618 BRIDGMAN. 



Ill by O.O5I3, and III is less compressible than IV by O.OgSS. This 

 verifies the abnormally high compressibility of IV again. Computa- 

 tions made in the same way give for the difference of specific heats 

 between II and III 0.16 cal. against Bellati and Romanese's ^^ direct 

 value 0.071, and for the difference IlI-IV 0.048 cal. against their 

 value 0.052. This verifies the abnormal sequence of the specific heats 

 Il-llI-IV, and the numerical values agree as well as one could expect 

 when one considers that the method of computation employed above 

 throws by far the larger part of the error into the specific heats. This 

 has already been fully explained in a preceding paper. 



The question of the time rate of the various transitions and the 

 lag phenomena will be dealt with in greater detail in another paper. 

 It will not be out of place to mention here, however, that many meas- 

 urements were made of the time rate, and at all points it was found 

 that the rate is greater with decreasing pressure, as has also been 

 foiuid for most other substances. 



Nitrates with no New Forms. 



The following nitrates were examined without result for other 

 forms between 1 and 12000 kg. at 20° and 200°; NaNOg, LiNOs+Aq, 

 Hg2(N03)o, Hg(N03)2, Pb(N03)2, A1(N03)3. 



Sodium, although chemically similar in many respects to potassium 

 and the other alkali metals whose nitrates are polymorphic, is in its 

 crystalline properties known to be so much dissimilar that its salts 

 do not form isomorphic mixtures. From this point of view, it is 

 therefore not surprising that no new forms were fovmd. NaNOs 

 (which is trigonal) does, however, crystallize isomorphously with the 

 high temperature form (trigonal) of AgNOs, and will crystallize in 

 small percentages isomorphously with the low temperature form. 

 We suspect, therefore, that NaN03 has a second modification, stable 

 at low temperatures at atmospheric pressm-e. It would be interesting 

 to search for this. Hissink ^ has found no new form between 270° and 

 — 50°. The transition, if it exists, is by analogy with AgNOs of the 

 ice type, the transition curve falling with rising pressure. It is natural 

 that no new form was found at high pressures at room temperature. 



LiN03 is probably not isomorphous with AgNOs or NaNOa, al- 

 though Retgers says that it is with the latter. There is no necessity 

 for expecting other forms. Arzruni, however, on authority which goes 



