ART. II-On a Method for distinguishing between Biaxial and Uniaxial Crystals when in thin plates,-and the results of the examination of several supposed Uniaxial Micas; by W. P. BLAKE. (Read before the American Association for the Advancement of Science, at Cincinnati, May, 1851.)

THE greater number of the "black micas" so-called, have a deep olive green or "bottle green" color, so deep as generally to be opaque when the thickness exceeds one-fourth of a millimetre.* The other colors most frequently observed are a dull brownish green, sometimes yellowish, and a fiery red. Of course their power of absorbing light varies greatly; thus, the mica from Greenwood Furnace, olive green, is opaque in plates over Sussex Co., N. J., (No. 115) bottle green, St. Lawrence Co., (No. 116) brownish "

(No. 113) olive green,

46

46

500 mm, thick.

222

66

*062"

*042

These thicknesses were taken with an instrument constructed by Soleil, for measuring the thickness of thin plates. It is called a spherometer and was designed by M. Cauchoix.

In stating the color of specimens, I have given that color which they present when a plate sufficiently thin to be transparent is held near the eye, while the light from the clouds or window is allowed to pass through it. It is not therefore possible to examine plates of these micas in the ordinary way, even when the thickness is less than stated above; and of course when so very thin, the images formed around the pole or poles of the resultant axes, are much extended and it would generally be impossible to distinguish them or the dark bars which cross the center; and when they can be seen, they so nearly resemble the symmetrical image formed in uniaxial crystals that they cannot be distinguished from them.

In order to enable me to give these dark micas a more complete and thorough examination, I have arranged, in addition to the combination of lenses and "Nichols Prism," which I described at the meeting of the American Association last August, a large convex lens, placed between the polarizing mirror of my instrument and the film of mineral to be examined; I also use, instead of the variable and uncertain light from the clouds, a broad gas flanie, which I bring near to the mirror, so that its reflected image is condensed by the large convex lens upon the mica; with this arrangement I have been able to discern the rings and cross in plates of the dark micas much thicker than could be examined by any former arrangement. It now became interesting to examine some of the micas whose optical uniaxial character was equivocal, and for this purpose I adopted the following test.

* Ordinary visiting cards are from 3 to 4 of 1 mm. thick.
Described in E. Peclet's Traité de Physique, 4e ed. Paris, 1847, i, 5.

It is well known that where a plate of Iceland-spar, cut with faces perpendicular to the axis, is placed in a polariscope (the eyepiece being "crossed") and then rotated in its own plane, the symmetrical cross and rings do not suffer any change of form or position, if, however, a plate of a biaxial crystal-as nitre-be rotated in a similar manner, the dark bars which form the cross, as in fig. 1, will, when the plate has been rotated 45°, "open out," and take the form and position represented in fig. 2. These

facts and figures, though familiar to the student of optics, I here repeat for the sake of the explanations which follow; and this is particularly important since observers of eminence have evidently mistaken the optical character of many of the so-called uniaxial or hexagonal micas. When the micas referred to are examined by my instrument I find the same evidence of a biaxial character. The thin plates, if the line joining the poles is parallel to the plane of primitive polarization, give to the eye symmetrical crosses and rings, as in fig. 3, that is, so nearly symmetrical that it is

Of these columns, the first, third and fourth are mere transcripts of the printed observations; with the hours of C's change (morning or afternoon) supplied to the first column. The second and fourth belong to the comparison. It is to be observed, farther, that the supposed rule claimed no more than the distinction between dry and (as it is called) falling weather. Thus, the existence of nimbus or rain-cloud on any quarter day, provided it did not rain, might be taken as coinciding with the weather on a term-day which was clear. But I have not so taken it, in fact; and have reckoned the existence of nimbus as equivalent to rain, and noted it as accordant or discordant, as the case may be. However hardly this may strain the supposed rule, I have no doubt of the soundness of its being insisted on. On the other hand, the propriety of considering the weather on 18th Aug., 1843, as coincident with the weather on 25th July, 1843, for instance, will, I suppose, be equally admitted.

With this exposition of the principles and manner of the comparisons, of which throughout the extract above is an average sample, the aggregate results may now be stated. Of the 31 lunations, then, contained in group (a),

16 agree throughout,

8 miss once,

4 miss twice,

2 miss thrice,

1 disagrees throughout.

31 in all.

