properties of the crystal.  We have just seen that in clinorhombic crystals the principal absorption directions of certain bands were completely different from the axis of optical elasticity of the crystal for the corresponding radiations.  If we examine this anomaly, we perceive that the crystals manifesting these effects are complex bodies, formed of various matters, one, or sometimes several, of which absorb light and give each different absorption bands.  Now, M. De Senarmont has shown that the geometric isomorphism of certain substances does not necessarily involve identity of optical properties, and in particular in the directions of the axes of optical elasticity in relation to the geometric directions of the crystal.  In a crystal containing a mixture of isomorphous substances, each substance brings its own influence, which may be made to predominate in turn according to the proportions of the mixture.  We may, therefore, admit that the molecules of each substance enter into the crystal retaining all the optical properties which they would have if each crystallized separately.  The principal directions of optical elasticity are given by the resultant of the actions which each of the component substances exerts on the propagation of light, while the absorption of a given region of the spectrum is due to a single one of these substances, and may have for its directions of symmetry the directions which it would have in the absorbing molecule supposing it isolated.  It may happen that these directions do not coincide with the axes of optical elasticity of the compound crystal.  If such is the cause of the anomaly of certain principal directions of absorption, the bands which present these anomalies must belong to substances different from those which yield bands having other principal directions of absorption.  If so, we are in possession of a novel method of spectral analysis, which permits us to distinguish in certain crystals bands belonging to different matters, isomorphous, but not having the same optical properties.  Two bands appearing in a crystal with common characters, but presenting in another crystal characters essentially different, must also be ascribed to two different bodies.

* * * * *

[Continued from SUPPLEMENT, No. 585, page 9345.]

HISTORY OF THE WORLD’S POSTAL SERVICE.

It is commonly believed in Europe that the mail is chiefly forwarded by the railroads; but this is only partially the case, as the largest portion of the mails is intrusted now, as formerly, to foot messengers.  How long this will last is of course uncertain, as the present postal service seems suitable enough for the needs of the people.  The first task of the mail is naturally the collection of letters.  Fig. 17 represents a letter box in a level country.

[Illustration:  FIG. 17.—­COUNTRY LETTER BOX.]

By way of example, it is not uninteresting to know that the inhabitants of Hanover in Germany made great opposition to the introduction of letter boxes, for the moral reason that they could be used to carry on forbidden correspondence, and that consequently all letters should be delivered personally to the post master.