Sepf. 23, 1880] 



NATURE 



499 



not of atmospheric supply. And the fact that all the other crops 

 will yield full agricultural results even on ordinary arable land, 

 when proper manures are applied, is surely very strong evidence 

 that it is with them, too, a question of soil, and not of atmospheric 

 iupply. 



But we have other evidence leading to the same conclusion. 

 Unfortunately we have not reliable samples of the soil of the 

 different experimental fields taken at the commencement of each 

 series of experiments, and subsequently at stated intervals. We 

 have nevertheless, in some cases, evidence suflicient to show 

 whether or not the nitrogen of the soil has suffered diminution 

 by the continuous growth of the crop without nitrogenous 

 manure. 



Thus we have determined the nitrogen in the soil of the con- 

 tinuously unmanured wheat plot at several succesUve period-, and 

 the results prove that a gradual reduction in the nitrogen of the 

 soil is going on ; and, so far as we are able to form a judgment 

 on the point, the diminution is approximately equal to the 

 nitrogen taken out in crops; and the amount estimated to be 

 received in the annual rainfall is approximately balanced by the 

 amount lost by the land as nitrates in the drainage water. 



In the case of the continuous root-crop soil, on which the 

 decline in the yield of nitrogen in the crop was so marked, the 

 percentage of nitrogen, after the experiment had been continued 

 for twenty-seven years, was found t j be lower where no nitrogen 

 had been applied than in any other arable land on the farm which 

 has been examined. 



In the case of the experiments on the mixed herbage of grass 

 land, the soil of the plot which, under the influence of a mixed 

 mineral manure, including potass, had yielded fuch a large 

 amount of leguminous herbage and such a large amount of 

 nitrogen, showed, after twenty years, a consideraijly lower 

 percentage of nitrogen than that of any other plot in the series. 



Lastly, determinations of nitrogen in the garden soil which has 

 yielded so much nitrogen in clover, made in samples collected in 

 the fourth and the twenty-sixth years of the twenty-seven of the 

 experiments, show a very large diminution in the percentage of 

 nitrogen. The diminution, to the depth of g inches, oidy repre- 

 sents approximately three-fourths as much as the amount esti- 

 mated to be taken out in the clover during the intervening period ; 

 and the indication is that there has been a considerable reduction 

 in the lower depths also. It is to be supposed however that 

 there would be loss in other ways than by the crop alone. 



I would ask, Have we not in these facts — that full amounts 

 of the different crops can be grown, provided proper soil con- 

 ditions are supplied ; that without nitrogenous manure the yield 

 of nitrogen in the crop rapidly declines ; and that, coincidently 

 with this, there is a decline in the percentage of nitrogen in the 

 soil — have we not in these facts cumulative evidence pointing to 

 the soil, rather than to the atmosphere, as the source of the 

 nitrogen of our crops ? 



In reference to this point I may mention that the ordinary 

 arable soil at Rothamsted may be estimated to contain about 

 3,000 lbs. of nitrogen per acre in the first nine inches of depth, 

 about 1,700 lbs. in the second nine inches, and about 1,500 lbs. 

 in the third nine inches — or a total of about 6,200 lbs. per acre 

 to the depth of twenty-seven inches. 



In this connection it is of interest to state that a sample of 

 Oxford clay obtaiced in the sub-Wealden exploration boring, at 

 a depth of between 500 and 600 feet {and which was kindly 

 given to me by the Pre.ident of the Association, Prof. Ramsay, 

 some years ago), showed, on analysis at Rothamsted, approxi- 

 mately the same percentage of nitrogen as the subsoil at 

 Rothamsted taken to the depth of about 4 feet only. 



