366 



NA TURE 



\_AMgust 1 8, 1887 



combinations, such as H2SO4 and Na, H2SO4 + Zn, HNO3 

 and K, &c., were tried, and in most cases irregular deflections 

 such as those above described were obtained. Ultimately I got 

 two constant and definite results : (i) Na thrown on strong and 

 pure acetic acid invariably left a positive charge on the insulated 

 dish, the escaping hydrogen being negative ; (2) a fragment of 

 zinc thrown into strong HCl invariably left a negative charge on 

 the insulated dish, the escaping hydrogen being positive. 



This last is a gross and unmistakable result. In fact its very 

 magnitude was for some time a source of embarrassment. I 

 shall not stop to describe the steps by which the next experiment 

 wis reached, but shall proceed at once to describe it ; and I shall 

 venture to give it somewhat in detail, as the title at the head of 

 the paper is mainly founded on it. 



The electi-ometer was not in a very sensitive state. The high- 

 resistance Daniell aforesaid gave a deflection of 38 divisions on 

 either side. A glass beaker 7^ inches high and 5 inches in 

 diameter was placed on the insulated stand. A porcelain dish, 

 2f inches in diameter and ig inches high, was nearly filled with 

 a 10 per cent, solution in distilled water of strong HCl, and 

 placed at the bottom of the glass beaker just described. The 

 insulated stand wa ; now connected tc one pair of quadrants, the 

 other pair were put to earth. The " spot " stood at 378 on the 

 scale. Three small fragments of granulated zinc were now 

 dropped into the dilute HCl in the insulated dish. A very slight 

 effervescence at once appeared. This gradually increased but 

 never became violent. No trace of spray could be detected at 

 the end of the experiment above the lower half of the beaker. 

 In 4 minutes from dropping the zinc the spot could be perceived 

 moving, and in 4^ minutes more it moved 28 divisions to the 

 left, indicating the charge on the dish negative and the escaping 

 hydrogen positive. The insulated stand, &c. , was now discon- 

 nected from the quadrants. The spot maintained its position on 

 the scale. In \\ minute after, the quadrants were again con- 

 nected to the insulated stand : the spot moved instantly 20 divi- 

 sions more to the left. In \\ minute more it had moved 10 

 divisions further to the left, but with a slower pace, and it 

 jiresently stopped and turned back, at first slowly, taking 5 

 minutes to go back the 68 divisions to the zero. In 4 minutes 

 more it had moved 80 divisions to the right. The insulated 

 stand was once more disconnected from the quadrants, and at 

 ihe end of 2 minutes they were re-connected, when the spot 

 instantly bounded up 55 divisions further to the right. It con- 

 tinued to move in the same direction until the effervescence 

 ceased owing to the acid being exhausted. A quantity of the 

 zinc survived. On short-circuiting the quadrants the spot returned 

 to within 4 divisions of the original zero. 



As the reaction between zinc and hydrochloric acid proceeds, 

 the quantity of chloride of zinc in solution continually increases, 

 and so it appears demonstrated that when hydrogen passes 

 through hydrochloric acid it acquires a positive charge, when it 

 passes through chloride of zinc it acquires a negative charge. I 

 I believe that this inference may be safely very much generalized, 

 but for the present I forbear. In confirmatioi of it, however, 

 it may be well to mention that at any stage of the last experi- | 

 ment a deflection may be obtained to right or left as required by 

 adding an excess of saturated chloride of zinc (for the first), or 

 of hydrochloric acid (for the second). 



When it is known that the sign of the charge on escaping 1 

 hydrogen depends upon the substance it has been in contact with, I 

 the very irregular results with K and Na already mentioned 

 become less mysterious. J. E.nright. 



Newton's Laws of Motion. 



There is a point in connection with Newton's laws of motion 

 which the text-books on dynamics, which found the science 

 upon those laws, seem to me to leave very inconveniently and 

 unnecessarily mysteriou-. The point to which I allude is the 

 meaning of the words "rest or uniform motion in a straight 

 line " in the first law. The difficult words are " uniform " and 

 " straight," which of course are each of them meaningless until 

 it is explained what the motion is with reference to ; but this 

 explanation is net given explicitly in any of the books on 

 dynamics which I am acqaainted with ; and a comparison of 

 their various statements leaves me in some doubt as to what is 

 intended to be implied. May I therefore appeal to those of 

 your readers who accept Newton's laws to say whether the 

 following is correct ? 



