5 1.0 0.5 0  FWHM (nm)  4:1 methanol-ethanol R.T 77 K  (b) 1.0 0.5 0 10 5 0 Pressure (GPa) R.T 77 K  Petroleum ether  (c) 1.0 0.5 0  1:1 n-pentane and isopentane R.T 77 K  (a)  1st.  2nd.  1st.  2nd. 1.0 0.5 0.0  (a)2-propanol R.T 77 K 1st. 2nd. 3rd.  1st.  2nd.  3rd. 1.0 0.5 0.0 FWHM(nm) (b) Glycerin R.T 77 K 1.0 0.5 0.0 10 5 0 Pressure (GPa) (c) Daphne 7474 R.T 77 K limit pressures at room temperature were determined by the ruby fluorescence method to be 7.3 and 10.5 GPa for the pentane and 4:1 M-E mixtures, respectively [10]. Petroleum ether (benzine or petroleum naphtha) is a group of liquid hydrocarbon mixtures whose main com- ponents are isohexan and n -pentane. It has been mainly used as an industrial washing solution but also in high- pressure studies for a long time [40, 41]. However, there have been no reports on the hydrostatic-limit pressure as far as is known. Figures 5 (a), (b) and (c) show the pressure dependences of the FWMHs of the three organic media at room temperature and 77 K. At room temperature, the FWHMs with the three me- dia do not show any strong increase. The FWHM for FIG. 5: (Color online)Pressure dependence of full-width at half maximum (FWHM) of the ruby R$_{1}$ luminescence line at room temperature and 77 K for (a)1:1 n -pentane and isopen- tane, (b) 4:1 methanol-ethanol, and (c) petroleum ether. FIG. 4: (Color online)Pressure dependence of full-width at half maximum (FWHM) of the ruby R$_{1}$ luminescence line at room temperature and 77 K for (a) 2-propanol, (b) glycerin and (c) Daphne 7474. the pentane mixture starts to weakly increase above the hydrostatic pressure of 7.3 GPa. The FWHM for the 4:1 M-E mixture starts to weakly increase above 8 GPa, smaller than the hydrostatic-limit pressure of 10.5 GPa determined at 297 K [10]. It has been reported that the liquid-glass transition pressure P$_{glass}$ is sensitive to tem- perature [42]. The value of P$_{glass}$ in the present exper- iments at T = 283 ± 3 K is estimated to be 8.5 ± 0.5 GPa, which is consistent with the present result. The FWHM for petroleum ether does not show any clear in- crease around P$_{glass}$ of 6 GPa [43]. It is suggested that the mechanical strength (yield and shear strength) of solid petroleum ether is not strong enough to distort the ruby crystal. At 77 K, the FWHMs for the three media