An Introduction to Chemical Science Part 25

326. Dextrin, a gummy substance used for the backs of postage stamps, is a carbo-hydrate, as in fact are gums in general.

Dextrin is made by heating starch with H2SO4 at a lower temperature than for dextrose.

327. Zylonite and Celluloid. -These two similar substances embody the latest use of cellulose in manufactured articles. For zylonite, linen paper is cut into strips two feet by one inch, soaked ten minutes in a mixture of H2SO4 and HNO3, a process called nitration, washed for several hours, then ground to a fine pulp, and thoroughly dried. It is then similar to pyroxiline.

Aniline coloring-matter of any desired shade is added, after which it is dissolved by soaking some hours in alcohol and camphor, the liquid is evaporated, and the substance is kneaded between steam-heated iron rollers, dried with hot air, and finally subjected to great pressure, to harden it, and cut into sheets. Zylonite is combustible at a low temperature, and when in the pyroxiline stage, explosively so. Ivory, coral, amber, bone, tortoise sh.e.l.l, malachite, etc., are so closely imitated that the imitation can only be detected by a.n.a.lysis. Collars, combs, canes, piano-keys, and jewelry, are manufactured from it, and it can be made transparent enough for windows.

CHAPTER LIX

CHEMISTRY OF FERMENTATION.

328. Ferments.--A large number of chemical changes are brought about through the direct agency of bodies called ferments; their action is called fermentation. Ferments are sometimes lifeless chemical products found in living bodies; but in other cases they are humble plants.

329. Yeast is one of the most common of living ferments, wild yeast being a microscopic plant found on the ground near apple- trees and grape-vines, and often in the air. The cultivated variety is sold by grocers. The temperature best suited to the rapid multiplication of the germs forming the ferment plant is 25 degrees to 35 degrees.

330. Alcoholic and Acetic Fermentation.--The changes which the juice of the apple undergoes in forming cider and vinegar are a good ill.u.s.tration of fermentation by a living plant. Apple-juice contains sucrose. Yeast germs from the air, getting into this unfermented liquor, cause it to "work." This process changes sucrose to glucose, and glucose to alcohol and CO2, and is known as alcoholic fermentation. The latter reaction, C6H12O6 = 2 C2H6O + 2 CO, is only partially correct, as other products are formed.

The juice has now become cider; the sugar alcohol. After a time, if left exposed, another organism finds its way to the alcohol, and transforms it into acetic acid, HC2H8O2, and H2O. This process is called acetic fermentation. C2H6O + O2 = HC2H3O2 + H2O. For this fermentation, a liquor should not have over ten per cent of alcohol. Mother of vinegar consists of the germs that caused the fermentation. Still a third species of ferment may cause another action, changing acetic acid to H2O and CO2. The vinegar then tastes flat. HC2H3O2 + 4 O = 2H2O + 2 CO2.

Some mineral acids, as H2SO4 and HCl, and some organic acids, are regarded as lifeless ferments. To this cla.s.s are thought to belong the diastase of malt and the pepsin of the stomach. This variety of ferments exists in the seeds of all plants, and changes starch to glucose.

331. Bread which is raised by yeast is fermented, the object being to produce CO2, bubbles of which, with the alcohol, cause the dough to rise and make the bread light.

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 princ.i.p.al points of distinction between the two great cla.s.ses 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 a.s.similated.

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 a.n.a.lysis, plants consist mainly of C, H, O, N, P, K. In proximate a.n.a.lysis, 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 a.n.a.lysis 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 const.i.tuents, except CO2 and a very small quant.i.ty 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.

337. Food of Man.--In the higher animals the object is not so much to increase the size as to supply the waste of the system. The princ.i.p.al elements in man's body are C, H, O, N, S, P.

