Oct. 1 8, 1888] 



NA TURE 



605 



Next as to systematic meteorites, those, that is, that are massed 

 in .swarms. 



Much still remains to be done before their greater density is 

 known. Prof. Newton has calculated that in the liiela swarm 

 the meteorites are thirty miles apart. 



J. Norman Lockyer. 



{To be continued.) 



DR. JANSSEN ON THE SPECTRUM OE 

 OXYGEN. 



""PIiE following is an abstract of the account given by M. 

 Janssen, in Section A of the Blitish Association, 

 of his researches into the different forms of oxygen, in the 

 direction of an inquiry into the molecular constitution of that 

 element. These experiments have been made in the labora- 

 tory which has been organized under Dr. Janssen's supervision, 

 and at the expense of the French Government, at Meudon. The 

 hall in which the observations have been carried out is 100 

 metres in length. It contains every requisite for studying the 

 optical properties of gases ; principally instruments so con- 

 structed that a long column of gas may be examined under a 

 high pressure. One of these is a set of steel tubes varying in 

 length from 0*42 metres to 60 metres, terminated at each end 

 by a glass plate, perpendicular to their axes, and constructed to 

 resist a pressure of 200 atmospheres. The chief result of this 

 work was the discovery of a new law of the selective absorption 

 by oxygen of any beam of light, quite independently of its origin, 

 whether from the sun or the electric light. It was proved that 

 oxygen produces two kinds of absorption-phenomena on the 

 spectrum of the light — first, the known rays ; and, secondly, a 

 system of dark bands which had not, up to this time, been 

 noticed. M. Janssen has demonstrated that the intensity of the 

 rays varies as the products of the length of the column into the 

 density ; while that of the bands varies as the products of the 

 length of the column into the square of the density. The prin- 

 cipal results obtained by M. Janssen are best displayed in the 

 following table : — 



Metres. 

 60 

 20 



5 



i'47 

 o75 

 0*42 



Atmospheres, 

 6 



10 to 12 

 23 

 38 



501055 

 70 to 75 



1. Length of the tube. 



2. Pressure observed. 



3. Pressure calculated by formula Ld- (product of length 

 of column into the square of the density). 



4. Pressure calculated by the formula Ld. 



These numbers are fixed by the point at which the band in 

 the yellow first appears, this phenomenon supplying the standard 

 term of comparison. It is easy to see how nearly the observed 

 results in the second column agree with the figures in the 

 third, and how far they differ from those in the fourth. 

 The law of the square has been discovered by an analytical 

 method, which will be published in full in the Proceedings of 

 the British Association. Dr. Janssen has proved the exact- 

 ness of this law in its application to the oxygen contained in the 

 atmosphere, in measuring the altitude of the sun necessary for 

 the first appearance of the band. He verified the same law by 

 experiments on oxygen in its liquid state, and found that a 

 thickness of 4 to 5 millimetres was sufficient. • The correctness 

 of this law must be considered as valid from o to 700 atmo- 

 spheres. For the flutings of the group B the law of variation 

 according to the formula Ld has been verified from o to 100 

 atmospheres by direct observation of the tubes. It is curious to 

 notice how by the systematic variation of length of column and 

 density it is possible to obtain either lines without ban Is, 

 bands without lines, or bands and lines together. Among the 

 astronomical applications of this law it is noted that a 

 nebula which might have a diameter of 2000 times the distance 

 of the earth from the sun, containing oxygen at a density of 

 one-millionth of an atmosphere, could be traversed by the light 

 of a star without causing the appearance of oxygen-bands in the 

 spectrum. M. Janssen stated that he is still pursuing these 

 investigations, and others attendant thereon, relative to the 



molecular construction of oxygen and its presence in the 

 atmosphere of the planets. 



At the conclusion of Dr. Janssen's paper, Sir Wm. Thomson 

 recapitulated the main facts to the audience, stating his opinion 

 that the discovery of the law of the square of the density was a 

 most brilliant achievement. 



