590 



NATURE 



\_Feb. 24, 1 88 1 



petent to reduce the planetary rotation without directly 

 affecting the satellite's orbital motion. 



It is then shown to be probable that solar tidal friction 

 was a more important cause of change when the planets 

 were less condensed than it is at present. Thus we nre 

 not to accept the present rate of action < f solar tidal 

 friction as indicating that which has held true in all past 

 time. 



It is also shown that if a planetary mass generates a 

 large satellite, the planetary rotation is reduced after the 

 change moie rapidly than before; nevertheless the 

 genesis of such a satellite is preservative of the moment 

 of momentum which is internal to the planetary sub- 

 system. This conclusion is illustrated by the compara- 

 tively slow rotation of the earth, and by the large amount 

 of angular momentum residing in the system of moon 

 and earth. 



An examination of the manner in which the difference 

 of distances of the various planets from the sun will have 

 affected the action of tidal friction leads to a cause for 

 t'le observ-ed distribution of satellites in the solar system. 

 According to the nebular hypothesis a planetary mass 

 cjntracts, and rotates quicker as it contracts. The 

 rapidity of the revolution causes its form to become un- 

 stable, or perhaps, as seems more probable, an equatorial 

 belt gradually detaches itself; it is immaterial which 

 of these really takes place. In either case the separa- 

 tion of that part of the mass which before the change 

 had the greatest angular momentum permits the central 

 portion to resume a planetary shape. The contraction 

 and increase of rotation proceed continually until another 

 portion is detached, and so on. There thus recur at 

 intervals a series of epochs of instability or of abnormal 

 change. 



Now tidal friction must diminish the rate of increase of 

 rotation due to contraction, and therefore if tidal friction 

 aad contraction are at work together the epochs of insta- 

 bility must recur more rarely than if contraction acted 

 alone. 



If the tidal retardation is sufticiently great, the increase 

 of rotation due to contraction will be so far counteracted 

 as never to permit an epoch of instability to occur. 



Now the rate of solar tidal friction decreases rapidlv as 

 we recede from the sun, and therefore these considera- 

 tions accord with what we observe in the solar system. 

 For Mercury and Venus have no satellites, and there is a 

 progressive increase in the number of satellites as we 

 recede from the sun. 



Whether this be the true cause of the observed distri- 

 bution of satellites amongst the planets or not, it is 

 remarkable that the same cause also affords an e.xplana- 

 tion of that difference between the earth with the moon 

 and the other planets with their satellites, which has 

 permitted tidal friction to be the principal agent of 

 change with the former, but not with the latter. 



In the case of the contracting terrestrial mass we may 

 suppose that there was for a long time nearly a balance 

 between the retardation due to solar tidal friction and the 

 acceleration due to contraction, and that it was not until 

 the planetary mass had contracted to nearly its present 

 dimensions that an epoch of instability could occur. 



If the contraction of the planetary mass be almost 

 completed before the genesis of the satellite, tidal friction, 

 due jointly to the satellite and the sun, will thereafter be 

 the great cause of change in the system, and thus the 

 hypothesis that it is the sole cause of change will give an 

 appro.ximately accurate e.xplanation of the motion of the 

 planet and satellite at any subsequent time. It is shown 

 in the previous papers of this scries that this condition is 

 fulfilled with the earth and moon. 



The paper ends with a short recapitulation of those 

 facts in the solar system which are susceptible of explana- 

 tion by the theory of the activity of tidal friction. This 

 series of investigations affords no grounds for the rejection 



of the nebular hypothesis, but while it presents evidence 

 in favour of the main outlines of that theory, it introduces 

 modifications of considerable importance. 



Tidal friction is a cause of change of which Laplace's 

 theory took no account, and although the activity of that 

 cause is to be regarded as mainly belonging to a later 

 period than the events described in the nebular hypothesis, 

 yet its influence has been of great, and in one instance 

 of even paramount, importance in determining the present 

 condition of the planets and their satellites. 



G. H. D. 



r 



INDIGO 



N July, 1S7S, an account was given in this journal of the 

 synthesis of indigo-blue from phenylacetic acid, accom- 

 plished by Prof. Baeyer of Munich (Nature, .xviii. 251). 

 The process there described did not permit of the suc- 

 cessful production of indigo-blue on a manufacturing 

 scale at reasonable cost. Since that time Prof. Baeyer 

 has continued to work at the problem, and he has so far 

 succeeded that he has now taken out a patent for the 

 artificial manufacture and application of indigo-blue. 



In a paper in the last number of the Baiiner Berichte 

 Baeyer gives an interesting tt'sniiu' of the steps whereby 

 progress has been slowly made, since 1865, in solving the 

 problem of the synthesis of indigo. 



Following up the work sketched in the article already 

 referred to, Backer attempted to prepare orthonitrophenyl 

 acetic aldehyde, expecting that this substance would yield 

 indol, which may be regarded as the parent substance of 

 the indigo group of compounds. But as the work pro- 

 ceeded Baeyer became more and more convinced that 

 the hypothesis which had guided his earlier work was 

 that which should still regulate his experiments. In 1S69 

 he had written, " In order to prepare indol synthetically 

 it is necessary — in accordance with the formula already 

 given — to introduce a pair of carbon atoms and one 

 nitrogen atom into benzene, and to link these together. 

 The necessary conditions are found in iiitro-cinnainic 

 acid, if one supposes carbon dio.xide and the oxygen of 

 the nitro-group to be removed. And indeed it has been 

 shown that nitro-cinnamic acid yields indol by fusion 

 with potash." The steps in the preparation of indigo- 

 blue, according to Baeyer' s patent, are these : — 



1. Cinnamic acid (or phenyl acrylic acid) — 



C0H5.C2H2.CO2H. 



2. Orthonitrociiinainic acid — 



QHjC 



'NO, 



\C2Hj.CO2H. 



3. Orthonitrocinnamic acid dibroinide — 



prepared by acting on No. 2 with gaseous bromine and 

 crystallising from benzene. 



The dibromide in alcoholic solution is then treated 

 with alcoholic potash, in the proportion of i : 2 molecules ; 

 and after dilution with water 



4. Orthonitrpinoncbi omcinnainic acid — 



C H /^^"- 



is precipitated. By again treating this acid with three 

 molecules of alcoholic potash 



5. Vrthotitrophenylpropiolic acid— 





is produced. When an aqueous solution of this acid is 

 warmed with such feeble reducing agents as grape- or 

 milk-sugar, in presence of caustic or carbonated alkali, 

 indigo-blue separates in crystals. It is not however 



