Chemistry and Physics. 61 



acids of the third group produce in the corresponding reaction an 

 evohition of heat. 



(7.) The tribasic acids exhibit differences similar to those of the 

 bibasic acids, citric acid belonging to the second group, arsenic 

 and orthophosphoric acid to the third. Thus we have 

 Sodic hydrate. Citric acid. Arsenic acid. Ortho-pli..sphoric acid. 



1. Molecule, 124 . . 150 . . 148 . . 



2. Molecule, 126 . . 126 . . 123 . . 



3. Molecule, 132 . . 83 . . 69 . . 

 ITciire the second or third molecule of sodic hydnite evolves wifli 

 citric acid mor^, with arsenic and ortho-phosphorio aciil Ic^s In ;il 

 thai, the first molecule. The difference hi tin- evohition ol' licat 

 in the second and third molecule of .sodic hydrate is !.c'-i(ks vcrv 



and phosphoric 

 _ ^ , •■ groups which I have esta1ilisli<<l in t}i<' cla.ss of 



bibasic acids, and of which two are found als<i in the class of tri- 

 basic acids, have their probable origin in the different ((institution 

 of the acids. In fact in the case of a bibasic aci<l the [.o^itiun of 

 the two atoms of hydrogen replaceable by sodium ni.iy be con- 

 ceived in three different ways. Thus sulphuric acid may be ex- 

 pressed by the following three formulas : 



^O^JH' ^*^3JH ' ^^2"iOH- 

 The first formula might be most probable if the acid were a hy- 

 drogen acid ; the second is most probable for an acid wliose anhv- 

 drid exhibits a slight affinity for water; the third is the fonnuhi of 

 a bibasic hydroxylic acid. The constitution expressed by the first 

 formula probably corresponds to that of a bibasic hydrogen acid, 

 as for instance fluosilicohydric and chlorplatinhydric acid. 



SiF.|g PtCl.|«. 



Sulphurous, selenious, carbonic, chromic and boric acids proba- 

 hly correspond to the second formula. Of these the first four at 

 least easily and spontaneously split into anhydrid and water, so 

 that the hydrates (acids proper) of several can scarcely be pre- 

 pared at all. We have therefore for the rational formulas of these 

 acids, SO^ . gH SeO^ . ^ CJO^ . ^, &c. 



This corresponds entirely with the behavior of the homologues of 

 carb(Jnic acid, like glycolic acid and the other members of the 

 lactic acid series, which are probably diatomic, but usually appear 

 monobasic, as the second atom cannot be easily replaced by so- 

 dium, but readily by alcoholic and acid radicals. To the third 

 formula correspond sulphuric, selenie, oxalic and tartaric acids, 

 the formulas of which according with the usual assumption, be- 

 come as follows : 



,^ OH ^ ^ on ^ ^ OH 



^2 -OH' ^^^3- OH' -^2^2 • OH' 



