Page:Science vol. 5.djvu/208

 [Vol. v.. Ko. 109.

��pU(^6 your elbowH at. a ciniveiiieiit dlsutice apari. This positlgn evidently leavca a spaoe between your cheat und the back, of the chair equal In tengUi to your (ore-arraB. which are extended hori/ont»lly. Miss Lulu novr tukes a position beside you, aud, tiold- ing her body bafl(, simply places the palmar surface of her band uu the back of the chair ou the side to- wardj your body. After a few momenta she seems to make the effort to detach her hand from the chair, whicb Utter you are privileged to push fonrards. The force at work, however, is too strong for you, and both yaurself and the cbair are carried back- wards, without her hand having clianged its poettion. Tbe chiir being a cane-backed one, It is evident that slie could in no way gain a bold upon It, and the back of her hand never could come In contact with your cliest, as the spanning of such a distance would at once be detected.

Professor Newcomb's conclusions, after having witnessed the test of lifting a chair with some one sitting in it, are to me far from satisfactory. I saw the girl lean over an ordinary chair, with a man weighing over two hundred pounds sitting In it, ami placing the palmar surfaces of her hands on the outer sides of the rear uprights near their middles, and tolt/touf an]/ contraction of the muscles of the arm or fore-arm, or increaEie of pulse (remained at SO) or respiratory cSort. or change of countenance due to exertion, so far lift that chair and its heavy contents from the Qoor as to compel the latter to get out of it; and this without fractur- ing any of the bones of her upuer exti'emlties, or the sides of the chair. Tbe simplest computation will prove that the lateral rrresmire required must be enor- mous in order to getahold, and preventsucb a weight absolutely slipping between her hands when the up- ward force comes to be exerted.

It. W. Shufbi,i>t, U.S.A. Fort Wlngale, Nvw Uiliio, l^eb. IS.

��THE MICROSCOPE IN GEOLOGY.

Mant persons have beard lliat the micro- scope, everywhere reti^nized as indispenaable in the iuvcatigation of organic nature, has also recently been made use of in geology ; but very few hare any distinct notion of the sort of problems to which it can there be applied, or of the way in which it can contribute toward their solution. The determination of the dif- ferent minerals which comiwse very flne grained rocks may doubtless appear, even to many geologists who have been accustomed to deal with only great areas and mountain masses, a. matter of small importance ; and they often fail to see that the methods which render such a determination possible, are capable, if prop- erly employed, of throwing much light on some of the moat difficult questions with which they have to deal.

The microscopic study of rock-sections is one of difficulty, and indeed quite discour- aging to a beginner who attempts it without proper guidance, no matter how familiar he mtiy be with mineralogy, or with the use of Uie

��microscope in other fields of reaearcli. This lact, coupled with the newness of the branch. sufflcienlly accounts for the number of workers in it still lieing so small in this country, whiob presents unrivalled opportunities for its cul- tivation.

Although the idea of preparing rooks transparent sections for the microscope t ginated with an Englishman, the fniitful lim' of research to which it gave rise has sinoc! been almost exclusively cultivated in Germany. Here the seed fell into soil made already fer- tile by the labors of older geologists, and sprang at once into a strong and rapid growlb. Tbe keen perception and great energy of Zirkel first made known the microscopic appearance of the common rock-forming minerals, as well ss discovered the wide distribution of others before considered inrities. Vogelsang, not contented merely to observe, was able to draw from hu studies the most suggestive conclusions, whidi he substantiated by ingenious and delicate experiments. It is, however, to Rosenbtisdl that the development of petrography as ence is most largely due. In his work, pub- lished in 1873, he showed in a masterly manner how what had been learned of the optical prt>j>- erlies of different crystals, especially their action oil polarized light, could be applied to their idcutiflcatioo in thin sections, thus ren- dering a rigid microscopic diagnosis for the Srst time possible. From this time on, the inter- est in this branch of investigation became in Germany lery general, and its growth propor- tionately rapid. The attainment of the long- desired separation of rock constituents, even when of the smallest size, by means of solu- tions of high specific gravity, and the perfeo-- tion of manj- miero-eheroical reactions of grt precision, followed each other in quick succi sion, until to-day the accuracy and beauty ol petrographical methods are hardly second those found in any other branch of natni

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��The geologists of other cotmtries on the con*] tinent, especially in France and Scandinavia, soon perceived the value of the German workjl and early availed themselves of its results tol start similar investigations in their own c tries. It is a surprising fact that the appreoi**^ tion of it among English-speaking people hai l)eeu so slow, that not one reliable text-bo<4c] on the subject of petrography exists i language of the man who gave the first im-i pulse to its modern development. Any knowlrl edge of the subject in America is recent, datinff ■ from the publication of Zirkel's ' Microecopi^'l petrography' in iS7f>. llow steadily the inter-]

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