Part 11 (1/2)
But we shall better learn the value of some of these discoveries by taking a general review of the contributions to chemical science of the man who spent most of his life at work in that room in Stockholm.
The German chemist Richter, in the first or second year of this century, had drawn attention to the fact that when two neutral compounds, such as nitrate of potash and chloride of lime, react chemically, the substances produced by this reaction are also neutral. All the potash combined with nitric acid in one salt changes places with all the lime combined with muriatic acid in the other salt; therefore, said Richter, these different quant.i.ties of potash and lime are neutralized by the same quant.i.ty of nitric acid; and, hence, these amounts of potash and lime are chemically _equivalent_, because these are the amounts which perform the same reaction, viz. neutralization of a fixed quant.i.ty of acid. If then careful a.n.a.lyses were made of a number of such neutral compounds as those named, the _equivalents_ of all the commoner ”bases” and ”acids”[9] might be calculated.
Richter's own determinations of the equivalents of acids and bases were not very accurate, but Berzelius was impressed with the importance of this work. The year before the appearance of Dalton's ”New System” (_i.e._ in 1807), he began to prepare and carefully a.n.a.lyze series of neutral salts.
As the work was proceeding he became acquainted with the theory of Dalton, and at once saw its extreme importance. For some time Berzelius continued to work on the lines laid down by Dalton, and to acc.u.mulate data from which the atomic weights of elements might be calculated; but he soon perceived--as the founder of the theory had perceived from the very outset--that the fundamental conception of each atom of an element as being a distinct ma.s.s of matter weighing more or less than the atom of every other element, and of each atom of a compound as being built up of the atoms of the elements which compose that compound,--Berzelius, I say, perceived that these conceptions must remain fruitless unless means were found for determining the number of elementary atoms in each compound atom.
We have already learned the rules framed by the founder of the atomic theory for his guidance in attempting to solve this problem. Berzelius thought those rules insufficient and arbitrary; he therefore laid down two general rules, on the lines of which he prosecuted his researches into chemical synthesis.
”One atom of one element combines with one, two, three, or more atoms of another element.” This is practically the same as Dalton's definitions of binary, ternary, etc., compounds (p. 132). ”Two atoms of one element combine with three and five atoms of another element.” Berzelius here recognizes the existence of compound atoms of a more complex structure than any of those recognized by Dalton.
Berzelius further extended the conception of atom by applying it to groups of elements formed, according to him, by the combination of various compound atoms. To his mind every compound atom appeared as built up of two parts; each of these parts might be an elementary atom, or might be itself built up of several elementary atoms, yet in the Berzelian theory each acted as a definite whole. So far as the building up of the complex atom went, each of the two parts into which this atom could be divided acted as if it were a simple atom.
If we suppose a patch of two shades of red colour to be laid on a smooth surface, and alongside of this a patch of two shades of yellow colour, and if we suppose the whole ma.s.s of colour to be viewed from a distance such that one patch appears uniformly red and the other uniformly yellow, we shall have a rough ill.u.s.tration of the Berzelian compound atom. To the observer the whole ma.s.s of colour appears to consist of two distinct patches of contrasted colours; but let him approach nearer, and he perceives that what appeared to be a uniform surface of red or yellow really consists of two patches of unlike shades of red or of yellow. The whole ma.s.s of colour represents the compound atom; broadly it consists of two parts--the red colour represents one of the const.i.tuent atoms, the yellow colour represents the other const.i.tuent atom; but on closer examination the red atom, so to speak--and likewise the yellow atom--is found to consist of parts which are less unlike each other than the whole red atom is unlike the whole yellow atom.
We shall have to consider in more detail the reasoning whereby Berzelius arrived at this conception of every compound atom as a _dual_ structure (see pp. 209-212). At present I wish to notice this conception as lying at the root of most of the work which he did in extending and applying the Daltonian theory. I wish to insist on the fact that the atomic theory could not advance without methods being found for determining the number of elementary atoms in a compound atom, without clear conceptions being gained of every compound atom as a structure, and without at least attempts being made to learn the laws in accordance with which that structure was built.
