Page:The New International Encyclopædia 1st ed. v. 04.djvu/646

CHEMISTRY. 564 CHEMISTRY. form : the dashes are omitted, the atoms ai'e comliined into groups, and these are written so that their relative arranfjcmeul in the niole- <!ule may he evident. Such abbreviated expres- sions are" usually employed, for convenience' sake, in preference to the full graphic formulas. Thus, acetic acid is represented bv the formula CII3.CO.OH (or simply CH,COOH. or CH3CO.fI ) : methyl formate is represented by the fornuila H.C().OCrL, ; etc. Remembering the valencies pe- culiar to the constituent elements, the chemist has no <iiHiculty in reconstructing the graphic formula-s from abbreviations of this kind. There is, however, another kind of constitutional for- mulas, which may I>e described as incomplete, or imperfect, because they are made up of atomiir group.s vvhicli can be represented, not by only one, but by two or more different graphical schemes. Such formulas are assigned to com- pounds when we do not know enough about their chemical nature. lIixriJRE.s. We have seen in a preceding para- grai)h that while the properties of a chemical compound are (piite dillVrcnt from those of its constituent elements, the properties of a mix- ture are made up by the alligation of those of the components. This is true in the case of all gaseous mixtures, and one of the general laws of gases is that the pressure in a vessel containing sev- eral gases is equal to the sum of the pressures that would be exerted by them if each was iso- lated in a similar vessel (Dalton's law). But in the case of homogeneous ("physiear) mi.xtures in the scdid or liquid state, the principle of the 'additivity of properties' is only a rough ap- proximation, for in such mixtures the properties of each component are often considerably affected by the presence of the other components. A class of mixtures whose theory forms one of the most ivnportant chapters of physical chemistry will be discussed in the article Solution. TR.VNSFOUJI.VTIONS. We have seen above that when heat is applied to a mixture of hydrogen and oxygen, or to a mixture of powdered iron and sulphur, changes set in which result in the formation of entirely new substances — water and sulphide of iron respectively. Clianges of this kind are temied chemical rraclioiis. On the other hand, there are changes of matter that are nuich less ])ro- found in their character and involve neither the disappearance of the given nor the, formation of new chemical substances. Such changes (mostly changes of state) are termed physical trans- formations. Thus, for example, the mere evap- oration of water may be spoken of as a physical transformation, because liquid water and water vapor are chemically identical. The science of chemistry deals with physical as well as with chemical transformations, mainly because the former often inlluence and accompany the latter. Following are (he laws governing the various transformations of matter: (1) CoN.sERVATioN OF M.ss. No transforma- tion is known to involve gain or loss of the mass of matter. When, for example, a candle burns up in the air, its material is not lost: it merely combines with the oxygen of the air to form two invisilile products — water vapor and carbonic acid : but these, too, are matter, for they have weight, and their mass is precisely equal to the original nfass of the candle, plus the mass of oxygen consumed. Hence the inductive principle known as the law of the indestructi- bility or conservation of mass. A strong argu- ment in favor of this law is found in the fact that in spit<' of the violent i)r<x'esses undoubted- ly taking place in the sun, its weight has not in the least changed within historical times: for an appreciable change in the mass of the sun would have involved a change in the length of the day, and such a change is positively known not 10 have taken place. Of course, the main evidence in favor of the principle of conservation is presented by the imuimerable quantitative proi'esses actually employed by chemists. Be- sides, accurate investigations have been insti- tuted for the special puri>i>se of testing the pre- cision of the principle, and have invariably failed to prove it incorrect. (2) Conservation of the Elements. Xo transformation is known to involve the transnui- tation of one chemical element into another. Combining this with the preceding principle, we get what is known as the law of conservation of the elements. According to this, no transforma- tion involves gain or loss of the mass of each of the chemical elements, and hence, while an cle- ment may exist either free or in a state of chemical combination with other elements, its total mass in the universe is inchangeable. (3) CoMiii.MNO Quantities. While we can mix substances in any desired |)roportion, chem- ical combination can only take place between certain definite relative quantities, which de- pend on the nature of the reacting substances. Thus, hydrogen and oxygen combine in the pro- portion of 1 part by weight of the former to 8 parts by weight of the latter; or, what is the same, they combine entirely when the volume of hydrogen is twice as great as the volume of oxygen (oxygen is 10 times as heaw as hydro- gen). If, instead of these relative quantities, we should mix. say, 1 part by weight of hydrogen with 9 parts of oxygen, we would still find tJiat only 8 parts of the latter have combined with all of the hydrogen into water, and that 1 part of oxygen has remained uncombined: the ease would be analogous if, instead of an excess of oxygen, we employed an excess of hydrogen. Hence, the conception of 'combining quantities.' Early in the Nineteentli Century, (iay Lussac discovered a remarkable fact — viz, that the rela- tive combining volumes of gases can in all cases be expressed in the form of simple arithmetical ratios. We have just seen that hydrogen and oxygen combine in the ratio of 2 volumes of the former to 1 of the latter ( the product is 2 volumes of water). In the case of the reacting pair, hy- drogen and chlorine, the ratio is still simpler, 1 volume of hydrogen combining with 1 volume of chlorine (the product is 2 volumes of hydro- chloric acid). It was this general fact that sug- gested to Avogadro his c<"lebrated hypothesis, in accordance with which we explain the fact that 2 volumes of hydrogen react with 1 of oxygen, by saying that every 2 molcrulfs of the fonner react with every 1 molecule of the latter. Chemical Equ.vtions. The three principles just stated are expressed syndiolically in those equations which chemists use to represent the various reactions of substances. Take, for ex- ample, the equation representing the combustion of marsh gas — that is, its combination with oxvgen — viz. : CH. + 20, = CO. + 2H,0