152 



NATURE 



[Dec. 19, 1889 



that " each division or tubercle of the [horny] molar is separately 

 developed, and they become confluent in the Course of growth." 

 By the way, no one can have been better acquainted with the 

 work of Home than his successor in the Hunterian Chair, Sir 

 Richard Owen ; and yet, in his numerous references to this 

 subject (Art. " Monotremata," "Cyclop. Anat. and Physio- 

 logy"; "Odontography"; "Comp. Anat. of Vertebrates," &c.), 

 no trace is shown of any knowledge of a discovery which could 

 not have failed to have interested him, if it had been made 

 before his time. 



If a cursory perusal of Sir Everard Home's first account of 

 the mouth of the Ornithorhynchus (in the Philosophical Trans- 

 actions for 1800), or any interpretation placed upon his figures, 

 might lead anyone to infer, with Dr. Merriam, that the real 

 teeth of the young animal had been discovered at that time, 

 the best possible authority may be conclusively cited against 

 such an idea, no other than that of Home himself, who, in his 

 later description of the same specimen (" Lectures on Compara- 

 tive Anatomy," 1814), describes the organs in question as "the 

 first set of cuticular teeth" — an expression quite incompatible with 

 their being the teeth described by Mr. Poulton and Mr. Oldfield 

 Thomas. It really seems superfluous to have to remind a 

 zoologist of such high repute as Dr. Hart Merriam that the 

 difference between teeth with the structure and mode of growth 

 which characterize these organs in the Mammalia generally, 

 and the horny epithelial plates of Ornithorhynchus, is not merely 

 one of " chemical composition." W. H. Flower. 



The Pigment of the Touraco and the Tree Porcupine. 



Attention has been lately again directed to the red pigment 

 in the wing feathers of the touraco, which has been stated by 

 several observers to be soluble in pure water. Prof. Church, 

 who was the first to experiment upon this pigment ( The Student, 

 vol. i., 1868 ; Phil. Trans., 1869), quotes Mr. Tegetmeier and 

 others, to the effect that this pigment can be washed out of the 

 feathers by water. Later, M. Verreaux (Proc. Zool. Soc, 1871) 

 confirmed these statements from his own experiments while 

 travelling in South Africa ; attempting to catch one of these birds 

 whose feathers were sodden with rain, he found that the colour 

 stained his hands "blood-red." A few years ago Prof. Kruken- 

 berg (" Vergl. Phys. Studien ") took up the study of turacin — as 

 Prof. Church termed the pigment — and added some details of 

 importance to Prof. Church's account ; Krukenberg, however, 

 contradicted certain of the statements quoted by Church with 

 reference to the solubility of turacin in pure water, remarking 

 that the pigment in the dead bird is insoluble in water. A 

 writer in the Standard of October 17 is able " partially to con- 

 firm " the assertion that turacin is soluble in pure water. Seeing 

 that there is some conflict of opinion with regard to this matter, 

 I think it worth while to state that I found it quite easy to ex- 

 tract with tap water (warm) some of the pigment from a spirit- 

 preserved specimen of the bird ; only a very small amount couid 

 be extracted in this way, and the feathers were not perceptibly 

 decolorized even after remaining in the water for a fortnight. I 

 also experimented upon a feather just shed from one of the speci- 

 mens now in the Zoological Society's Gardens ; this was steeped 

 in water for some time without any effect being visible, but after 

 a period of two days the water became stained a very faint pink. 



The touraco, however, is not a unique instance of a terrestrial 

 animal with an external colouring matter soluble in water. I 

 am not aware whether other cases have been recorded, but I find 

 a pigment of a similar kind in a South American tree porcupine 

 {Sphingurus villosus). 



This porcupine has bright yellow spines which are for the 

 most part concealed by abundant long hair. The spines them- 

 selves are parti-coloured, the greater part being tinged with a 

 vivid yellow ; the tip is blackish-brown. I was unable to extract 

 this pigment with chloroform, or with absolute alcohol even 

 when heated ; like so many other colouring substances which are 

 insoluble in these fluids, the pigment could be extracted by 

 potash or ammonia ; I found also that tap water, warm or cold, 

 dissolved out the yellow colour ; the action was slower than 

 when the water was first rendered alkaline by the addition of 

 ammonia, but, unlike the touraco, the pigment was nearly, if 

 not quite, as completely dissolved. The skin, from which the 

 spines were taken, was a dried skin of an animal recently living 

 in the Zoological Society's Gardens ; it had not been preserved 

 in alcohol or treated in any way which might lead to the sup- 

 position that the pigment was chemically altered. There is, 



therefore, a considerable probability that in the living animal 

 the pigment is also soluble in water. I believe that this yellow 

 pigment is undescribed, but I have not yet completed my study 

 of it ; in any case, it is not zoofulvin or picifulvin, or any 

 "lipochrome." Frank E. Beddard. 