Before the atomic weight of oxygen could be determined it was necessary that the number of oxygen and of hydrogen atoms in the atom of water should be known; otherwise all that could be stated was, the atomic weight of oxygen is a simple multiple of 8. Berzelius did much to advance chemical science by the introduction and application of a few simple rules whereby he determined the number of elementary atoms in various compound atoms. But as the science advanced, and as more facts came to be known, the Berzelian rules were found to be too narrow and too arbitrary; chemists sought for some surer and more generally applicable method than that which Berzelius had introduced, and the imperious demand for this method at last forced them to recognize the importance of the great generalization of the Italian naturalist Avogadro, which they had possessed since the year 1811, but the meaning of which they had so long failed to understand.
Berzelius made one great step in the direction of recognizing Avogadro's distinction between atom and molecule when he accepted Gay-Lussac's generalization that ”equal volumes of gases contain equal numbers of atoms:” but he refused to apply this to other than elementary gases. The weights of the volumes of elementary gases which combined were, for Berzelius, also the weights of the atoms of these elements. Thus, let the weight of one volume of hydrogen be called 1, then two volumes of hydrogen, weighing 2, combine with one volume of oxygen, weighing 16, to form two volumes of water vapour; therefore, said Berzelius, the atom of water consists of two atoms of hydrogen and one atom of oxygen, and the atom of the latter element is sixteen times heavier than the atom of the former.
Three volumes of hydrogen, weighing 3, combine with one volume of nitrogen, weighing 14, to form two volumes of ammonia; therefore, said Berzelius, the atom of ammonia consists of three atoms of hydrogen combined with one atom of nitrogen, and the nitrogen atom is fourteen times heavier than the atom of hydrogen.
While Berzelius was applying these rules to the determination of the atomic weights of the elements, and was conducting the most important series of a.n.a.lyses known in the annals of the science, two great physico-chemical discoveries were announced.
In the year 1818 the ”_law of isomorphism_” was stated by Mitscherlich: ”Compounds the atoms of which contain equal numbers of elementary atoms, similarly arranged, have the same crystalline form.” As thus stated, the law of isomorphism affirms that if two compounds crystallize in the same form, the atoms of these compounds are built up of the same number of elementary atoms--however different may be the nature of the elements in the compounds--and that these elementary atoms are similarly arranged. This statement was soon found to be too absolute, and was accordingly modified; but to go into the history of the law of isomorphism would lead us too far from the great main path of chemical advance, the course of which we are seeking to trace.
Berzelius at once accepted Mitscherlich's law, as an aid in his researches on atomic weights. The help to be derived from this law may be ill.u.s.trated thus: let us a.s.sume that two compounds have been obtained exhibiting ident.i.ty of crystalline form; let it be further a.s.sumed that the number of elementary atoms in the atom of one of these compounds is known; it follows, by the law of isomorphism, that the number of elementary atoms in the atom of the other is known also. Let the two compounds be _sulphate of potash_ and _chromate of potash_; let it be a.s.sumed that the atom of the first named is known to consist of two atoms of pota.s.sium, one atom of sulphur, and four atoms of oxygen; and that the second substance is known to be a compound of the elements pota.s.sium, chromium and oxygen; then the atom of the second compound contains, by Mitscherlich's law, two atoms of pota.s.sium, one atom of chromium and four atoms of oxygen: hence the relative weight of the atom of chromate of potash can be determined, and hence the relative weight of the atom of chromium can also be determined.
A year after the announcement of Mitscherlich's law, the following generalization was stated to hold good, by two French naturalists, Dulong and Pet.i.t:--”The atoms of all solid elements have the same capacity for heat.”
If the amount of heat required to raise the temperature of one grain of water through one degree be called _one unit of heat_, then the capacity for heat of any body other than water is the number of units of heat required to raise the temperature of one grain of that substance through one degree. Each chemical substance, elementary and compound, has its own capacity for heat; but, instead of comparing the capacities for heat of equal weights, Dulong and Pet.i.t compared the capacities for heat of weights representing the weights of the atoms of various elements. Thus, equal amounts of heat are required to raise, through the same interval of temperature, fifty-six grains of iron, one hundred and eight grains of silver, and sixty-three and a half grains of copper; but the weights of the atoms of these three elements are in the proportion of 56:108:63-1/2.