Lastly, in a letter received from Boussingault some years ago, 

 referring to the sources whence the nitrogen of vegetation is 

 derived, he says : — 



" From the atmosphere, because it furnishes ammonia in the 

 form of carbonate, nitrates, -or nitrites, and various kinds of 

 dust. Theodore de Saussure was the first to demonstrate the 

 presence of ammonia in the air, and consequently in meteoric 

 waters. Liebig exaggerated the influence of this ammonia on 

 veget.ation, since he went so far as to deny the utility of the 

 nitrogen which forms a part of farmyard manure. This influ- 

 ence is nevertheless real, and comprised within limits which have 

 quite recently been indicated in the remarkable investigations of 

 M. Schlbsing. 



"From the soil, which, besides furnishing the crops with 

 mineral alkaline substances, provides them with nitrogen, by 

 ommonia, and by nitrates, which are formed in the soil at the 



expense of the nitrogenous matters contained in diluvium, which 

 is the basis of vegetable earth ; compounds in which nitrogen 

 exists in stable combination, only becoming fertilising by the 

 effect of time. If we take into account their immensity, the 

 deposits of the last geological periods must be considered as an 

 inexhaustible reserve of fertilising agents. Forests, prairies, and 

 some vineyards, have really no other manures than what are 

 furnished by the atmosphere and by the soil. Since the basis of 

 all cultivated land contains materials capable of giving rise to 

 nitrogenous combinations, and to mineral substances, assimilabl" 

 by plants, it is not necessary to suppose that in a system of culti- 

 vation the excess of nitrogen found in the crops is derived from 

 the free nitrogen of the atmosphere. As for the absorption of 

 the gaseous nitrogen of the air by vegetable earth, I am not 

 acquainted with a single irreproachable observation that esta- 

 Wishes it ; not only does the earth not absorb gaseous nitrogen, 

 but it gives it ofT, as you have observed in conjunction with Mr. 

 Lawes, as Reiset has shown in the case of dung, as M. Schlbsing 

 and I have proved in our researches on nitrification. 



"If there is one fact perfectly demonstrated in physiology it 

 is this of the non-assimilation of free nitrogen by plants ; and I 

 may add by plants of an inferior order, such as mycoderms and 

 mushrooms (translation)." 



If, then, our soils are subject to a continual loss of nitrogen 

 by drainage, probably in many cases more than they receive of 

 combined nitrogen from the atmosphere — if the nitrogen of our 

 crops is derived mainly from the soil, and not from the atmosphere 

 — and if, w-hen due return is not made from without, we are 

 di'awing upon what may be termed the store of nitrogen of the 

 soil itself— is there not, in the case of many soils at any rate, 

 as much danger of the exhaustion of their available nitrogen 

 as there has been supposed to be of the exhaustion of their 

 available mineral constituents ? 



I had hoped to say something more about soils to advance our 

 knowledge respecting which an immen-e amount of investigation 

 has been devoted of late years, but in regard to which we have 

 yet very nmch more to learn. I must however now turn to 

 other matters. 



( To lie continued. ) 



IMPROVED HELIOGRAPH OR SUN SIGNAL'' 

 "T^HE author claims to have contrived a heliograph, or sun 

 telegraph, by which the rays of the sun can lie directed 

 on any given point with greater ease and certainty than by those 

 at present in use. 



When the sun's rays are reflected at a small plane surface 

 considered as a point, the reflected r.ays form a cone, whose 

 vertex is at the reflector and whose vertical angle is equal to 

 that subtended by the sun. Adding to the size of the mirror 

 adds other cones of light, whose bounding rays are parallel with 

 those proceeding from other points of the mirror, and only 

 distant from them the same distance as the points on the mirror 

 from w-hich they are reflected. Hence increasing the size of the 

 mirror only adds to the field to which the sun's rays are reflected 

 a diameter equal to the diameter of the miiTor, and this at any 

 distance at which the sun signal would be used is quite inap- 

 preciable. Adding to the size of the mirror adds to the number 

 of rays sent to each point, and hence to tlie brightness of the 

 visible flash, but not to the area over which it is visible. 



By the author's plan an ordinary field-glass is used to find 

 the position of the object to be signalled to, and to it is attached, 

 in the position of the ordinary sun-shade, a small and light 

 apparatus, so arranged that when the mirror is turned to direct 

 the cone of rays to any object within the field view of the glass, 

 an ima^e of the sun appears in the field, at the same time as the 

 image of the distant object, and magnified to the same degree, 

 and°the part of the field covered by this image is exactly that 

 part to which the rays are reflected, and at which some part of 

 the sun's disk is visible in the mirror. 



A perfectly plane silvered mirror, A, takes up the rays of the 

 sun, and when in proper position reflects them parallel with the 

 axis of D, which is one barrel of an ordinary field-glass. The 

 greater part of the light passes away to the distant object, but 

 some is taken up by the small silvered mirror, E, which is 

 placed at an angle of 45° to the axis of D, and reflected at a 

 right angle through the unsilvered plane mirror, F, and the 

 convex lens, K, by which it is brought to a focus on the white 



' Paper read at the British Association by Tempest Anderson, M.D., B.Sc. 



