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THE POPULAR SCIENCE MONTHLY 



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 com- 

 paratively small portion of these wide sub- 

 jects can be usefully brought to bear on 

 engineering, and to these he must restrict 

 himself. The methods of applying these 

 sciences to engineering problems consti- 

 tute a large subject, aud one that is neces- 

 sary for him to study. Then there are 

 those manual operations which are essen- 

 tial to bring his knowledge to a practical 

 issue, and in which a long course of train- 

 ing 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 pro- 

 ceed 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 recog- 

 nize 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 Par- 

 is Academy of Sciences the results of a new 

 series of experiments which prove that, un- 

 der the influence of atmospheric electrici- 

 ty, free nitrogen is absorbed by the proxi- 

 mate principles of plants. The apparatus 

 used in these experiments consists of a 

 system of tubes in which the organic sub- 

 stances 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 con- 

 ditions pure nitrogen, or the nitrogen of the 

 atmosphere, is invariably fixed by the or- 

 ganic matter employed, viz., wet filtering- 

 paper or a solution of dextrine. The amount 

 of nitrogen that is thus fixed is consider- 

 able. Tims these experiments bring to light 

 a natural cause, hitherto overlooked, in con- 

 sidering the question of the fixation of ni- 

 trogen by vegetable tissues. It is now de- 

 monstrated that this fixation is brought 

 about by the incessant action of the electrici- 

 ty of the atmosphere. 



Development of Electricity by Light. 



To determine experimentally the action of 

 light in the development of electricity, Han- 

 kel 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 ves- 

 sel 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 illumi- 

 nated strip was strongly negative ; on dark- 

 ening again, the deflection gradually disap- 

 peared ; behind red glass the action was 

 less ; behind light-yellow glass the plate 

 was first positive, then negative ; on dark- 

 ening, 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 illumi- 

 nated 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 strong- 

 ly 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 dimin- 

 ished ; on darkening, a strong negative de- 

 flection 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. 



