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It is observable that Kopp makes the oxide of silver to differ from the oxides of lead and mercury, and the chloride of mercury to differ from the chlorides of lead and silver. It is questionable how far, in a subject confessedly but approximative, Kopp was justified in assigning different atomic volumes to the same element when forming perfectly analogous compounds.

Chloride of mercury is a somewhat insoluble salt, chloride of lead still more insoluble, chloride of silver one of the most insoluble salts with which we are acquainted. All three chlorides, however, are much more soluble in solutions of alkaline chlorides, owing to the formation of crystallizable double salts. The mercurial double salts are well known. Wetzlar and Becquerel have described a very definite chloride of silver and sodium. The sulphates, iodides, and most other protosalts of the three metals show a marked resemblance in their characters. The metals of this group are remarkable for the facility with which they replace the hydrogen of neutral and faintly acid organic compounds. The only marked discrepancy, in addition to the difference in the atomic heats, consists in the varied behaviour of the oxides and hydrates with solutions of caustic alkali; lead being soluble in potash, silver in ammonia, and mercury in neither solvent. This group is associated

1. With the sodic group, by the analogy of sulphate of soda to sulphate of silver, and by the circumstance of silver having the same atomic heat as sodium and potassium.

2. With the calcic group, by the very general isomorphism of plumbic with calcic and barytic salts.

3. With the ferric group, by the general analogy of cuprous and mercurous salts.

Moreover, we have several illustrations of a multiple isomorphism of the plumbic with the above three groups.

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*This last series of empirical formulæ illustrates curiously the parity of relations subsisting between those monobasic, bibasic, and terbasic salts which have the same number of oxygen atoms. That this isomorphism is

The not unfrequent isomorphism of unequal numbers of atoms of different elements has been before alluded to*.

GROUP XIII.

Palladium, Platinum,-Gold.

The propriety of associating gold with the platinum group is very questionable. Palladium appears to present a relation of parity with rhodium and ruthenium,-platinum with iridium and possibly with osmium, though indeed many osmic reactions are altogether special. The double chlorides of platinum, iridium, and osmium are isomorphous.

The atomic weight of palladium, or 53.2, is rather more than half that of platinum, or 986; or the two metals may be looked upon as members of an uncompleted triad, the increment of atomic weight being 45.4.

Palladium and platinum are both white, hard, ductile, tenacious, scarcely fusible, and very unoxidizable metals. They both crystallize in octahedra. The two protoxides are of a black. colour; the two binoxides are brownish, substances which dissolve in alkaline solutions forming ill-defined salts.

The sulphides and chlorides are analogous. Each of the bichlorides becomes converted into a protochloride at a temperature considerably below the boiling-point of mercury. Each metal gives rise to numerous analogous double chlorides, with the chlorides of ammonium and the basylous metals, &c. The platinum bases are very numerous, but several corresponding palladium bases have been also obtained. Platinum and palladium have the same atomic volume, 57, and sensibly the same atomic heat, namely 3·15 and 3·19.

Gold differs from the other two members of this group in its colour, softness and fusibility. The protoxide and protochloride of gold correspond with the proto-compounds of palladium and platinum; the bisulphide of gold with the bisulphide of platinum; and the teroxide and terchloride of gold with the manifested by quadroxides as well as by teroxides, is shown in the following examples :

Isomorphous
teroxides.

Isomorphous

quadroxides.

MNO3.
M2 CO3
M3 SbO3
MCIO1
M2 SO4

M3 PO4

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Na NO3 Nitrate of soda.

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Again, in the hydrated salts,

MČIO1. 7H2O

M2 SO. 7H2O

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Fe2 SO. 7H2O

sulphate of iron. Arseniate of soda.

M3 AsO1. 7H20 . HNa2 AsO4.7H2O

These last two salts both crystallize in the oblique prismatic system, in closely approximating, if not in identical forms.

* Laurent's 'Chemical Method,' Cavendish Society's translation, p. 140.

teroxide and terchloride of iridium? The atomic weight, the atomic heat, and the atomic volume of gold are nearly the doubles of those of platinum.

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The atomic volume and atomic heat of silver are the same as those of gold. These two last groups comprise all the metals whose oxides are reduced to the metallic state by heat alone.

It is observable that we have altogether thirteen triads of similar elements, the ferric triad, and probably several others, being double from the existence of twin elements, and the platinic triad being incomplete.