An ill.u.s.tration of the transformation of mineral foods by plants before they can be used by animals is found in the Ca3(PO4)2 of bones. This is rendered soluble; plants absorb and transform it; animals eat the plants and obtain the phosphates. Thus man is said to "eat his own bones." The food of mankind may be divided into four cla.s.ses (1) proteids, which contain C, H, O, N, and often S and P; (2) fats, and (3) amyloids, both of which contain C, H, O; (4) minerals. Examples of the first cla.s.s are the gluten of flour, the alb.u.men of the white of egg, and the casein of cheese. To the second cla.s.s belong fats and oils; to the third, starch, sugar, and gums; to the fourth, H2O, NaCl and other salts. Since only proteids contain all the requisite elements, they are essential to human food, and are the only absolutely essential ones, except minerals; but since they do not contain all the elements in the proportion needed by the system, a mixed diet is indispensable. Milk, better than any other single food, supplies the needs of the system. The digestion and a.s.similation of these food-stuffs and the composition of the various tissues is too complicated to be taken up here; for their discussion the reader is referred to works on physiological chemistry.

338. Conservation.--Plants, in growing, decompose CO2, and thereby store up energy, the energy derived from the light and heat of the sun. When they decay, or are burned, or are eaten by animals, exactly the same amount of energy is liberated, or changed from potential to kinetic, and the same amount of CO2 is restored to the air. The tree that took a hundred years to complete its growth may be burned in an hour, or be many years in decaying; but in either case it gives back to its mother Nature, all the matter and energy that it originally borrowed. The ash from burning plants represents the earthy matter, or salts, which the plant a.s.similated during its growth; the rest is volatile. In the growth and destruction of plants or of animals, both energy and matter have undergone transformation. Animals, in feeding on plants, transform the energy of sunlight into the energy of vitality. Thus "we are children of the sun."

CHAPTER LXI.

THEORIES.

339. The La Place Theory.--This theory supposes that at one time the earth and the other planets, together with the sun, const.i.tuted a single ma.s.s of vapor, extending billions of miles in s.p.a.ce; that it rotated around its center; that it gradually shrank in volume by the transformation of potential into kinetic energy; that portions of its outer rim were thrown off, and finally condensed into planets; that our sun is only the remainder of that central ma.s.s which still rotates and carries the planets around with it; that the earth is a cooling globe; that the other planets are going through the same phases as the earth; and finally that the sun itself is destined like them to become a cold body.

340. A Cooling Earth.--The sun's temperature is variously estimated at many thousands, or even millions o degrees. Many metals which exist on the earth as solids -e.g. iron- are gases in the dense atmosphere of the sun. Thus the earth, in its early existence, must have been composed of gases only, which in after ages condensed into liquids and solids. So intense was the heat at that time, that substances probably existed as elements instead of compounds, i.e. the temperature was above the point of dissociation. We have seen that Al2O3, CaO, SiO2, etc., are dissociated at the highest temperatures only. If the temperature were above that of combination, compounds could not exist as such, but matter would exist in its elemental state. On slowly cooling, these elements would combine. It is, then, a fair inference that such compounds as need the highest temperatures to separate them, as silica, silicates, and some oxides, were formed from their elements at a much earlier stage of the earth's history than were those compounds that are more easily separable, such as water, lead sulphide, etc., and that the most infusible substances were solidified first.

341. Evolution.--As the earth slowly cooled, elements united to form compounds, gases condensed to liquids, and these to solids.

At one time the entire surface of our planet may have been liquid. When the cooling surface reached a point somewhat below that of boiling water, the lowest forms of life appeared in the ocean. This was many millions of years ago. Most scientists believe that all vegetable and animal life has developed from the lowest forms of life. There is also a theory that all chemical elements are derivatives of hydrogen, or of some other element, and that all the so-called elements are really compounds, which a sufficiently high temperature would dissociate. As evidence of this, it is said that less than half as many elements have been discovered in the sun as in the earth, and that comets and nebula, which are less developed forms of matter than the sun, have a few simple substances only.

It is easy to fancy that all living bodies, both animal and vegetable, are only natural growths from the lowest forms of life; that these lowest forms are a development, with new manifestations of energy, from inorganic matter; that compounds are derived from elements; and that the last are derivatives of some one element; but it must be borne in mind that this is only a theory.

