252 PHENOMENA, ATOMS, AND MOLECULES 



of chemists in the similarity of the constitutions of these compounds, there 

 seem to be no convincing data indicating similarity of crystalline form in 

 these cases. For example, Groth points out that potassium hydrogen 

 carbonate and potassium hydrogen sulfite both belong to the monoclinic 

 system, and that their crystals have the same habit. An examination of the 

 data shows that both form crystals which are elongated in the direction of 

 the &-axis. The following crystallographic data, however, prove that the 

 substances are very far from isomorphous : 



a. b. c. p. 



KHCO3 2.6770 I 1.3115 103° 25' 



NaHCOs 0.7645 I 0.3582 93° 19' 



KHSO5 0.9276 I 2.2917 94° 46' 



The ratios of the axes could hardly be more different, and there is a 

 diff'erence of 9° in the inclination of the axes. The sodium acid carbonate is 

 also monoclinic but has quite different constants from either of the other 

 compounds. No other anhydrous sulfites are given by Groth. 



The following hydrated sulfites -and carbonates given by Groth are 

 the only ones that are comparable in constitution. Ammonium sulfite, 

 (NH4)oSO.'?.H20, is monochnic while sodium carbonate, Na2C03.H20, is 

 orthorhombic. The compound Na2C03.7H20 is orthorhombic but Na2S03.- 

 7H2O is monoclinic. Finally, MgC03.3H20 crystallizes in the orthorhombic 

 system, while MgS03.3H20 is "ditrigonal pyramidal." All of the available 

 crystallographic data thus- seem to indicate conclusively that sulfites and 

 carbonates do not have similar constitutions. 



The chlorates and nitrates usually have different crystalline forms, but 

 in many cases it has been found possible to make mixed crystals containing 

 as much as 10 or 15% of one or the other of the constituents. Sodium 

 chlorate crystallizes in the cubic system isomorphous with sodium bromate, 

 while sodium nitrate is trigonal. This form of sodium chlorate is normally 

 produced either from aqueous solution or by cooling the molten mass. 

 By slow evaporation of a strongly supersaturated aqueous solution it is 

 possible to obtain sodium chlorate in a trigonal form with angles very much 

 like those of sodium nitrate, and also having negative double refraction like 

 that of sodium nitrate ; this trigonal form of sodium chlorate is very un- 

 stable, and soon goes over into the cubic form even at room temperature. 

 It was found possible to prepare mixed crystals of sodium nitrate and 

 chlorate containing as much as 22.5% of the chlorate. An unstable mono- 

 clinic form of sodium chlorate has also been prepared. 



Potassium chlorate crystallizes only in the monoclinic form with axes 

 inclined 109°, while potassium nitrate is orthorhombic. Notwithstanding 

 the fact that the crystals belong to different systems, Groth states that 



