Perhaps still the best experimental evidence for the increase with temperature of the 3.5 Å "fingerprint" dense structure at the expense of the ordinary outer O···O structure at 4.5 Å comes from the early isochoric temperature differential x-ray scattering (ITD) experiments of Bosio, et al. [31]. Recently, it has been shown [32] that the experimentally observed ITD’s (for D₂O) are a direct consequence of a temperature dependent mixture of ice-I-type and ice-II-type bonding [6], with temperature dependent proportions derived from the density [33]. An example of such a composite ITD is illustrated in Fig. 1.

What is currently known experimentally about structural changes in the liquid under the influence of pressure? It is worth noting that the experimental results [34,35] involving pressure effects were predicted by the outer bonding model long before the experiments were actually performed. See sect 6 (Pressure Effects) of Ref. 5 and subsequent papers. The most explicit of the experimental papers [34] beautifully confirms the decrease of open 4.5 Å structure with a concurrent increase of the compact ~ 3.5 Å structure as the pressure is raised to 7.7 kbar. In spite of the clear disclosure of this ~3.5 Å outer O···O structure in liquid water, both in the Bosio, et al. ITD data and the Okhulkov, et al. pressure studies, as well as in a few earlier x-ray investigations [36,37], our designation of this outer 3.5 Å feature as a peak [38] was soundly denounced in the discussions following our Faraday presentation in Ref. [39].

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