Page:The New International Encyclopædia 1st ed. v. 04.djvu/642

* CHEMISTRY. 560 CHEMISTBT. pirival laic. Thus, the iirinciplc of the consen-a- tion of matter is an eiiipiiical law. Perhaps this law may suggest itself « priori; but as a law of science it has been induced from facts es- tablisliod by the balance. On the other liand, there are problems which cannot be attacked by experiment. Thus, the problem of the ulti- mate structure of matter lies far beyond our power of direct observation : yet it is intimately connected with the correlation of substances, and tbeicfore cheraistri- is compelled to consider it for purely practical reasons. In cases of tliis nature, cbemistiy, like any other science, and like speculative philosopliy, makes some plaus- ible assumption, termed a hypothesis. Like speculative philosophy, it develops the hypotlie- sis, combines it, if necessary, with other assump- tions, and thus builds up a theoi'i/. But at this point, where speculative research reaches its noil plus ultra, the work of the scientist really begins. The general principles forming part of the theoiy are busily applied to phenomena capa- ble of direct observation, and then, if their cor- rectness is indicated by actmil experiment, they become theoretical laws. A scientific theory has for its object, first, to correlate seemingly difl'erent facts, and, secondly, to throw light on the road of invest igiition and lead to the estab- lishment of new facts. Thus, the atomic theory- of chemistry has correlated the various chemical substances with regard to their composition and constitution, and it has revealed the possible existence of innumerable compounds many of which have since been actually prepared — an achievement not unlike the discovers- of Xei)tune by theoretical astronomy. SUBSTAKCES. The Cuemicai, Element.s. The ultimate de- composition of all known forms of matter has produced over seventy substances which could not be further decomposed, and which are there- fore regarded as fonning the qualitative basis of all things. None of the substances which we term elements has been broken up into diflferent constituents or changed without gain of weiglit into some other substance, in any of the innu- merable processes ever employed by man. More- over, spectroscopic study of the sun and the stars shows that the elements of the earth exist un- decomposed also in the heavenly bodies, in spite of the high temperatures prevailing on many of them. Of course it seems strange, and certainly does not apjK'al to our imagination, that the universe should be made up of several kinds of matter essentially dilTerent from one anotlier. We -should like to think of the elements as repre- senting outwardly different forms of .some m- inary matter pervading all space and making up all things. As a matter of speculation, views of tliis nature are even now held by many emi- nent chemists, although no actual step has yet been taken in the direction of making them part of science. Yet the view that the chem- ical elements are related to one another in some mysterious way is supported not only by specula- tion, but also by the fact, long known, that cer- tain elements are quite similar in their proper- ties. Chemists have well known for many de- cades how very closely chlorine, bromine, and iodine resemble one another. Sodium and potas- sium, too, are so much alike that for most pur- poses it makes no difference whether we use the hydroxide of the one (caustic soda) or the hydroxide of the other (caustic potash). A large number of facts of this nature have been systematized by JIcndelfefT and Lothar Meyer, who have shown (180!)) that not only the ele- ments just mentioned, but the rest of the ele- ments, too, can be arranged in groups, the members of which are quite similar in their ]>roperties. This remarkable relation, extending as it does to all the well-known elements, can hardly be considered accidental ; it must have a cause, and that cause must lie in some hidden relationship uniting the elements themselves, and at the same time rendering them quite dis- tinct from all other forms of matter — i.e. from substances that can be decomposed by the ordi- nary physical and chemical agencies. See Peri- odic Law. Chemical Co.mi'OUNds. The conception of the qualitative unity of a chemical element defines clearly the distinction between elementary sub stances on the one hand and compounds and mixtures on tlie otlier. The distinction between a chemical compound and a mixture is not easy to define pi-ecisely and generally. But before we proceed to formulate this latter distinction, it may be well to consider two simple cases illus- trating the formation of true chemical com- pounds. First, when hydrogen and oxygen are mixed in certain proportions at ordinary temi)eratures, a gas is obtained which still exhibits the prop- erties of the two constituents, each of which be- haves just as if the other was not present. If the walls of the vessel containing them are por- ous, both diffuse out. but the hydrogen dilTuses out more rapidly : and so it oukl be if the two gases were confined in porous vessels separately. Now, if we should apply to otir mixture sutli- cient heat, a remarkable change would suddenly set in — an explosicm would take i>lace, the hydro- gen as well as the oxygen would cease to exist as such, and a new substance (water) would be found in their place. By suitable methods (e.g. by the use of a g-alvanic current) water may be decomposed into hydrogen and oxygen; bu* ".vbile it exists as water, it has a set of properties all its own, and does not exhibit any of the prop- erties of either hydrogen or oxygen. Secondbj, if finely divided iron and finely di- vided sulphur should be carefully mixed in cer- tain proportions and left at ordinary tempera- tures for any length of time, the two would con- tinue alongside each other, iron as iron, sulphur as sulphur. . microscope would show two difTer- ent kinds of ])articles. A magnet would separate out the iron and leave the sulphur. But, again, if we should heat the mixture, a ehange would take place, accompanied by an evolution of heat and light, and. as a result, we would find a substance (sulphide of iron) which has none of the prop- erties of either sulphur or iron, although these substances may be obtained from it by suitable methods of decomposition. It is therefore clear that a chemical compound is an 'iiKlividual' with properties peculiar t/) it- self and different from those of its com|)onents : in a mixture, each constituent retijins its own individuality, and may be recognized by its own properties. A compound is invariably found to be homogeneous, even if examined with a power- ful microscope; a mixture may be homogeneous, us in the case of hydrogen and oxygen, or it may be heterogeneous, as in the case of iron and