Scpi. 23, 1880] 



NATURE 



497 



candle at different distances (2i and 3i metres) from the selenium 

 gave tolerably concordant results vhen calculated on the supposi- 

 tion that the effect upon the selenium varies as the square root of 

 the liglit intensity. The influence of about 350 square centimetres 

 of the luminous sheet on the selenium was found equal to that of 

 O'00l4 standard candle, or 0-04 standard candle per square 

 metre. 



In conclusion I wish to remark that the above must be consi- 

 dered only as preliminary experiments, and the figures given as 

 only approximate. I am now engaged in making further ex- 

 periments on this subject with the endeavour to obtain more 

 accurate results and to extend these researches, as it seems 

 probable that the sensitive selenium plate may render similar 

 services to the study of phosphorescent light as the thermopile 

 has rendered to the study of radiant heat. EUGEN Obach 



A GRICUL TURAL CHEMISTR V ' 

 II. 

 T T has been shown that the plant may receive abundance of 

 nitrogen, may produce abundance of chlorophyll, and may be 

 subject to the influence of sufficient light, and yet not assimilate 

 a due amount of carbon. On the other hand, it has been seen 

 that the mineral constituents may be liberally provided, and yet, 

 in the absence of a sufficient supply of nitrogen in an available 

 condition, the deficiency in the assimilation of carbon will be 

 still greater. In fact, assuming all the other necessary conditions 

 to be provided, it was seen that the amount of carbon assimilated 

 depended on the available supply of nitrogen. 



In a certain general sense it may be said that the success of 

 the cultivator may be measured by the amount of carbon he 

 succeeds in accumulating in his crops. And as, other conditions 

 being provided, the amount of carbon assimilated depends on 

 the supply of nitrogen in an available form within the reach of 

 the plants, it is obvious that the question of the sources of the 

 nitrogen of vegetation is one of first importance. Are they the 

 same for all descriptions of plants ? Are they to be sought 

 entirely in the soil, or entirely in the atmosphere, or partly in 

 the one and partly in the other ? 



These are questions which Mr. Lawes and myself have dis- 

 cussed so frequently that it might seem some apology was due 

 for recurring to the subject here, especially as I considered it in 

 some of its aspects before this Section at the Sheffield meeting 

 last year. But the subject still remains one of first importance 

 to agi'iculture, and it could not be omitted from consideration in 

 such a review as I have undertaken to give. Moreover, there 

 are some points connected with it still unsettled, and some still 

 disputed. 



It will be remembered that De Saussure's conclusion was that 

 plants did not assimilate the free or uncombined nitrogen of the 

 atmosphere, and that they derived their nitrogen from the com- 

 pounds of it existing in the atmosphere, and especially in the 

 soil. Liebig, too, concluded that plants do not assimilate 

 nitrogen from the store of it existing in the free or uncombined 

 state, but that ammonia was their main source, and he assumed 

 the amount of it annually coming down in rain to be much more 

 than we now know to be the case. 



Referring to our previous papers for full details respecting 

 most of the points in question, I will state, as briefly as I can, 

 the main facts known — first in regard to the amount of the 

 measurable, or as yet measured, annual deposition of combined 

 nitrogen froai the atmosphere ; and secondly as to the amount 

 of nitrogen annually assimilated over a given area by different 

 crops — so that some judgment may be formed as to whether the 

 measured atmospheric sources are sufficient for the requirements 

 of agricultural production, or whether, or where we must look 

 for other supplies? 



First, as to the amount of combined nitrogen coming down as 

 ammonia and nitric acid in the measured aqueous deposits from 

 tlie atmospliere. 



Judging from the results of determinations made many years 

 ago, partly by Mr. Way, and partly by ourselves, in the rain, 

 &c., collected at Rothamsted ; from the results of numerous 

 determinations made much more recently by Prof. Frank land in 

 the deposits collected at Rothamsted, and also in rain collected 

 elsewhere ; from the results obtained by Boussingault in Alsace ; 

 from those of Marie-Davy at the Meteorological Observatory at 



^ Opening Address ia Section B (Chemical Science), at the Swansea 

 meeting of the British Association, by J. H. Gilbert, Ph.D., F.R.S., 

 V.B.C.S., F.L.S.. President of the Section. Continued from p. 476. 



