38 ABSORPTION The absorption of gases by other liquids than water is a subject still open for investigation, and has thus far only been determined for a few gases. So Dr. Vander Weyde of New York found in regard to laughing gas, that alkaline solutions absorb more than pure water, and alcoholic liquors most, strong alcohol over five tunes its volume ; solutions of neutral salts in general absorb the same amount as water, ex- cept the sulphates, which absorb much less of the gas, while acids absorb the least, especially diluted sulphuric acid, which absorbs only 0'3 to 0'05 of its volume, according to its strength. III. Absorption of Heat. The capacity of bodies to absorb heat is in direct proportion to their capacity to emit heat. Light-colored, polished, or smooth surfaces possess this capacity in the least degree, while dark-colored and rough sur- faces absorb heat very readily. However, ac- cording to the late researches of Melloni, this effect depends less upon the apparent color than upon the nature of the coloring material. He also finds that when the heat-giving body is not luminous, the color is without influence ; but when it is luminous, the color has great influence. Melloni has also determined the capacity of absorption of heat by different transparent substances. He found that while transparent rock salt absorbed only 8 per cent, of the heat passing through with the light, fluor spar absorbed from 25 to 50, Iceland spar and glass 60, alum 90, and ice 94 per cent. ; while for heat emitted from a non-luminous body, the latter substances were totally opaque, absorbing all the heat and transmitting none. Recently Tyndall and Magnus have made re- searches on the absorbent power of gases, and found that under the pressure of one atmos- phere, the source of heat being a copper ball heated to 518 F., the absorption by dry air being accepted as the unit, hydrogen was also 1, chlorine 39, carbonic acid 90, nitrous acid 355, marsh gas 403, sulphurous oxide 710, olefiant gas 970, and ammonia 1,195 ; which means that the latter two gases absorb respectively 970 and 1,195 times more of the heat transmitted through them than is the case with dry air. IV. Absorption of Light. The apparent color of all objects is caused by the elective absorption of certain colored rays in the white light, while the remaining are reflected and determine the color of the object. Even the purest white and the most perfectly polished surfaces absorb some of the light. It is the same with the most transparent substances; they all absorb light more or less. In many of these an elec- tive absorption also takes place ; colored gems and glass or liquid solutions absorb certain colored rays and let others pass ; those which pass determine the color of the substance. Sometimes, besides the absorption of several colors, a color is reflected complementary to that transmitted ; in a thin layer of aniline red, red rays are transmitted, while green rays are reflected; a similar action takes place in a solution of litmus and several other sub- stances. Some crystals possess the power of absorbing different colors when light passes through them in different directions; this is called dichroism and polychroism. Thus the mineral iolite, a gem consisting of alumina, magnesia, and iron, shows different colors ac- cording as the light falls 'along the axis of crystallization or in a transverse direction. Many artificial crystals exhibit the same re- markable property; for instance, the double chloride of platinum and potassium, which ap- pears either deep red or bright green. The investigation of this peculiar kind of absorp- tion of light has recently given rise to the in- vention of a new modification of the micro- scope by Haidinger, by which this property may be examined in the minutest crystals; this invention is called the dichroscope and dichroic microscope. V. Absorption Spectrum. The elective absorption of transparent gases, liquids, and solids is determined by means of the spectroscope. This instrument proves in- deed that the cause of this absorption is simply the incapacity of the transparent substance to transmit luminous waves of a certain length, and thus that it is opaque for such waves. The result of such partial opacity is the for- mation of the so-called absorption bands, in case such a substance is placed between the light and the slit of the spectroscope. The Fraunhofer lines in the solar spectrum are in fact nothing but absorption bands produced by the passage of the light through the solar atmosphere; our own atmosphere also pro- duces such bands, which spectroscopists call the atmospheric lines. The absorption spec- trum differs in each substance which we may submit to examination. Thus iodine vapor and nitrous acid vapor produce very characteristic absorption spectra when placed before the slit of the spectroscope (figs. 1 and 2), while differ- Fio. 1. Absorption Spectrum of Iodine Vapor. ent solutions of apparently the same color may be unmistakably distinguished from each other by the difference in the absorption spectra Fio. 2. Absorption Spectrum of Nitrons Add Gas. which they produce. The most striking illus- tration is given by the black absorption bands produced by a perfectly clear and colorless so- lution of any salt of the rare metal didyminni, so that in this way the merest traces of this metal in any solution may be detected, as lately