THE GROWTH OF ROOT-CROPS. 1 



"T"IIIS is a pamphlet of extremely closely written matter, 

 - 1 which purports to be a lecture delivered on July 27, 1887, 

 to agricultural students in Cirencester College. Viewing it as a 

 lecture we should accord it qualified praise, because a lecture 

 must be regarded as oral instruction, and ought to be sufficiently 

 dilute and sufficiently moist to allow of the process of mental 

 deglutition. The pamphlet is really a treatise upon the effect of 

 fertilizers on the growth of roots and their composition, and it 

 would be presumption on our part to do more than bow respect- 

 ful acquiescence to each statement made by so learned and so 

 experienced a specialist. 



Dr. Gilbert has studied turnips ever since 1843, and probably 

 long before then, and his knowledge of their habits, their require- 

 ments, and their uses, is unequalled by that of anyone else in this 

 country. Anyone who will read through the pamphlet now before 

 us will find his ideas with regard to these esculents enlarged and 

 dignified. Dr. Gilbert chiefly treats his subject from a chemical 

 point of view — the fertilizers best suited for producing a crop, 

 and the composition of the crop after it is grown. The extra- 

 ordinary dependence of the turnip upon artificial help is shown 

 by many tables, and the erroneous idea that the turnip acts as a 

 renovator or restorer of fertility is exposed and disproved. If any 

 crop is capable of completely exhausting a soil of all its available 

 fertility, it is a turnip crop manured with superphosphate of lime. 

 So far from being a renovator it is a waster. Still, circumstances 

 control cases, and the special circumstances which accompany 

 turnip cultivation are of an ameliorating sort. True, if your 

 turnip is sold off the farm it may be looked upon by the landowner 

 as a burglar making off with his goods and chattels, but con- 

 sumed "on the premises" it yields up its wealth and becomes 

 beneficent. Like John Barleycorn, it springs up again after ever 

 such rough usage, and its spirit lives in succeeding corn crops. 



The superiority of swedes over turnips is shown by the much 

 smaller proportion of leaf existing in them in comparison with 

 white turnips ; and also in the larger proportion of dry matter 

 in the root. White turnips, especially when dressed with nitro- 

 genous matter, gave 600 parts in weight of leaf to 1000 of root. 

 Swedes gave under similar circumstances 228 parts of leaf to 

 1000 of root. White turnips were found to contain from 7 '66 to 



8 "54 per cent, of dry matter, while swedes contained from 10*83 

 to 1 2 '04 percent, of dry matter. In both swedes and turnips the 

 effect of superphosphate of lime in increasing the crop is remark- 

 able when there is a sufficient stock of nitrogen in the soil. A 

 single crop will, however, deplenish the excess of nitrogen, and 

 fresh applications of superphosphate will not act with the same 

 energy. Take, for example, the series of root-crops grown in 

 rotation with other crops, but recurring at intervals of four years 

 in 1848, 1852, 1856, &c. The portion unmanured yielded 



9 tons per acre the first year, but the fifth, ninth, thirteenth, 

 and seventeenth, it only yielded from helf a ton to one ton per 

 per acre. Similarly, superphosphate gave a crop of 14^ tons 

 in 1848, and of II tons in 1852 ; but in 1856, i860, and 1864, the 

 yields produced by the same top dressing varied from lh to 6f 

 tons per acre. In no crop more than in the turnip crop is a full 

 supply of nitrogenous and mineral plant foods more essential, 

 and hence the importance of farm -yard manure for its thorough 

 development. 



But the most interesting portion of the lecture is the second 

 part, in which the effect of fertilizers upon the proportion of sugar 

 and albuminoids in root-crops is dealt with. The effect of nitro- 

 genous dressings in increasing the power of the plant to take car- 

 bon from the air, and especially to elaborate it into sugar, is much 

 enforced. It is, however, evident that the effect of the nitrogenous 

 manure, especially in the case of mangel-wurzel, consists in in- 

 creasing the crop, and the crop being increased the amount of 

 sugar and dry matter generally, will naturally increase also. So 

 far indeed as percentage goes, it is higher where no nitrogenous 

 manure is used than in any other cases. In fact, wherever 

 nitrogenous manures are employed, the percentage of sugar is 



1 "The Growth of Root-Crops." by J. H. Gilbert, M.A., LI..!)., F.R.S., 

 Sibthorpian Professor in the University of Oxford. 



