+---------------------+------++----------------------+------+ | _tc_ | _pc_ || _tc_ | _pc_ | +---------------------+------++----------------------+------+ | N_{2} -146° | 33 || H_{2}S +108° | 92 | | CO -140° | 39 || C_{2}N_{2} +124° | 62 | | O_{2} -119° | 50 || NH_{3} +131° | 114 | | CH_{4} -100° | 50 || CH_{3}Cl +141° | 73 | | NO -93° | 71 || SO_{2} +155° | 79 | | C_{2}H_{4} +10° | 51 || C_{5}H_{10} +192° | 34 | | CO_{2} +32° | 77 || C_{4}H_{10}O +193° | 40 | | N_{2}O +53° | 75 || CHCl_{3} +268° | 55 | | C_{2}H_{2} +37° | 68 || CS_{2} +278° | 78 | | HCl +52° | 86 || C_{6}H_{6} +292° | 60 | | H_{2}O +365° | 200 || C_{6}H_{5}F +287° | 45 | | CH_{3}OH +240° | 79 || C_{6}H_{5}Cl +360° | 45 | | C_{2}H_{5}OH +243° | 63 || C_{6}H_{5}Br +397° | 45 | | CH_{3}COOH +322° | 57 || C_{6}H_{5}I +448° | 45 | +---------------------+------++----------------------+------+ Young and Guy (1891) showed that _tc_ and _pc_ clearly depend upon the composition and molecular weight. [30] I came to this conclusion in 1870 (_Ann. Phys. Chem._ 141, 623). [31] Pictet, in his researches, effected the direct liquefaction of many gases which up to that time had not been liquefied. He employed the apparatus used for the manufacture of ice on a large scale, employing the vaporisation of liquid sulphurous anhydride, which may be liquefied by pressure alone. This anhydride is a gas which is transformed into a liquid at the ordinary temperature under a pressure of several atmospheres (_see_ Note 27), and boils at -10° at the ordinary atmospheric pressure. This liquid, like all others, boils at a lower temperature under a diminished pressure, and by continually pumping out the gas which comes off by means of a powerful air-pump its boiling point falls as low as -75°. Consequently, if on the one hand we force liquid sulphurous anhydride into a vessel, and on the other hand pump out the gas from the same vessel by powerful air-pumps, then the liquefied gas will boil in the vessel, and cause the temperature in it to fall to -75°. If a second vessel is placed inside this vessel, then another gas may be easily liquefied in it at the low temperature produced by the boiling liquid sulphurous anhydride. Pictet in this manner easily liquefied carbonic anhydride, CO_{2} (at -60° under a pressure of from four to six atmospheres). This gas is more refractory to liquefaction than sulphurous anhydride, but for this reason it gives on evaporating a still