If I were endeavoring to sustain the supposed rule, coûte qui coûte, as a matter of argument or opinion, instead of honestly seeking for information, the uniformity of bisection in this last series and the correspondence between the numbers and characters of the discordances, might be plausibly urged according to the received doctrine of probabilities as being of great weight. Of the 12 lunations in group (b),

7 agree throughout,

5 miss once.

12 in all.

Of the 6 lunations in group (c),

5 agree throughout,

1 misses twice.

6 in all.

The single lunation, classed (d), which contains but one quar

ter-day, misses.

and the chances would be in every 100 times, 56 in favor of the supposed rule and 44 against it; or the final probability would be 1,27, etc.: 1, for the rule.

But this comparison by lunations is defective; inasmuch as it does not allow a distinction in the character and degree of the discordances. For this, as well as for the proportion of missing observations, it is better to make comparison by the actual epochs or quarter-days; designating separately the aggregate number of those which coincide with the term-days, of those which differ, and of those which have failed to be observed at all. Of course, every such epoch affords occasion for a coincidence; and in proportion to the number of actual coincidences out of a given number of such occasions will be the probability of the supposed rule. Counting, then, by epochs, the aggregate are as under:

If we leave the epochs not observed out of the question, the chances stand as rather more than 4 to 1 in favor of the rule; or its probability is a little more than .

If we consider that among those unobserved epochs the chances of coincidence are even, then they will stand as 76 25 out of 100 in favor of the rule; or its probability is rather more than . The lowest of these probabilities is greater than is sometimes taken as sufficient ground for human belief or human actions.

I may terminate this notice in the words of Carlini upon a similar occasion:*"Queste conclusioni non devono considerarsi..... come quelle che con verebbe dedurre dal paragone delle osservazioni continuate per un gran numero d'anni. Ciò nulla ostante, non ci è sembrato del tutto inutile il presentar quì i risultati delle poche osservazioni delle quali potevamo far uso; sperando che da essi possa trarsi qualche norma par istabilire un piano generale d'osservazioni meteorologiche che riesca il più opportuno allo studio de' fenomeni atmosferici."

* Sulla Legge delle Variazoni Orarie del Barometro, etc., Modena, 1848.

ART. III.-On some of the Thermal Waters of Asia Minor; by Dr. J. LAWRENCE SMITH, of New Orleans, Prof. Chem. in the University of Louisiana.

Part I.-The Thermal Waters of Broosa.

THERE are few countries where Thermal Waters are so numerous, and cover so extensive a surface as in Western Asia Minor; many of them still bear marks of the estimation in which they were held by the ancient Romans and Greeks for the purpose of supplying their baths.

Owing to the difficulty of obtaining proper vessels or corks at or near the springs, coupled with the risk of breakage by the necessary transportation on the backs of horses over rough and mountainous roads, travellers have been deterred from collecting these waters for the purpose of analysis. In my travels through certain parts of this country, I took along with me bottles and corks, and collected between twenty and thirty specimens of dif ferent localities, some of them in considerable quantity; and of that number fifteen or sixteen have arrived safely to my laboratory, where most of them have been already examined.

In my remarks upon them I will first allude to the thermal waters of Broosa or Prusia, which are the most important at the present day, and the most accessible from Constantinople. The spot itself is hallowed by many interesting historical associations. The city was founded by Hannibal during a friendly visit which this great Carthagenian general made to Prusias, the king of Bythinia, whose name was given to it. Like all other cities of so ancient date, it has gone through many changes, passing successively into the hands of the Greeks, Romans, and Turks. Since 1326 the Turks have continued masters of this part of Asia Minor, it having been conquered by Osman just prior to his death, for many years after which event it remained the capital of the Ottoman empire.

Broosa is readily reached from Constantinople by a steamer that goes from this latter place to Modania, on the gulf of the same name, about seventy miles from Constantinople. From Modania a ride of about twenty miles on horseback brings you to Broosa, at the foot of the Bythinian Olympus. The warm baths of this place have been celebrated from the earliest epochs, and the visit of Constantine with his wife in 797, is recorded in history as having resulted favorably in restoring the latter to health. And at a still later period Sultan Soleman the Great visited these baths on account of an attack of gout, and to commemorate his cure he had a large dome constructed over the source to which he attributed the beneficial effects derived by him; the dome still stands.