[Illustration:  FIG. 3.]

[Illustration:  FIG. 4.]

[Illustration:  FIG. 5.]

This arrangement, which is adopted in Mr. Kapp’s instruments, gives very good results, as may be easily seen by reference to Figs. 3 and 4, in which the current intensities or differences of potential are referred as ordinates and the degrees of deflection of the needle as abscissas.  The unbroken lines represent the curves obtained with the apparatus just described, while the dotted ones give the curve of deflection of an ordinary tangent galvanometer.  These curves show that for strong intensities of current Mr. Kapp’s instrument is more advantageous than the tangent galvanometer.  Mr. Crompton has constructed an amperemeter upon the same principle, which is shown in Fig. 5.—­La Lumiere Electrique.

* * * * *

THE CHEMICAL ACTION OF LIGHT.

Professor A. Vogel, in a communication to the “Sitzungsberichte der Munchener Akademie,” brings into prominence the fact that the hemlock plant, which yields coniine in Bavaria, contains none in Scotland.  Hence he concludes that solar light plays a part in the generation of the alkaloids in plants.  This view is corroborated by the circumstance that the tropical cinchonas, if cultivated in our feebly lighted hothouses, yield scarcely any alkaloids.  Prof.  Vogel has proved this experimentally.  He has examined the barks of cinchona plants obtained from different conservatories, but has not found in any of them the characteristic reaction of quinine.  Of course it is still possible that quinine might be discovered in other conservatory-grown cinchonas, especially as the specimens operated upon were not fully developed.  But as the reaction employed indicates very small quantities of quinine, it may be safely assumed that the barks examined contained not a trace of this alkaloid, and it can scarcely be doubted that the deficiency of sunlight in our hothouses is one of the causes of the deficiency of quinine.

It will at once strike the reader as desirable that specimens of cinchonas should be cultivated in hothouses under the influence of the electric light, in addition to that of the sun.

If sunlight can be regarded as a factor in the formation of alkaloids in the living plant, it has, on the other hand, a decidedly injurious action upon the quinine in the bark stripped from the tree.  On drying such bark in full sunlight the quinine is decomposed, and there are formed dark-colored, amorphous, resin-like masses.  In the manufacture of quinine the bark is consequently dried in darkness.

This peculiar behavior of quinine on exposure to sunlight finds its parallel in the behavior of chlorophyl with the direct rays of the sun.  It is well known that the origin of chlorophyl in the plant is entirely connected with light, so that etiolated leaves growing in the dark form no chlorophyl.  But as soon as chlorophyl is removed from the sphere of vegetable life, a brief exposure to the direct rays of the sun destroys its green color completely.