RECENT ADVANCES IN SCIENCE 447 



Constitution. — From a consideration of the conditions of 

 dehydration, of the reactions of P 2 3 C1 4 , of ethyl pyrophosphate, 

 and of NaAg 3 P 2 7 , as well as the reaction between PC1 5 and 

 Na 4 Po0 7 , BalarefT (Zeit. anorg. Chem. 191 4, 88, 133) comes 

 to the conclusion that pyrophosphoric acid must have a con- 

 stitution represented by the following unsymmetrical formula : 



OH\ 



:: >P— OH 



OH' 



o o 



0=P— OH 



The first two hydrogen ions, moreover, have practically 

 identical ionisation constants, whilst the constants of the 

 third and fourth are widely different from these, but near 

 to one another. The unsymmetrical formula would account 

 for these differences. Further, Rosenheim and Triataphyllides 

 (Ber. 191 5, 48, 582), show that metallo-derivatives of this acid 

 can be prepared of the general formula R(R'P 2 7 ), where R 

 is a monovalent alkali metal (K, Na, Am) and R' a trivalent 

 metal (Mn, Cr, Mo), in which the grouping R'P 2 7 acts as an 

 acidic complex and does not give the ordinary reactions of 

 the metal R'. 



These complex metallo-derivatives are always poly- 

 hydrates. 



Generally speaking, noticeably more attention is being paid 

 to the structural arrangement of the water molecules in hy- 

 drates, and when a definite colour is associated with the 

 hydrated state, one of the best methods of investigating the 

 problem is to be found in measurements of absorption spectra. 

 In some cases marked differences in the ease of dehydration 

 of one or more molecules of the hydrate water is being taken 

 as a criterion of a different structural position. MgS0 4 . 7H0O, 

 for example, loses one molecule of water on long desiccation 

 over calcium chloride, and Johnsen (Centr. Min. 191 5, 289), 

 consequently, represents the constitution by the formula 

 [Mg(H 2 0) 6 ] . S0 4 ,H 2 with the possibility of the six molecules 

 being polymerised into (H 4 2 ) 3 or (H 6 3 ) 2 . The problem of 

 hydrates must, however, be taken in conjunction with general 

 complex radicle formation, in which the water molecules 



