Graphic formulas often have an abbreviated form: the dashes are omitted. the atom are combined into groups, and these an. written no that their relative arrangement in the mole cule may be evident. Such abbreviated expres sions are usually employed. for convenience' sake. in preference to the full graphic formulas. Thus, acetic acid is represented by the formula Y1I,.CO.( )11 (or simply ClIC0011, or ) : methyl formate is represented by the formula 11.01.0CI ,; etc. Remembering the valencies pe culiar to the constituent elements, the chemist ha- no difficulty in reconstructing the graphic formulas from abbreviations of this kind. There is, however, another kind of constitutional for mulas, which may be described as incomplete, or imperfect, because they are made lip of atomic groups which 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.
NiXTUIRES. 'We have seen in a preceding para graph that while the properties of a chemical are quite different from those of its constituent elena.nts, the properties of a mix ture are made up by the alligat ion of those of the components. This is true in the ease of all gaseous mixtures, and one of the general laws of gases is that the pressure in a vessel containing sev 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 ('physical') mixtures in the solid or liquid state, the principle of the `additivity of properties' is only a rough ap proximation, for in 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 important chapters of physical chemistry will be discussed in the article SOLUTIoN.
TEA NSFOn 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. Changes of this kind are termed chemical react imm. On the other hand, there are changes of matter that are much less pro found in their character and involve neither the disappearance of the given nor the formation of new chemical substances. Snell changes (mostly changes of state I are termed physical trans formations. Thus, for 4.xample, 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 :IS well as with ehemical transformations, mainly because the former often influence and accompany the latter. Following are the laws governing, the various of matter: (1) Cossmtv.yrrox or :\IAss. No transforma tion is known to involve gain or loss of the mass of matter. When. for example. a candle burns lip in the air, its material is not lost; it merely combines with the oxygen of the air to form two invisilde products—water vapor and carbonic acid; but these, too, are matter, for they have weight, and their mass is precisely equal to the original mass of the candle. phis the
mass of oxygen consumed. Ilence 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 ill spite of the violent undoubted ly taking Wave in the sun, its weight has not in the least changed within historical times; for an appreciable change ill the mass of the sun would have involved a change in the length of the day. and such a change is positively known not to have taken place. (If course, the main evidence in favor of the principle of conservation is presented by the innumerable quantitative processes actually employed by chemists. Be sides. al-curate investigations ha ye been insti tuted for the special purpose of testing the pre cision of the principle, and have invariably failed to prove it incorrect.
(2) CON sEnvATIoN m"ruk; ELEMENTS. No transformation is known to involve the transmu tation of one chemical element into another. 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 ele ment mav exist either free or in a state of chemical combination With other elements, its total mass in the universe is unehangeable.
(31 Coins I NO QUANTITIES. While We can mix substances in any desired proportion, chem ical combination call 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 I part by weight of the former to S parts IT weight of the latter; or, what is the same, they combine entirely when the volume of hydrogen is twice as great as the volunle of oxygen (oxygen is 16 times as heavy 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 that only S 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. the conception of 'combining quantities.' Early in the Nineteenth Century, t;ay 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 emnbine in the ratio of 2 volumes of the former to 1 of the latter (the product is 2 volumes of water). Tn 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 celebrated hypothesis. in accordance with which we explain the fact that 2 vohnnes of hydrogen react with 1 of oxygen, by saying that every 2 molecules of the former react with every 1 PIO/CC/de of the latter.