I find that Law III. is interpreted by the most influential 



authorities, such as Maxwell and Tait, to mean that force occurs 

 only as one side of a mutual action, consisting of two equal 

 and opposite forces between two portions of nntter. I am 

 therefore led to suppose that the freedom from force action, 

 which is spoken of in Law I., should be explained (by means of 

 Law III.) as meaning isolation from the influence of all other 

 matter ; and that Law I. must be considered as containing a 

 definition of an arbitrary meaning to be given in dynamics to 

 the words "rest or uniform motion in a straight line," namely, 

 that it is the motion possessed by any particle isolated from the 

 influence of all other matter, which intluence is to be traced by 

 its mutual character. Law I. would then go on to say, as an 

 experimental result, that all isolated particles move with refer- 

 ence to one another in a way consistent with this definition. 



In order to reach this conclusion I find it necessary to inter- 

 pret some statements in text-books in a somewhat awkward 

 fashion {e.g. Maxwell, "Matter and Motion," article xl.), and 

 to suppose some others to be incorrect ; hence my d jubts, and 

 my appeal for their resolution. W. 



August 9. 



On the Constant P in Observations of Terrestrial 

 Magnetism. 



On page 304 of vol. ii. of their excellent treatise on " Practical 

 Physics," Messrs. Stewart and Gee give the usual expression for 

 the constant depending upon the distribution of magnetism in a 

 pair of magnets employed for measuring terrestrial horizontal 

 force ; namely — 



A - A' 



P = 



A' 



Instead of this awkward and troublesome form, I would 

 suggest 



.V / _ A'N 

 - rA A 



P = 



which can be readily deduced from Gauss's original equations, 

 and is much better adapted to logarithmic computation ; espe- 

 cially when ;- and r.^ remain constant throughout a series of 

 observations, and Gaussian logarithms are used to form the 

 factor (i - A'/ A). Wm. Harkness. 



Washington, D.C., August i. 



The Stature of the Human Race. 



In your "Notes" of last issue, p. 348, you mention General 

 Pitt-Rivers conducting a party of the Royal Archaeological Insti- 

 tute to Woodcutts, where skeletons dug out show that the people 

 who inhabited the ancient Romano-British village were of very 

 inferior stature, the males being only on an average 5 feet 2 

 inches, and the females 4 feet 10 inches. I think it would be a 

 very interesting inquiry to ascertain the average height of the 

 human race in the past, as it appears to me from such data as I 

 have been able to collect that the human race has continuously 

 increased in average stature. I have measured agreat many Roman 

 coffins, where I happened to come across them, and my average 

 shows that the Roman could not have greatly exceeded 5 feet 

 5 inches. In taking measurements of ancient armour, I find that 

 the English aristocracy have decidedly increased in average height 

 within 500 years. For a paper I read before our local Society, I 

 measured twenty-five mummies in the British Museum as nearly 

 as I could through the cases, making estimate for wrap- 

 ping, and I found the average height of males 61 inches, 

 females 55 inches. The mummy of the celebrated Cleopatra 

 measures about 54 inches, about the height of the pres^ 

 average European girl of 13. The most ancient mumm^ 

 an Egyptian king yet discovered measui-ed 52 inches. 

 re^earch I have no doubt interesting data could be obtainec] 

 on this subject. Limiting the matter to my o.vn ob ervations, If 

 have formed the idea that the average stature of the human rac 

 increases at about the rate of i"25 inches per 1000 years. 



Wm. F. Stanley, 



Cumberton, South Norwood, August 13. 



resenj 

 mya 



A Spider al'owing for the Force of Gravity 



The manoeuvres of the small hunting spider, so commo: 

 the West Coast of Africi, are always attractive, and my int^ 



1 



i 



