126 



NATURE 



[October i8, 191 7 



composition of all kinds of matter, the great 

 chemists of the period were of necessity analysts, 

 and the analytical branch of chemistry stood in 

 high repute. Latterly there has been some 

 neglect, and less fundamental knowledge of 

 analysis has been demanded of chemists. This is 

 partly due to the great development of organic 

 chemistry and physical chemistry, which have 

 held out better promise of new discoveries. The 

 lecturer contends, however, that the field of re- 

 search in analytical chemistry is by no means an 

 exhausted one. He instances, in support of his 

 opinion, the possibility of finding uses for the 

 rarer elements, such as gallium and indium ; the 

 influence of minute quantities of elements on the 

 properties of materials; and the importance of 

 exact analytical methods in physico-chemical re- 

 searches. A national institution of analytical 

 chemistry is wanted, of high scientific authority 

 and in touch with industry : the best conditions 

 for establishing and maintaining such an institu- 

 tion are discussed by Dr. Hillebrand at some 

 length. C. S. 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



Plated Teeth of Sheep. 



I HAVE read with interest the letters from various 

 correspondents in Nature (vol. xcix., pp. 264, 284, and 

 306) and the abstract of Prof. Liversidge's paper (ibid., 

 p. 290). Since the latter paper was published, 

 operating on a much larger sample than Prof. Liver- 

 sidge had at his disposal, 1 have been able to make a 

 complete quantitative analysis of the deposit, not only 

 from the teeth of sheep, but also from those of oxen 

 and of man, both of which in composition agree closely 

 with that from the sheep. A full account of my in- 

 vestigations will shortly be published in the Proc. 

 Linnean Society of New South Wales. 



Some of the writers in Nature attribute the deposit 

 to iron pyrites, either derived direct from fragments of 

 mineral occurring about the pastures, or formed by 

 interaction of iron oxide with sulphates. It would 

 be interesting to know if any of these correspondents 

 have applied tests to prove the presence of iron 

 sulphide. Under the conditions ruling in a sheep's 

 mouth, the formation of iron sulphide from the oxide 

 and a sulphate is quite impossible, nor could iron 

 pyrites be caused to spread itself on, and adhere to, the 

 teeth when chewed along with its food by the animal. 



As indicated by Prof. Liversidge, the deposit con- 

 sists mainly of calcium phosphate with organic matter. 

 As a matter of fact, such deposits, commonly known 

 as " tartar," are of very general occurrence, being 

 found not only on the teeth of sheep, but also on those 

 of all mammals, including marsupials, which I have 

 examined. Sometimes the coating is in a very thin 

 layer, and brown or black in colour, but it varies up 

 to quite a heavy coating one-eighth of an inch or 

 more in thickness. In the case of sheep it does not 

 always present the metallic appearance which has been 

 the cause of so much speculation ; it is sometimes 

 nacreous and sometimes chalky. In man it consti- 



NO. 2503, VOL. 100] 



tutes the "tartar," which is removed from the teeth 

 by dentists in the operation of "scaling." 

 ' It is not derived from the food as such, or from any 

 accidentally eaten mineral, but is a true salivary cal- 

 culus, precisely analogous to the calculi so commonly 

 occurring in the urinary tract, and is derived, probably 

 entireh', from the saliva. 



It is extremely unlikely that the deposit found in 

 England differs from that occurring in Australia. I 

 may say that I have handled and examined many 

 hundreds of sheep's jaws, as well as those of other 

 animals, in consignments of bones received at a large 

 bone charcoal factory in Sydney, and in every case the 

 deposit, when present, has been as above described. 

 I have also on several occasions found good examples 

 of the deposit on the teeth in sheep's heads purchased 

 in retail shops. Thos. Steel. 



Sydnev, N.S. Wales, August 8. 



An Optical Phenomenon. 



I HAVE never seen the following phenomenon de- 

 scribed; perhaps a physiologist can give the explana- 

 tion. If the eye is fixed on a stream of water for 

 twenty or thirty seconds, and is then turned on to 

 a fixed object, the part of the field of view that had 

 previously been occupied by the stream appears to 

 move in a contrary direction to that in which the 

 water had been moving; the apparent motion slows 

 down rapidly and ceases in from five to ten seconds. 

 This is seen not only with lateral motion, but also 

 with up-and-down motion, as when a stream is looked 

 down on from a bridge. The phenomenon is perhaps 

 best seen with running water, but it may be observed 

 with other bodies in motion — a passing train, for in- 

 stance. The effect is very curious, as only part of 

 the field of view appears to move, and it is remarkable 

 to see objects apparently in motion, yet not changing 

 their position relativelv to objects above or below. 



C. J. P. Cave. 



Lynmouth, October 4. 



The Fireball of October 1 last. 



Since forwarding to you the results of a preliminary 

 discussion of the observations of this fine object, a 

 large amount of additional data has come to hand. 

 A re-examinafion on the basis of forty-four reports 

 shows that the heights of the fireball as given in my 

 contribution to Nature of October 11 are as nearly as 

 possible correct. But the radiant point should be 

 further N.E., and the position of the object over Eng- 

 land was more probably from forty-five miles E. by 

 N. of Hull to twelve miles S. by E. of Wolsingham, 

 Durham. 



There were evidently two fireballs on the same night, 

 one at 6h. 37m., the other at loh. 46m. (October i), 

 and they appear to have been both directed from a 

 radiant point near the star 7 Piscium. It was this 

 radiant which furnished the brilliant detonating 

 meteorite that fell in a field near Wigan on October 13, 

 1914. It has also vielded many large fireballs in Sep- 

 tember. W. F. Denning. 



44 Egerton Road, Bristol, October 12. 



The Autumn Moon. 



In the attractive paper on "The Autumn Moon" in 

 Nature of September 27 Sir Geo. Greenhill refers to 

 the mistakes of poor common folks, and of poets and 

 painters even, when they deal with things astronomical. 

 He instances Coleridge's reported intention (in the 

 first draft of " Christabel ") to seat a star within the 

 horns of the crescent moon. The idea seems to have 