342. New Theory of Chemistry. We have seen that heat lies at the basis of chemical as well as of physical changes. By the loss of heat, or perhaps by the change of potential into kinetic energy, in a nebulous parent ma.s.s, planets were formed, capable of supporting living organisms. Heat changes solids to liquids, and liquids to gases; it resolves compounds, or it aids chemical union. In every chemical combination heat is developed; in every case of dissociation heat is absorbed. Properly written, every equation should be: a + b = c + heat; e.g. 2 H + 0 = H2O + heat; or, c - a = b - heat; e.g. H2O - 2 H = 0 - heat. Another ill.u.s.tration is the combination of C and O, and the dissociation of CO2, as given on page 82. C + O2 = CO2 + energy. CO2 - O2 = C - energy. In fact, there are indications that the present theory of atoms and molecules of matter, as the foundation of chemistry, will at no distant day give place to a theory of chemistry based on the forms of energy, of which heat is a manifestation.

Chapter, LXII.

GAS VOLUMES AND WEIGHTS.

343. Oxygen.

Experiment 134.--Weigh accurately, using delicate balances, 5 g.

KClO3, and mix with the crystals 1 or 2 g. of pure powdered MnO2.

Put the mixture into a t.t. with a tight-fitting cork and delivery-tube, and invert over the water-pan, to collect the gas, a flask of at least one and a half liters' capacity, filled with water. Apply heat, and, without rejecting any of the gas, collect it as long as any will separate.

Then press the flask down into the water till the level in the flask is the same as that outside, and remove the flask, leaving in the bottom all the water that is not displaced. Weigh the flask with the water it contains; then completely fill it with water and weigh again.

Subtract the first weight from the second, and the result will evidently be the weight of water that occupies the same volume as the O collected. This weight, if expressed in grams, represents approximately the number of cubic centimeters of water,--since 1 cc. of water weighs lg,--or the number of cubic centimeters of O.

At the time the experiment is performed the temperature should be noted with a centigrade thermometer, and the atmospheric pressure with a barometer graduated to millimeters.

Suppose that we have obtained 1450 cc. of O, that the temperature is 27 degrees, and the pressure 758 mm.; we wish to find the volume and the weight of the gas at 0 degrees and 760 mm.

According to the law of Charles--the volume of a given quant.i.ty of gas at constant pressure varies directly as the absolute temperature. To reduce from the centigrade to the absolute scale, we have only to add 273 degrees. Adding the observed temperature, we have 273 degrees + 27 degrees = 300 degrees. Applying the above law to O obtained at 300 degrees A, we have the proportion below. Since the volume of O at 273 degrees will be less than it will at 300 degrees, the fourth term, or answer will be less than the third, and the second term must be less than the first. 300 : 273 :: 1450 : x. This would give the result dependent upon temperature alone.

By the law of Mariotte - Physics, - the volume of a given quant.i.ty of gas at a constant temperature varies inversely as the pressure. Applying this law to the O obtained at 758mm, we have the following proportion. The volume at 760mm will be less than at 758mm; or the fourth term will be less than the third; hence the second must be less than the first. 760: 758:: 1450: x. This would give the result dependent on pressure alone.

Combining the two proportions in one:--

300: 273 ):: 1450: x = 1316cc.

760: 758 )

1316cc=1.316 liters. It remains to find the weight of this gas. A liter of H weighs 0.0896g. The vapor density of O is 16. Hence 1.316 liters of O will weigh 1.316 X 16 X 0.0896 =1.89g.

(KClO3 = KCl + O3) From the equation (122.5 48) we make a proportion, ( 5 x)

122.5: 5:: 48: x = 1.95, and obtain, as the weight of O contained in 5g of KClO3, 1.95g. The weight we actually,obtained was 1.89g. This leaves an error of 0.06g, or a little over 4 per cent of error (0.06 / 1.95 = 0.03 +). The percentage of error, in performing this experiment, should fall within 10.

Some of the liabilities to error are as follows:--