網頁圖片
PDF
ePub 版

and has now been accepted by most chemists. These new atomic weights not only greatly simplify Dulong and Petit's law, but are also in harmony with many other facts, most of which were observed after the change had been made. Thus the formulæ of corrosive sublimate, bichloride of tin, and zinc methyl are, according to the old system, HgCl ; SnCl2 ; and ZnC,H, ; and H=1. According to the new system, they are HgCla; SnCl. ; and Zn(CH3)2.

It will be at once observed that the second set of formulæ represent just twice the quantity represented by the first; now the second formulæ express the molecular weights of the substances according to Avogadro's law (see ATOMIC THEORY). Further, if we adopt the old atomic weights, we see no reason why oxide of lead should readily form basic salts, while oxide of silver does not. This peculiarity is to some extent explained by the new atomic weights; thus we have nitrate of silver-old formula AgNO, new formula AgV03; nitrate of lead-old formula PbN06, new formula Pb(NO3)2; basic nitrate of lead-old formula PbO,PbN06, new formula Pb2O(NO3)2. The contrast will be better seen if we put the new formulæ into a graphic form, Nitrate of Silver. Nitrate of Lead.

Basic Nitrate of Lead.

[ocr errors][ocr errors][ocr errors]

We at once see the connection between the dyad character of lead (Pb), and the occurrence of basic salts.

Similarly let us notice the effect of treating potassium oxide and calcium oxide with equal quantities of hydrochloric acid. The result in one case is two molecules of potassium chloride and one molecule of water ; while in the other, it is one molecule of calcium chloride and one molecule of water. Putting the symbols in graphic form we have:

HCI
K)
KCI

H)
+ ço =

t _{0,
HCI

КСІ
HCI

(

CH)
+ CaO =
HCI

[ocr errors]
[ocr errors]
[ocr errors]

Where we see why we have, in the first case, two molecules of potassium chloride, and in the second, one molecule of calcium chloride, the reason being that calcium is a dyad, and one atom of it has the combining capacity of two atoms of potassium. Many other examples might be given, but these may suffice as an indication of the reasons which have induced chemists to prefer the atomic weights given in the second column in the table in the article ATOMIC WEIGHTS.

Assuming, then, these atomic weights, let us return to the subject of Chemical Structure. This may be defined in various ways, but most conveniently as the indication by a graphic formula, or something equivalent to it, of all the chemical changes by which the substance can be formed or decomposed. This will best be illustrated by means of a few examples, and we shall select these from among organic compounds, that is. compounds of carbon, because the structure of these compounds has been most fully investigated.

Acetic acid has (on the new system, which will be exclusively used in the remainder of this article) the formula C,H,Og. If it is treated with caustic potash, it yields acetate of potash according to the equation C,H,O, + KHO = C,H,KO, +1,0. Here one atom of hydrogen has been replaced by one atom of potassium, and we find that further treatment with caustic potash does not cause any further replacement of hydroyen by potassium. We may therefore write the formula of acetic acid thus : H-(C 11,0,). and this formula indicates the replaceability of one atom of hydrogen by metal, and explains (as far as such formulæ can explain anything) the occurrence of such compounds as acetate of lead (C2H30,)-Pb-(C,H,0,), and all the other acetates. The question now remains, what is the structure of the group C,H,02, which is united in acetic acid to hydrogen, and in the acetates to metal?

To answer it we must examine some other reactions of acetic acid. When treated with pentachloride of phosphorus, it loses an atom of oxygen, the place of which is taken by two atoms of chlorine—the pentachloride of phosphorus taking the oxygen in exchange for the chlorine; but instead of obtaining a compound (C,H,OCI,), we find that the result is expressed by the equation: C,H,O,+ PC1, = C,H,OCl + HCl + POC),. We thence conclude that in acetic acid the atom of dyad oxygen removed in the action given above was united to an atom of hydrogen, and to the group C,H,O, and represent the change thus:

[ocr errors][ocr errors][ocr errors]

the replacement of the dyad oxygen by two atoms of the monad chlorine necessitating the falling asunder of the compound. The reactions of chloride of acetyl (C,H,OCI) lead us to the further conclusion that the atom of hydrogen replaceable by metal is the atom not present in chloride of acetyl, so that the formula (C,,01–0-A is a fuller and more explanatory form of (C,H,O,)-H. Again, if we heat acetate of potash with caustic potash, we have marsh gas (CH4) given off, and the residue consists of carbonate of potash.

н

[ocr errors][ocr errors][merged small][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors]
[ocr errors]
[ocr errors]

the dotted lines separating in the diagram the symbols of the parts of the molecules which change places.

We have considered only a few of the reactions of acetic acid, but the formula just given is equally consistent with all the others. It is therefore said to exhibit the structure of acetic acid. This word “structure” is perhaps a little misleading—we must recollect the precise sense in which it is used, as a concise representation of many reactions. It is conceivable that it may have some relation to the actual relative position of the atoms in a molecule of acetic acid, but we have not as yet any means of ascertaining whether this is so or not.

We may illustrate the meaning of chemical structure further by a somewhat more complex case. Asparagine, a colorless crystalline substance extracted from asparagus, and also from the blanched shoots of other plants, has the composition expressed by the formula C,H.N,0g. When treated with caustic potash it yields ammonia and a body called aspartate of potash--the potash salt of aspartic acid. The change is obviously

(H an exchange of K-O- and -N and may be thus indicated :

(H

[ocr errors][ocr errors][ocr errors][ocr errors]
[graphic][subsumed][subsumed][subsumed][subsumed][ocr errors][ocr errors][ocr errors][subsumed][ocr errors][subsumed][graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][merged small]

CHEMISTRY.–1. Pipette. 2. Trituration tubes. 3. Burette. 4. Liebig's potash apparatus. 5. Appan,

cium tube. 8. Determination of nitrogen. 9. Production of phosphate of calcium. 10. Produc. sulphuric acid gas. 15. Production of phosphoric acid gas ; 16, of sulphuric acid ; 17, of annon

[graphic][subsumed][ocr errors][graphic][graphic][ocr errors][ocr errors][subsumed]

atus for organic analysis with carbon. 6. Gas-stove for organic analysis. 7. Chloride of calstion of nitrogen ; 11, of sulphurated hydrogen. 12, 13. Test for arsenic. 14. Decomposition of ia. 18. Interior of a laboratory.

« 上一頁繼續 »