Montsouris, Paris ; and from those of many others made in 

 France and Germany — we concluded, some years ago, that the 

 amount of combined nitrogen annually so coming down from the 

 atmosphere would not exceed S or 10 lbs. per acre per annum in 

 the open countiy in Western Europe. Subsequent records 

 would lead to the conclusion that this estimate is more probahly 

 too high than too low. And here it may be mentioned in 

 passing, that numerous determinations of the nitric acid in the 

 drainage water collected from land at Rothamsted, which had 

 been many years unmanured, indicate that there may be a 

 considerable annual loss by the soil in that way ; indeed, prob- 

 ably sometimes much more than the amount estimated to be 

 annually available from the measured aqueous deposits from 

 the atmosphere. 



It should be observed, however, that the amount of combined 

 nitrogen, especially of ammonia, is very much greater in a given 

 volume of the minor aqueous deposits than it is in rain ; and 

 there can be no doubt that there would be more deposited within 

 the pores of a given area of soil than on an equal area of the 

 non-porous even surface of a rain-gauge. How much, however, 

 might thus be available beyond that determined in the collected 

 and measured aqueous deposits, the existing evidence does not 

 afford the means of estimating with any certainty. 



The nsxt point to consider is — What is the amount of nitro- 

 gen annually obtained over a given area, in different crops, when 

 they are grown without any supply of it in manure ? The field 

 experiments at Rothamsted supply important data relating to 

 this subject. 



Thus, over a period of 32 years (up to 1875 inclusive), wheat 

 yielded an average of 20"7 lbs. of nitrogen per acre per annum, 

 without any manure ; but the annual yield has declined from an 

 average of more than 25 lbs. over the first 8, to le;s than 16 lbs. 

 over the last 12, of those 32 years ; and the yield (it is true with 

 several bad seasons) has been still less since. 



Over a period of 24 years barley yielded lS"3 lbs. of nitrogen 

 per acre per annum, without any manure ; with a decline from 

 22 lbs. over the first twelve, to only I4'6 lbs. over the next 12 

 years. 



With neither wheat nor barley did a complex mineral manure 

 at all materially increase the yield of nitrogen in the crops. 



A succession of so-called "root-crops" — common turnips, 

 Swedish turnips, and sugar-beet (with 3 years of barley inter- 

 vening after the first 8 years)— yielded, with a complex mineral 

 manure, an average of 26'S lbs. of nitrogen per acre per annum 

 over a period of 31 years. The yield declined from an average 

 of 42 lbs. over the first eight years, to only 13 'I lbs. (in sugar- 

 beet) over the last 5 of the 31 years ; but it has risen some- 

 what during the subsequent 4 years, with a change of crop to 

 mangolds. 



With the leguminous crop, beans, there was obtained, over a 

 period of 24 years, 31 '3 lbs. of nitrogen per acre per annum 

 without any manure, and 45-5 lbs. with a complex mineral 

 manure, including potass (but without nitrogen). Without 

 manure the yield declined from 4S'I lbs. over the first 12 years 

 to only I4'6 lbs. over the last 12; and with the complex mineral 

 manure it declined from 6r5 lbs. over the first 12, to 29^5 lbs. 

 over the last 12, years of the 24. 



Again, an ordinary rotation of crops of turnips, barley, clover, 

 or beans, and wheat, gave, over a period of 28 years, an average 

 of 36'S lbs. of nitrogen per acre per annum without any manure, 

 and of 45'2 lbs. with superphosphate of lime alone, applied 

 once every four years, that is for the root crop. Both without 

 manure, and with superphosphate of lime alone, there was a 

 considerable decline in the later courses. 



A very remarkable instance of nitrogen yield is '^the follow 

 ing — in which the results obtained when barley succeeds barley 

 that is when one gramineous crop succeeds another, are con 

 trasted with those when a leguminous crop, clover, intervenes 

 between the two cereal crops. Thus after Ae growth of six 

 grain crops in succession by artificial manures alone, the field so 

 treated was divided, and, in 1873, on one half barley, and on the 

 other half clover, was grown. The barley yielded 37-3 lbs. of 

 nitrogen per acre, but the three cuttings of clover yielded 15 1 "3 

 lbs. °In the next year, 1874, barley succeeded on both the barley 

 and the clover portions of the field. Where barley had previously 

 been grown, and had yielded 37 3 lbs. of nitrogen per acre, it 

 now yielded 39-1 lbs. ; but where the clover had previously been 

 grown, and had yielded 151 '3 lbs. of nitrogen, the barley suc- 

 ceeding it gave 69'4lbs., or 30-3 lbs. more after the removal of 

 151-3 lbs. in clover, than after the removal of only 37'3 lbs. in 

 barley. 



