but his main object should be to learn how the one can be brought to bear upon the other. Mathematics and natural science are indispensable, but he must not expect to become a master of either. Only a comparatively small portion of these wide subjects can be usefully brought to bear on engineering, and to these he must restrict himself. The methods of applying these sciences to engineering problems constitute a large subject, and one that is necessary for him to study. Then there are those manual operations which are essential to bring his knowledge to a practical issue, and in which a long course of training is necessary to acquire the requisite skill, such as mechanical drawing, and the use of measuring and surveying instruments. To acquire a useful knowledge of these various branches will require three or at least two years. The student will then proceed with his practical training, which should include as great a range of work as possible. In this he will find the knowledge he has acquired of very great help; he will recognize much that he sees, and be able to judge of the most important things to which to direct his attention.
Absorption of Nitrogen by Plants.—The chemist Berthelot has submitted to the Paris Academy of Sciences the results of a new series of experiments which prove that, under the influence of atmospheric electricity, free nitrogen is absorbed by the proximate principles of plants. The apparatus used in these experiments consists of a system of tubes in which the organic substances come into contact either with pure nitrogen or with atmospheric air, the whole communicating with a source of electricity at a tension precisely the same as that of atmospheric electricity. Under these conditions pure nitrogen, or the nitrogen of the atmosphere, is invariably fixed by the organic matter employed, viz., wet filtering paper or a solution of dextrine. The amount of nitrogen that is thus fixed is considerable. Tims these experiments bring to light a natural cause, hitherto overlooked, in considering the question of the fixation of nitrogen by vegetable tissues. It is now demonstrated that this fixation is brought about by the incessant action of the electricity of the atmosphere.
Development of Electricity by Light.—To determine experimentally the action of light in the development of electricity, Hankel took two bright strips of copper, one of which he fixed in a porous clay cell by means of a cork stopper. The cell was filled with water, and placed in a larger glass vessel containing the same water, in which was immersed the other strip, so that one of its surfaces was turned toward the source of light. The two strips were connected with the wire of a galvanometer. The glass with its contents was now placed in a black case having a slide, by means of which direct sunlight or colored light could be admitted to the outer strip of copper. The results were as follows: On access of free sunlight the illuminated strip was negative to the one in darkness, but only moderately so; behind a red glass the action was extremely small; behind yellow, a little stronger; behind green and dark blue successively, still stronger; behind dark violet it became less again.
The copper strips were now oxidized by moderate heating, and the following results were obtained: In free sunlight the illuminated strip was strongly negative; on darkening again, the deflection gradually disappeared; behind red glass the action was less; behind light-yellow glass the plate was first positive, then negative; on darkening, it first became still more negative, and then the action disappeared; behind dark-green glass the behavior was similar, but the first positive deflection was less; behind bright-blue, dark-blue, and violet glass, the plate was equally negative.
Strongly oxidized copper strips were next tested. In free sunlight the illuminated strip was first strongly positive, then weakly negative; on darkening, it was first strongly negative, then the action ceased. Behind red glass the plate was pretty strongly positive, but the deflection of the needle soon fell off considerably; behind bright yellow glass the strip was very strongly positive, but very soon the action diminished; on darkening, a strong negative deflection occurred. Behind dark-green glass the plate was first weakly positive, and then negative; behind dark-blue glass the cop per was also negative, and this change was more considerable than with free sunlight; behind violet glass, the action was similar.