Exact Thermometry. 



In the account which Prof. Mills has given (Nature, Decem- 

 ber 5, p. 100) of M. Guillaume's " Traite pratique de la Thermo- 

 metrie de precision," the permanent ascent of the zero-point of 

 a mercurial thermometer, after prolonged heating to a high tem- 

 perature, is stated to be due to compression of the bulb — rendered 

 more plastic by the high temperature — by the external atmo- 

 spheric pressure. 



The constant slow rise of the zero-point of a thermometer at 

 the ordinary temperature is mentioned by Prof. Mills ; and the 

 late Dr. Joule's observation of this change in a thermometer 

 during twenty-seven years is specially alluded to. It may, I 

 imagine, be taken for granted that after the lapse of a sufficient 

 length of time — possibly many centuries — a final state of equili- 

 brium would be attained ; and it has always appeared to me 

 that the effect of heating the thermometer to a high temperature 

 is simply to increase the rate at which this final state is 

 approached. It is my impression that, owing to the more rapid 

 cooling of the outer parts of the bulb after it has been blown, 

 the inner parts are in a state of tension, as, to a very exaggerated 

 degree, in the Prince Rupert's drops ; and that it is the gradual 

 equalization of the tension throughout the glass that causes the 

 contraction ; in other words, that the process is one of slow 

 annealing. 



This explanation appears to be supported by the facts — (i) 

 that when a thermometer is exposed for a long time to a high 

 temperature, the zero-point rises rapidly at first, then more and 

 more slowly, and finally becomes constant or nearly so ; (2) that 

 the higher the temperature the more rapidly is this state of 

 equilibrium attained. I do not know of any experimental 

 evidence that the rate of ascent is influenced by changes of 

 external pressure, and it seemed to be desirable to test the 

 point. 



In order to do this I have exposed three thermometers, A, B, 

 and C, constructed by the same maker and of the same kind of 

 glass, to a temperature of about 280° for several days in the same 

 vapour-bath, under the following conditions : — The thermo- 

 meters were all placed in glass tubes closed at the bottom (C 

 being suspended from above), and the tubes were heated by the 

 vapour of boiling bromonaphthalene. One of the tubes — that 

 containing thermometer C — was exhausted so as to reduce the 

 exterhal pressure on the bulb to zero ; the others were open to 

 the air. In thermometer A there was a vacuum over the 

 mercury, but air was admitted into B and C to increase the 

 internal pressure. Consequently, the bulb of A was exposed to 

 a resultant external pressure equal to the difference between the 

 barometric pressure and that of the column of mercury in the 

 stem of the thermometer ; the internal and external pressures on 

 the bulb of B were approximately equal ; lastly, the internal 

 pressure on the bulb of C was the sum of the pressures of the 

 column of mercury in the stem and of the air above it, while the 

 external pressure was zero. 



The following results were obtained : — 



A. Rise. B. Rise. C. Rise- 



. 0-15 0"10 -O'lO 



0-35 0-25 0-40 



.. 0-50 0-35 0-30 



o"8o 075 Q-So 



After an additional 5 J hours' 

 heating i'30 no I'lo 



Zero before heating . . . 

 After 2 hours' heating 



Total rise of zero- point... I"i5 I'oo i'20 



The thermometers were heated until 5 p.m. each day, and 

 the zero-points read on the following morning. 



If the diminution of volume of the thermometer bulb, usually 

 observed, were due to external pressure, the zero- point of A 

 should have risen, that of B should have remained nearly 

 stationary, while that of C should have fallen. Instead of this, 

 however, the zero-points of all three thermometers rose at nearly 

 the same rate ; therefore the yielding of the bulbs to pressure^ 

 owing to the plasticity of the glass, if it occurred at all, had no 

 sensible effect on the result. Sydney Young. 



University College, Bristol, December 12. 