Dulong and Pet.i.t based their generalization on measurements of the capacities for heat of thirteen elements; further research has shown that their statement most probably holds good for all the solid elements. Here then was a most important instrument put into the hands of the chemist.
It is only necessary that the atomic weight of one solid element should be certainly known, and that the amount of heat required to raise through one degree the number of grains of that element expressed by its atomic weight should also be known; then the number which expresses the weight, in grains, of any other solid element which is raised through one degree by the same amount of heat, likewise expresses the relative weight of the atom of that element. Thus, suppose that the atomic weight of silver is known to be 108, and suppose that six units of heat are required to raise the temperature of one hundred and eight grains of this metal through one degree; then suppose it is found by experiment that six units of heat suffice to raise the temperature of two hundred and ten grains of bis.m.u.th through one degree, it follows--according to the law of Dulong and Pet.i.t--that 210 is the atomic weight of bis.m.u.th.
The modified generalization of Gay-Lussac--”Equal volumes of _elementary_ gases contain equal numbers of atoms;” the laws of ”isomorphism” and of ”atomic heat;” and the two empirical rules stated on p. 163;--these were the guides used by Berzelius in interpreting the a.n.a.lytical results which he and his pupils obtained in that memorable series of researches, whereby the conceptions of Dalton were shown to be applicable to a wide range of chemical phenomena.
The fixity of composition of chemical compounds has now been established; a definite meaning has been given to the term ”element;” the conception of ”atom” has been gained, but much remains to be done in the way of rendering this conception precise; and fairly good, but not altogether satisfactory methods have been introduced by which the relative weights of the atoms of elements and compounds may be determined. At this time chemists are busy preparing and describing new compounds, and many new elements are also being discovered; the need of cla.s.sification begins to be felt more and more.
In the days of Berzelius and Davy strenuous efforts were made to obtain some generalizations by the application of which the many known elements and compounds might be divided into groups. It was felt that a cla.s.sification might be founded on the composition of compounds, or perhaps on the properties of the same compounds. These two general principles served as guides in most of the researches then inst.i.tuted; answers were sought to these two questions: Of what elements is this compound composed?
and, What can this compound do; how does it react towards other bodies?
Lavoisier, as we know, regarded oxygen as the characteristic element of all _acids_. This term _acid_ implies the possession, by all the substances denoted by it, of some common property; let us shortly trace the history of this word in chemistry.
Vinegar was known to the Greeks and Romans, and the names which they gave this substance tell us that sourness was to them its characteristic property. They knew that vinegar effervesced when brought into contact with chalky earths, and that it was able to dissolve many substances--witness the story of Cleopatra's draught of the pearl dissolved in vinegar. Other substances possessed of these properties--for instance oil of vitriol and spirits of salt--as they became known, were cla.s.sed along with vinegar; but no attempts were made to clearly define the properties of these bodies till comparatively recent times.
The characteristics of an acid substance enumerated by Boyle are--solvent power, which is exerted unequally on different bodies; power of turning many vegetable blues to red, and of restoring many vegetable colours which had been destroyed by alkalis; power of precipitating solid sulphur from solutions of this substance in alkalis, and the power of acting on alkalis to produce substances without the properties of either acid or alkali.
But what, one may ask, is an alkali, of which mention is so often made by Boyle?
From very early times it had been noticed that the ashes which remained when certain plants were burned, and the liquid obtained by dissolving those ashes in water, had great cleansing powers; that they removed oily matter, fat and dirt from cloth and other fabrics. The fact that an aqueous solution of these ashes affects the coloured parts of many plants was also noticed in early times. As progress was made in chemical knowledge observers began to contrast the properties of this plant-ash with the properties of acids. The former had no marked taste, the latter were always very sour; the former turned some vegetable reds to blue, the latter turned the blues to red; a solution of plant-ash had no great solvent action on ordinary mineral matter, whereas this matter was generally dissolved by an acid. In the time of the alchemists, who were always seeking for the principles or essences of things, these properties of acids were attributed to _a principle of acidity_, while the properties of plant-ash and substances resembling plant-ash were attributed to a _principle of alkalinity_ (from Arabic _alkali_, or _the ash_).