VAPORIZATION 265 boil when sustaining an atmospheric pressure equal to a mercurial column 8 in. high. If the experiment is varied by depressing the tube in the reservoir, it will be seen that, although at the beginning nothing but the invisible vapor of ether occupied the upper part of the tube, when the degree of depression is sufficient, liquid ether will make its appearance. Until this point is reached the mercury will continue to descend to a lower level during the depres- sion of the tube ; but as soon as the conden- sation of vapor into liquid ether begins, the mercury will remain at the same level until all the vapor is condensed, and the upper portion of the tube is filled with liquid ether. "When the tube is raised, the mercury will continue to stand at the same level as long as there is any liquid ether in the upper part ; but as soon as it all vaporizes, the mercury will begin to rise, and continue to do so with the elevation of the tube. When such a quantity of vapor is contained in a space that it cannot be con- densed, the temperature remaining the same, without a portion passing into the liquid state, it is said to be saturated for that temperature. A certain volume of saturated vapor, therefore, when it is heated, ceases to be saturated, and when exerting a pressure equal to that of the at- mosphere does not represent the boiling point of the liquid ; this it does only at the points of saturation and of equal pressure. (The boiling points of alcohol, ether, water, and other fluids are given in BOILING POINT, vol. ii., pp. 793 and 796.) The condition of saturation of a vapor is therefore that of its maximum tension at the same temperature, since it cannot be com- pressed with partial condensation into a liquid. The more a vapor is removed from its satura- tion point, either by expansion or by increase of temperature, the more nearly does it re- semble in physical properties a permanent gas ; and it has therefore been concluded that the so-called permanent gases are only va- pors which exist at ordinary temperatures far above their points of saturation, and that by si- multaneously lowering their temperature and subjecting them to pressure they could be re- duced to a point below that of saturation, and therefore that they could be partially liquefied. In many instances experiment has verified the correctness of this conclusion, as in the lique- faction of nitrous oxide, carbonic anhydride, ammonia, and several other gases. (See HEAT, vol. viii., p. 578.) The passage of a vapor into a vessel containing a permanent gas, or into the atmosphere, follows the laws of the dif- fusion of gases (see GAS, vol. vii., p. 633), and it is found that the combined tension of the gas and the vapor is nearly equal to the sum of the separate tensions of the gas and vapor, when contained in the same space at the same temperature. This is indicated in the increased tension of the atmosphere when it contains a more than usual quantity of invisible watery vapor, and in its diminished tension when such invisible vapor begins to condense and form clouds, as evidenced in the fall of the barom- eter. The vaporization of liquids under cir- cumstances in which their surfaces are not in contact with the air or any gaseous space, as when water globules are enveloped in oil, pre- sents many remarkable phenomena, some of which are described and explained in the article BOILING POINT, as also the effect which sub- stances in solution have upon the vaporization of liquids, as well as the relation of tl^e chemi- cal constitution of a liquid to its boiling point, and an outline of the mode of measurement of the tension of vapors. (See also EXPANSION, and STEAM.) The following table, from Mil- ler's "Chemical Physics," gives some of the results of Eegnault's experiments upon the tension of vapors of several liquids at equal temperatures. The tension is measured by the height of a column of mercury which each vapor will support at a given temperature, the .degrees being given on both the centigrade and Fahrenheit scales. TEMPEBATURE, deg. C. Sulphuric ether. Bisulphide of carbon. Chloroform. Alcohol. Oil of turpentine. Water. TEMPEBA- TUBE, teg. F. -20... 2-725 O'lSl 0'086 4 -10 4-856 S'llO 0-256 0-082 14
7-176 5-008 0-501 0-082 0-182 82 10 11-278 7-846 0-948 0-090 0-361 50 20 17-117 11-740 1-732 0-168 0-686 63 80 25-078 17-110 8-086 0-275 1-245 86 40 85-971 24-810 14-380 5-159 0-460 2-168 104 50 49-920 38-57 20-641 8-673 0-675 8-631 122 60 68-121 ' 48-71 29-054 18-776 ' 1-058 5-874 140 70 90-92 60-98 88'48 21-228 1-628 9-208 158 80 116-08 79-94 53*85 82-00 2-408 18-998 176 90 158-50 103-27 71-81 46-86 8-582 20-740 194 100 193-72 180-75 92-70 66-33 5-810 80-00 212 no 246-02 162-84 118'91 92-59 7-372 42-45 230 120 201-58 150-81 126-26 10-117 58-87 248 ISO....' 246-47 185-86 170-51 18-660 80-14 266 140 221-95 18-199 107-27 284 150 285-73 28 '798 141 -86 802 160 30-596 188-61 820 170 88-98 285-32 888 180 48-41 297-87 856 190 59-62 872-71 874 aoo... 78-45 461-88 892
