Grapes and other fruits ferment and produce wines, etc., from which distilled liquors are obtained.
332. Lactic Fermentation changes the sugar of milk, lactose, to lactic acid, i.e. sour milk. In canning fruit, any germs present are killed by heating, and those from the air are excluded by sealing the can. Milk has been kept sweet for years by boiling, and tightly covering the receptacle with two or three folds of cotton cloth.
333. Putrefaction is fermentation in which the products of decay are ill-smelling. Saprophytes attack the dead matter, feed on it, and cause it to putrefy. This action, as well as that of ordinary fermentation, used to be attributed solely to oxygen. Germs bring back organic matter to a more elementary state, and so have a very important function. By some scientists, digestion is regarded as a species of fermentation, probably due to the action of lifeless ferments; e.g. sucrose cannot be taken into the system, but is first fermented to glucose.
334. Most Infectious Diseases are now thought to be due to parasites of various kinds, such as bacteria, microbes, etc., with which the victim often swarms, and which feed on his tissues, multiplying with enormous rapidity. Such diseases are small-pox, intermittent and yellow fevers, etc. Consumption, or tuberculosis, is believed to be caused by a microbe which destroys the lungs. In some diseases not less than fifteen billions of the organisms are estimated to exist in a cubic inch. These multiply so rapidly that from a single germ in forty-eight hours may be produced nearly three hundred billions. These germs do not spring into life spontaneously from inorganic matter, but come from pre-existent similar forms. Parasites are not so rare in the system even of a healthy person as is generally supposed. They are found on our teeth and in many of the tissues of the body.
Several infectious diseases are now warded off or rendered less virulent by vaccination, the philosophy of which is that the organisms are rendered less dangerous by domestication; several crops, or generations, are grown in a prepared liquid, each less injurious than its parent. Some of the more domesticated ones are introduced into the system, and the person has only a modified form of the disease, often scarcely any at all, and is for a more or less limited time insured against further danger.
Dust particles and motes floating in the air are in part germs, living or dead, often requiring only moisture and mild temperature for resuscitation. Most of these are harmless.
Chapter LX.
CHEMISTRY OF LIFE.
335. Growth.—The chemistry of organic life is very complex, and not well understood. A few of the principal points of distinction between the two great classes of living organisms, plants and animals, are all that can be noted here. Minerals grow by accretion, i.e. by the external addition of molecules of the same material as their interior. A crystal of quartz grows by the addition of successive molecules of SiO2, arranged in a symmetrical manner around its axis. The growth of crystals can be seen by suspending a string in a saturated solution of CuSO4, or of sugar. In plants and animals the growth is very much more complex, but is from the interior, and is produced by the multiplication of cells. To produce this cell-growth and multiplication, food-materials must be furnished and assimilated. In plants, sap serves to carry the food-materials to the parts where they are needed. In the higher animals, vari- ous fluids, the most important of which is the blood, serve the same purpose.
336. Chemistry of Plants.—In ultimate analysis, plants consist mainly of C, H, O, N, P, K. In proximate analysis, as it is called, they are found to contain these elements combined to form substances like starch, sugar, etc. Water is the leading compound in both animals and plants. One of the most important differences between animals and plants is, that all plants, except parasitic ones, are capable of building up such compounds as starch from mineral food-stuffs, while animals have not that power, but must have the products of proximate analysis ready prepared, as it were, by the plant. Hence plants thrive on minerals, whereas animals feed on plants or on other animals. The power which plants have of transforming mineral matter is largely due to sunlight, the action of which in separating CO, was described. The reaction in the synthesis of starch from CO2 and H2O in the leaf, is thought to be as follows: 6 CO2 + 5 H2O = C6H10O5 + 12 O. C6H10O5 is taken into the tree as starch; 12 O is given back to the air. All the constituents, except CO2 and a very small quantity of H2O, are absorbed by the roots, from the soil, from which they are soon withdrawn by vegetation. To renew the supply, fertilizers or manures are applied to the soil. These must contain compounds of N, P, and K. N is usually applied in the form of ammonium compounds, e.g. (NH4)2SO4, (NH4)2CO3, and NH4NO3. The reduction and application of Cas(PO4)2 for this purpose was described. K is usually applied in the form of KCl and K2SO4.