3:2 Casium Thallic Chloride, Cs,T,Cl,.-The conditions under which this salt can be made are very wide, 5 to 29 g. of cæsium chloride form a heavy white precipitate when added to a solution of 40 g. of thallic chloride. This dissolves readily in the solution upon heating and crystallizes in slender hexagonal prisms terminated by the pyramid. When the ratio of the cæsium chloride to the thallic chloride is 30 g. to 50 g. a salt is obtained which crystallizes in hexagonal plates. Analyses of the plates do not agree very closely with theory, but it is evident that they are the same as the prismstic salt with another crystalline habit. The high percentage of cæsium and the corresponding low percentage of thallium is probably due to the slight inclusions held by the crystals, which could be seen with the microscope. This salt is white, permanent in the air and recrystallizes unchanged from water. The analyses given below are of separate crops made under very different conditions.

The water found in these analyses was probably held mechan

ically by the crystals.

The prismatic variety of this salt showed only the forms of the prism, 1010, and pyramid, 1011.

Axis c 0.82566 0001 1011 = 43° 37′ 50′′

p ap, 1011, 11
map, 10101011

Measured.
*40° 21'

46 21; 46° 22′

Calculated.

46° 22'

Sections parallel to the basal plane show in convergent polarized light the normal uniaxial interference figure, with weak negative double refraction. The crystals served very well as 60° prisms for the determination of the indices of refraction with the following results:

3:1 Rubidium Thallic Bromide, Rb,TlBr. H,0.-This salt was formed, when 1.5 to 24 g. of thallic bromide were

added to a very concentrated solution of 50 g. of rubidium bromide. It crystallizes in beautiful golden yellow crystals, which are very soluble in water, giving the 1:1 salt on recrystallizing. Careful efforts were made to obtain a 2:1 and 3: 2 rubidium thallic bromide, but without success. Several separate products, made under very different conditions, were analyzed with the results which follow:

3.

The somewhat high percentage of rubidium and the low percentage of thallium found in the first four analyses is probably due to the large excess of rubidium bromide in the concentrated solutions from which the crystals were obtained. As more thallic bromide was added, better crystals were obtained in more dilute solutions, which give percentages agreeing very well with the calculated.

a

p

m

The crystallization of this salt is tetragonal. Doubly terminated crystals were obtained up to a length of 6mm.

The forms observed are:

2

3:2 Casium Thallic Bromide, Cs, Tl, Br,.-This salt was observed, as yellowish red crystals, when 1 to 15 g. of thallic bromide were added to a solution of 50 g. of cæsium bromide. It was always obtained in small striated crystals, which were

not adapted for measurement. It is permanent in the air and recrystallizes unchanged from water.

Analyses of separate

products gave the following results,

1:1 Casium and Rubidium Thallic Bromides, CsTlBr, and RbTlBr. H,O.-These two salts are of nearly the same color, pale yellow. The rubidium compound which retains its luster and color much better than the other, recrystallizes unchanged from water, while the cæsium salt gives Cs,Tl, Br,, when its solution is evaporated to crystallization. The cæsium salt was observed when 2 to 10 g. of cæsium bromide were added to 40 g. thallic bromide, and the rubidium salt when 3 to 24 g. of rubidium bromide were added to 40 g. thallic bromide. Analyses of several different crops gave the following results:

The crystallization of these two salts is isometric, the cube being the only form observed.

1:1 Casium and Rubidium Thallic lodides, CsTII, and RUTU.2H0.-Both of these salts were prepared from solutions containing a large excess of thallic iodide and also from solutions containing a large excess of the alkali iodide, so that no other type of double iodides with these two metals could be obtained. As the thallic iodide was very difficultly soluble in water, alcoholic solutions were used where the thallic iodide was in excess. The salts are ruby red, with a brilliant luster, which is slowly lost in the air. Both are decomposed by water. The analytical results obtained from several different crops are given below.

These salts crystallize in the isometric system, the habit being usually the cube truncated by the octahedron.

3:1 Sodium and Lithium Thallic Chlorides, Na,TICI.. 12H,O and Li,TICI ̧.8H2O.—Only one type of double salts could be obtained with these metals and it does not seem possible that others exist, for the ground was covered very carefully and systematically. On account of the extreme solubility of these salts, especially that of the lithium compound, the solutions had to be kept very concentrated, in a more or less syrupy condition, which accounts for the high alkali metal and low thallium found. These salts are transparent and colorless when first taken from the mother-liquor, but, upon exposure to the air, the sodium salt becomes opaque and the lithium compound deliquesces. Analyses of different products gave the following results:

On account of the instability of the sodium and lithium salts no crystallographic determinations were made.

Repeated attempts to prepare lithium and sodium thallic bromides were entirely without success, hence no attempt was made to prepare the iodides.

The author wishes to express his indebtedness to Prof. H. L. Wells for valuable advice in connection with the chemical part of this work, and to Prof. S. L. Penfield for suggestions concerning the crystallography.

Sheffield Scientific School, December, 1894.

*By difference.

ART. XXXII.-Argon, Prout's Hypothesis, and the Periodic Law; by EDWIN A. HILL.

IF Argon be an element, its properties indicate that its place in the periodic classification is between F and Na, with an atomic weight of 20. Its non-metallic acidic electro-negative character, and low melting and boiling points, link it to Series 2 ending with F rather than Series 3 beginning with Na; just as Fe is more closely allied to Mn than Cu. Its resemblance to the members of transitional Group VIII, into which it would therefore fall, is shown in many ways. All the members of this group have high specific gravities, small atomic volumes, very weak chemical affinities, are inert, and with basic or acidic properties very weakly developed if at all. Argon is as truly transitional from Na to F as Group VIII in general is transitional between the two halves of Mendeléef's long periods, and belonging in a short period, is cut off from the other long period members of Group VIII by the same differences in boiling points, melting points, atomic volumes, specific gravities, and other properties, which separate the Series F, O, N, from Mn, Cr, V. To assign it an atomic weight of 40, thus usurping the place of calcium, and placing it among elements to which it bears no analogies whatever, would violate all the principles of the periodic law as now understood; and the great mass of accumulated evidence, upon which that generalization rests, requires us to accept any reasonable explanation of the supposed inconsistency, between the specific heat ratio of 1.66 and the diatomicity of the molecule, rather than the conclusion that it is monatomic.

That is to say the burden of proof is on those who oppose the conclusions drawn from the periodic law.

The argument for monatomicity, briefly stated, is this: The Argon molecule, if diatomic, being eccentric, would by molecular contacts acquire rotational energy, which it does not possess, as proved by the specific heat ratio; hence its molecule must be monatomic, and its atomic weight 40. The whole argument is based on the assumption that a molecular encounter involves an actual contact of atoms, or is of the nature of a collision between two elastic balls. This, however, is not a necessary assumption, nor was it Maxwell's view.* As pointed

.

* "I have concluded (he says) from some experiments of my own that the collision between two hard spherical balls is not an accurate representation of what takes place, a better representation of such an encounter will be obtained by supposing the molecules to act on one another in a more gradual manner, so that the action between them goes on for a finite time during which the centers of the molecules first approach each other and then separate." And again: "We have evidence that the molecules of gases attract each other at certain small dis tances, but when they are brought still nearer they repel each other."