis, on the contrary, un- 

 like species may unite, and by a subsequent heterogeneous dif- 

 ferentiation give rise to new species, constituting what is called 

 double decomposition, the results of which, differently inter- 

 preted, have given origin to the hypothesis of radicals and the 

 notion of substitution by residues, to express the relations be- 

 tween the parent bodies and their progeny. The chemical his- 

 tory of bodies is then a record of their changes ; it is, in fact, 

 their genealogy ; and in making use of typical formulae to indi- 

 cate the derivation of chemical species, we should endeavour 

 to show the ordinary modes of their generation*. 



Keeping this principle in mind, let us now examine the theory 

 of the formation of acids. As we have just seen, I taught in 

 1848 that the monobasic, bibasic, and tribasic acids are derived 

 respectively from one, two, and three molecules of water, H 2 O 2 . 

 M. Wurtz, seven years later (in 1855), put forth a similar view. 

 He supposes a monatomic radical PO 4 ', a diatomic radical PO 3 ", 

 and a triatomic radical PO 2 "', replacing respectively one, two, 

 and three atoms of hydrogen in H 2 O 2 , H 4 O 4 , and H 6 O 6 , thus 

 (PO 4 ' H) O 2 , (PO 3 " H 2 ) O 4 , and (PO 2 "' H 3 ) O 6 . These radicals 

 evidently correspond to PO 5 which has lost one, two, and three 

 atoms of oxygen in reacting upon the hydrogen of the water 

 type ; and these acids may be accordingly represented as formed 

 by the substitution of the residue PO 5 — for H, &c. 



To this manner of representing the generation of polybasic 

 acids we object that it encumbers the science with numerous 

 hypothetical radicals, and that it moreover fails to show the 

 actual successive generation of the series of acids in question. 



When phosphoric anhydride, P 2 O l0 =(PO 4 ) 2 O 2 , is placed in 

 contact with water, it combines with one equivalent, H 2 O 2 . The 

 union is followed by homogeneous differentiation, and two equi- 

 valents of metaphosphoric acid result ; 



(PO 4 ) 2 2 + H 2 2 =2(P0 4 H)0 2 . 

 Two equivalents of this acid with one of water at ordinary tem- 

 peratures are slowly transformed into two of pyrophosphoric 

 acid by a reaction precisely similar to the last, 



2(PH0 6 )[=(PH0 5 ) 2 2 ]+H 2 2 =2(PH0 5 H)0 2 ; 



* See " On the Theory of Chemical Changes," Amer. Journ. of Science, 

 vol. xv. p. 226 ; Lond. Edinb. and Dub. Phil. Mag. [4] vol. v. p. 526 ; and 

 Chera. Centralblatt, 1853, p. 849. Also, "Thoughts on Solution," Amer. 

 Joum. of Science, vol. xix. p. 100 ; and 'Chemical Gazette,' 1855, p. 92. 



