March 29, 1888] 



NATURE 



511 



down to zero, and at the instant the train comes to rest the 

 constant (negative) acceleration abruptly ceases. 



How would this show itself in our feelings ? If I were sitting 

 with my back to the engine, leaning against the wall of the 

 carriage, I should feel a slight mutual pressure between my back 

 and the wall. This would remain constant to the end, and then 

 abruptly cease. It would be but slight (for any possible brake 

 acceleration must be but a small percentage of that of gravity) 

 in comparison with that which we feel when we lie down. Would 

 its abrupt termination — there being by supposition no sudden 

 change of velocity — be really unpleasant ? 



To take a fanciful example. If, as I sit in my chair, gravity 

 were suddenly annihilated (for me) : I should note the cessation 

 of pressure, and, in so far as my body is elastic, there would be 

 some change of conformation. The pressure in the blood-vessels 

 would also be changed, &c. But, dealing with even such a large 

 acceleration as this, would the instantaneous change be at all 

 comparable with that of a sudden trifling change ol velocity? 



Anyone who has been in a lift at the moment the cord 

 broke might be able to tell us what all this feels like : but he 

 must be careful to distinguish between the sensations due to 

 the first moment of his passage from those due to the last. 



Caius College, Cambridge. J. Venn. 



The Definition of Force and Newton's Third Law. 



Perhaps your correspondents now engaged in discussing the 

 value of dynamic terms could extend the range of their con- 

 troversy a little, and deal with a subject of great importance 

 which no text-books touch. 



It seems to me that the definition of force as that which causes 

 or alters motion is not reconcilable with Newton's law which 

 asserts that every force is always opposed by an equal and 

 opposite force. 



How can a force opposed by an equal and opposite force per- 

 form work, or affect the motion of anything ? We have here 

 either a fallacy or an indefiniteness, and the matter is worth 

 clearing up because it incessantly worries students who think. 



March 23. Nemo. 



Green Colouring-matter of Decaying Wood. 



Anyone who lives in a fairly wooded part of the country 

 must be familiar with the fact that at certain stages of decay 

 fallen branches of trees are often to be observed among the dry 

 forest-litter coloured more or less through their tissue with 

 various shades of green. After an examination of thin sections 

 with the microscope, I am unable to trace this to any sapro- 

 phytic organism. Chemical analysis, on the other hand, reveals 

 the presence of iron as the base of tlie green colouring-matter 

 (using fairly strong nitric acid as a solvent), which —so far as the 

 evidence at present goes — seems to be some organic salt of iron, 

 the organic acid being probably furnished by the slow decomposi- 

 tion of the woody tissue. In the hope that some further light may 

 be thrown on the origin of the green-colouring matter of many 

 Tertiary green earths, I would ask the favour of being allowed to 

 solicit references to any foreign literature of the subject with 

 which any of the numerous readers of Nature may be acquainted. 



A. Irving. 



Wellington College, Berks, March 17. 



THE HITTITES, WITH SPECIAL REFERENCE 

 TO VERY RECENT DISCOVERIES.^ 



r\Y late much has been said concerning the Hittites, 

 ^^ and, as might be expected in relation to such a 

 subject, there have been fanciful hypotheses and wild 

 vagaries, repugnant alike to the scientific method and the 

 scientific spirit. But most persons, one would suppose, 

 who have given serious attention to the subject, must have 

 become convinced that there is a great vacancy in that 

 map of the past which ancient history presents. Mighty 

 kings, dynasties enduring, it may be, through thousands 



» Based on Lectures delivered by Mr. Tho.nas Tyler at the British Museum 

 in January 1888. 



of years, great peoples who had made no slight advance 

 in civilization, have passed away without leaving any 

 chronicle equal even to those which were extant concern- 

 ing Egypt and Assyria before the decipherment of the 

 hieroglyphics and the cuneiform characters. The recent 

 change in public opinion concerning the Hittites is not 

 due merely to the discovery of monuments and inscrip- 

 tions in various parts of Asia Minor : a large proportion 

 of these had been known to exist for a considerable time, 

 some for a very long time. It must be referred rather to 

 the recognition of an identity or similarity of character in 

 these monuments and inscriptions. And thus has arisen 

 the idea of an empire stretching from the Euphrates to the 

 .(^Igean Sea. It is, however, doubtful whether — if we use 

 the word " empire " in such a sense as we employ it when 

 we speak, for example, of the Empire of Russia or the 

 Empire of China — there is any ground for believing that a 

 Hittite Empire ever existed. Most likely there were in 

 Asia Minor many States, or even single cities, which were 

 usually to a great extent independent, and the peoples of 

 which were not, perhaps, altogether homogeneous in race, 

 but which, under pressure of the necessities of war, 

 formed a federation. This view accords with the passages 

 in the First and Second Books of Kings which speak of 

 the "kings of the Hittites" (2 Kings, vii. 6) and of "all 

 the kings of the Hittites " for whom Solomon's merchants 

 brought up out of Egypt chariots and horses (i Kings, x. 

 28, 29). The testimony of these passages in relation to 

 the greatness of the Hittite peoples has been till recently 

 but little regarded. 



That the Hittites thus spoken of in the Old Testament 

 are to be identified with the Khita of the Egyptian monu- 

 ments, and with the peoples of the land of Khatti in the 

 Assyrian records, is coming out more and more clearly ; 

 and as an especial hnk joining together the peoples thus 

 designated by the Egyptians and Assyrians may be 

 mentioned the city of Carchemish. Holding the Upper 

 Euphrates, the Hittites stood between the Egyptians and 

 the powerful and warlike peoples of Mesopotamia. On a 

 superficial view this may seem not to be the direct route 

 from Egypt to Mesopotamia ; but to lead an army by the 

 apparently more direct way across the Syrian desert 

 would have been difficult or wholly impracticable. More- 

 over, it would not have been easy for an army to make 

 the passage of the Euphrates towards the mouth of the 

 river. But by the upper course of the Euphrates, at or 

 near the site of Jerablus, the river could be crossed with 

 comparative ease. On the site of Jerablus (from which 

 the British Museum obtained a few years ago most of the 

 Hittite monuments now in the collection) stood in all 

 probability the renowned city of Carchemish. This 

 identification, attributed to Mr. Skene, was accettfed by 

 the late Mr. George Smith, who visited the place shortly 

 before his death. It was this city of Carchemish (not to 

 go back further in Assyrian or Babylonian history) of 

 which Tiglath-Pileser, about 1100 years before Christ, 

 says, " The city of Kargamis, belonging to the country of 

 the Khatti, I smote in one day. Their fighting-men I 

 slew, their movables, their wealth, and their valuables I 

 carried off." He records further that he pursued the 

 portion of the Hittite army which fled ; that he crossed 

 the Euphrates in boats covered with bitumenized skins ; 

 and that he returned triumphantly to his city of Ashur. 

 The conflict between the Hittites and Assyrians was, 

 however, destined still to continue for 400 years, during 

 which, time, though repeatedly sustaining defeat, the 

 Hittite^ made again and again a determined resistance. 

 It was the fortune of Sargon to end the conflict by the 

 capture, in 717 B.C., of Carchemish and its king, Pisiris. 



Previous Egyptian monarchs had engaged in conflict 

 with the Hittites with more or less conspicuous success ; 

 but it was the renowned son of Seti, the great Rameses 

 II., about 1330 B.C., whose war with the Khita, and the 

 great battle fought with them at Kadesh, appear to have 



