Koine Physical Properties of Nitric Acid Solutions. 07 



cent, acid, or approximately of the composition HN0 3 .2H 2 ; further, 

 Pickering (loc. cit.) has obtained crystals of this composition by 

 freezing. As regards the contractions, the change at 70 per cent, 

 appears to be more marked than that at 63 per cent., and it might be 

 a matter of judgment if the results from 54 to 70 per cent, might not 

 equally well be represented as upon one curve and not upon two 

 straight lines. The latter view has been adopted to bring the results 

 obtained within these limits into uniformity with the rest. 



As regards the abnormality at 95 to 100 per cent., it is probable 

 this is the result of a wholly different cause ; a similar phenomenon 

 was observed in the electric conductivity, and the suggestion was put 

 forward of an initial decomposition into water and nitric anhydride, 

 but since the latter, when passed into nitric acid of about 100 per 

 cent, concentration, forms a substance of composition 2HN03.NoO r , or 

 2N 2 5 .HoO (a colourless liquid of specific gravity 1*642 at 18, crystal- 

 lisable at - 5), both the contraction and the electric conductivity 

 numbers might be expressed in terms of this substance, and not in 

 terms of nitric acid 2HN0 3 or N^0 5 .HoO. At present the physical 

 constants of the crystallisable compound have not been sufficiently 

 examined. 



IX. The Degree of Discontinuity. 



Though in the diagrams of curves the mean straight lines are pro- 

 duced until they intersect, yet the object thereof is to determine each 

 separate origin of co-ordinates; it is not ^intended to represent an 

 irregularity occurring suddenly or an abrupt change of events. The 

 results in themselves show one or two possibilities at each critical 

 point, namely, firstly, a very gradual transition from one phase to 

 another, so that the observation points within about a 2 per cent, 

 limit could equally well be represented on either of the lines ; 

 secondly, a transition stage, in which the observation points lie 

 upon neither of the straight lines, but one generally lower than 

 both. Examples of each of these two possibilities will be discussed 

 seriatim. 



Firstly, a gradual transition. An example of this can be illustrated 

 from the results obtained at 24'2 for acids of percentage values from 

 30-17 to 33*87, and in the following table (p. 98) the observed results 

 for the contractions are compared with those calculated both from the 

 21 per cent, and the 33'3 per cent, origins of co-ordinates. 



It is evident that the result for the 30-17 per cent, acid belongs to 

 the straight line drawn from the 4 per cent, origin, and the result 

 for 33-87 per cent, acid, on the other hand, belongs to the straight 

 line drawn from the 33-3 per cent, acid, while those intermediate 

 could within the degree of experimental error be equally represented 

 as belonging to either line. 



