Page:The American Cyclopædia (1879) Volume XVI.djvu/803

 YEAR may be added that the interval between suc- cessive passages of the perihelion is not, as might be supposed, equal in length to the in- terval between successive passages of the aphe- lion. This is easily shown. The perihelion is at present advancing at the rate of 11 -24" per annum ; so that, neglecting the minute gain while the earth is traversing this arc, the actual anomalistic year, estimated from the perihelion, exceeds the sidereal year by the time occupied by the earth in traversing an arc of 11-24" with her perihelion motion of 1 1' 9-9" per diem. Now if the anomalistic year were estimated from aphelion, the same would be true, only the earth's daily aphelion motion of only 57' 11-5" must be substituted. Since, with the more rapid motion, the arc 11-24" would be traversed in less time, the anomalis- tic year estimated from the perihelion is slight- ly shorter than the anomalistic year estimated from aphelion. The length of the mean anom- alistic year, however, is at present that above indicated, and therefore it follows that at pres- ent the actual interval between the earth's successive passages of perihelion is slightly shorter than the mean anomalistic year. Thirdly, the year may be measured with refer- ence to the earth's figure and position. For example, the plane of the earth's equator cuts the plane of the ecliptic in a straight line, which is carried round with the earth, moving almost exactly parallel to itself. Twice in- each circuit, therefore, this line passes through the sun's centre, and the year may be measured from one or other of these epochs. One cor- responds to the vernal equinox, the other to the autumnal equinox. Astronomers have se- lected the former to measure from, and the year thus measured is called the tropical year, or year of seasons, and is defined as the inter- val between the earth's successive passages of her vernal equinox. Since this point (when the earth viewed from the sun is at =2= and the sun viewed from the earth is at T) retro- grades, the tropical year is less than the si- dereal year. Its mean length is 365d. 5h. 48m. 48'6s. As in the case of the anomalistic year, the actual length of the tropical year depends on the point from which it is measured ; for the earth's diurnal motion at the passage of her vernal equinox is not precisely equal to Her motion at the passage of her autumnal equinox. Thus the length of the tropical year is not absolutely constant. Its length at pres- ent exceeds by nearly 13 sec. the length which it had in the time of Hipparchus. In like manner the length of the anomalistic year is slowly variable. The lengths of the four sea- sons, astronomically measured, are very nearly as follows: from the vernal equinox to the summer solstice, 92d. 22Jh. ; from the latter to the autumnal equinox, 93d. 13h. ; from this to the winter solstice, 89d. 16|h. ; from this to the vernal equinox, 89d. lh. For an ac- count of the years and calendars of different nations, see CALENDAR, and CHRONOLOGY. YEAST 775 IE1ST, the froth which rises upon the sur- face of beer and other liquors during fermen- tation, consisting principally of microscopic globules of a fungoid plant. This plant is also found in that variety of yeast which is developed in sedimentary fermentation. (See BREWING.) The history of this plant begins with its discovery in beer by Leeuwenhoek in 1680 by microscopical examination. Fabroni in 1787 regarded yeast as a vegeto-animal sub- stance residing in peculiar utricles in grapes as well as in corn, but does not seem to have at- tached great importance to the existence of the yeast globules discovered by Leeuwenhoek. ThSnard in 1803 recognized a relation between yeast cells and fermentation, but most of the chemists of that day who investigated the sub- ject of fermentation seem to have regarded the functions of yeast as having more of a chemi- cal than a physiological nature. In 1825 Des- mazi^res found organisms in yeast which he re- garded as animals. It was not till about 1887 that Cagniard de la Tour took up the micro- scopical observations of Leeuwenhoek, and, as has been said, "rediscovered the yeast plant." He declared that by its influence the equi- librium of the molecules of sugar was broken up, and measured the diameter of the cells, which he placed at -^^ of an inch, and also observed that they developed by budding. Schwann of Jena made independently, about the same time, similar discoveries. Their observations were confirmed by Quevenne, Mitscherlich, and Turpin ; the last placed the organism in the genus torula of Persoon, and this classification has been recognized until very recently. Yeast has also received the name of mycoderma vini. The torula has a mycelium, and it is held that ferments never have. Meyen, considering yeast to be a fun- gus, created a new genus for it under the name of saccharomyces. Kutzing and others placed it among algff, and in a separate ge- nus called cryptococcus. Whether yeast is the cause or the effect, or simply an accompani- ment of fermentation, has long been disputed, and it has not been positively decided that its presence is necessary for the commencement of the process of vinous fermentation; but the great weight of opinion leans toward the affirmative. (See FERMENTATION.) The most prominent advocate of this theory is Pasteur, who has made numerous elaborate experi- ments, not only to elucidate the nature of yeast, but to oppose the theory of sponta- neous generation. Reess and others have di- vided the yeast genus of fungi into several spe- cies. Of these, saccharomyces cere-cm^ or the yeast of beer, is again divided into two varie- ties sedimentary or bottom and surface yeast; but' the drawings of them have much resem- blance, and in fact it is known that the one variety is readily convertible into the other by cultivation. The sedimentary yeast i veloped at a considerably lower temperatui than surface yeast, and with much less evo-