In each triad, the intermediate term is possessed of intermediate properties, and has an exactly intermediate atomic weight. The mean differences or increments of atomic weight in the different groups, are approximatively as follow:

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Moreover, with each of several of the triads is associated an analogous element having an atomic weight approximatively onehalf that of the first member, or double that of the last member of the triad. The former relation obtains in the chloric, sulphuric, phosphoric, and possibly in the ferric groups-the latter in the phosphoric, the silicic, the molybdic, the platinic, and possibly in the ferric groups.

The phosphoric triad is the only one that does not present any relation in the atomic volumes of its members, a result possibly due to the allotropism of phosphorus. In the chloric, sulphuric, ferric, and platinic groups, we have the relation of equality; in the sodic, calcic, zincic, and plumbic groups, the relation of sequence.

With the atomic weights that I have employed, we find the atomic heats of the elements to be as follow:

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It is observable that the atom of an element having the number 6 for its atomic heat, is frequently isomorphous with two atoms of an element having the number 3, several instances of which have been specially alluded to.

FROM

LXV. Chemical Notices from Foreign Journals.
By E. ATKINSON, Ph.D.

[Continued from p. 276.]

ROM a series of experiments, Boussingault* was led to the opinion, in opposition to Ville, that the nitrogen of the atmosphere was never assimilated by plants in their germination. He held that atmospheric air could only furnish traces of ammonia, and that the true source of nitrogen in plants was rather to be sought for in the manures added to the soil, and in the ammonia contained in the water furnished by rain, fog and dew. A Commission appointed by the French Academy to report on the investigations of Ville arrived at different results. The experiments were made by Ville, and Chevreul+ reported on the result. The experiments were conducted in a manner similar to that in which Boussingault's were made. Cress-seeds were sown and allowed to germinate and grow in a soil free from ammonia, and placed in an air-tight glass case. The soil was composed of calcined sand, to which the ashes of cress-seeds were added, and it was kept moist by means of distilled water, the quantity of ammonia in which was specially determined. The air in the enclosed space was constantly renewed by means of an aspirator. The air before it passed into the case had 2 per cent. of carbonic

* Annales de Chimie et de Physique, vol. xliii. p. 149.
+ Comptes Rendus, vol. xli. p. 757.

acid added to it, and was purified from ammonia by being passed through sulphuric acid and through bicarbonate of soda. The weight of the cress-seeds was determined, and the quantity of nitrogen contained therein ascertained, by comparative experiments. When the plant was fully developed, it was weighed, and the quantity of nitrogen contained in it was estimated. The latter was always greater than the quantity of nitrogen which could have been derived from the soil and from the quantity of ammonia contained in the water used for irrigation. The determinations of the nitrogen contained in this water were rather uncertain, but the Commission thought itself fully justified in concluding, that the free nitrogen of atmospheric air was assimilated by plants.

That plants could assimilate nitrogen from nitrates, was shown by the success with which the use of nitrate of soda in manure has been attended. Liebig* believed that the nitrogen of nitric acid, like the carbon of carbonic, and the sulphur of sulphuric acid, could become a constituent of the vegetable organism. Salm-Horstmar arrived at the conclusion, that in the development of plants the alkaline nitrates could replace ammonia. Kuhlmann thought that the nitrates were first reduced to ammonia by the deoxidizing influence of decomposing organic substances. George Wilson was of opinion that the nitrogen was obtained by the reducing action of the plants on the nitrates.

Boussingault+ made a series of experiments on the action of nitrates on vegetation. If the presence of putrescible organic matters in the soil were not necessary for the assimilation of the nitrogen of a nitrate by a plant, two conclusions might be drawn : the first, that the nitrogen of nitric acid need not previously be transformed into ammonia outside the plant in order to be fit to be assimilated by the organism; secondly, that the nitrates do not merely act in virtue of their being potash or soda salts. The method adopted consisted in growing a plant in a soil rendered barren by calcination, and to which a known quantity of an alkaline nitrate was added. When the plant was developed, the quantity of nitrogen which it had absorbed was estimated, as well as the residual nitrate in the soil.

In an experiment made with sunflower in a soil free from ammonia and organic matter under the conditions indicated, he came to the conclusion that the nitrogen of the nitrate is assimilated by the plant, and that for each equivalent of nitrogen assimilated by the organism, an equivalent of alkali is assimilated; that almost the entire quantity of the still unabsorbed nitre is * Jahresbericht, 1855.

† Annales de Chimie et de Physique, January 1856.

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