March 22, 1888] 



NATURE 



487 



shortly before that of the Ulloa's ring, 38" 48' ± 48', and 38° 28' 

 ± 22' Thus the agreement between theory and observation is 

 singularly perfect. James C. McConnel. 



St. Moritz, Switzerland. 



"The Teaching of Elementary Chemistry." 



Ih reply to Prof. M. M. P. Muir's letter, I wish to say 

 that, jndging from his answer, Prof. Muir does not seem to 

 consider it necessary in books of which he is senior author to 

 secure that accuracy of which, from his criticisms of the writings 

 of others, one would expect to find him the champion. 



The first extract from the books mentioned sounds curiously 

 to chemists. I consider the statement misleading inasmuch as 

 it appears to convey an idea as to the constitution of caustic 

 soda which is not that generally entertained by chemists ; that 

 this is not the intention of the authors, however, is manifest 

 from p. 247 of the " Elementary Chemistry," where the usual 

 view is stated. 



It is utterly untrue and misleading to state that, " inasmuch as 

 the i^esult of passing chlorine over yellow mercuric oxide dried 

 at about 100° is to evolve oxygen without forming chlorine mon- 

 oxide, .... it may still be justly said that in making chlorine 

 monoxide ' we carry out a reaction in which oxygen is produced 

 in presence of chlorine.'" 



The facts are briefly these : — 



{a) When chlorine gas is passed at ordinary temperature over 

 yellow mercuric oxide, which has been previously heated to 

 300°-40o'', chlorine monoxide is obtained. 



{l>) When a large quantity of chlorine gas at ordinary tem- 

 perature comes rapidly into contnct with yellow mercuric oxide 

 which has been previously dried at ordinary temperature, a 

 violent reaction, accompanied with evolution of light and heat, 

 ensues, and nearly pure oxygen is the only gaseous product. If 

 both the chlorine and the mercuric oxide be kept cool by means 

 of a freezing mixture, chlorine monoxide is the only gaseous 

 product obtained. With intermediate conditions of tempera- 

 ture, &c., mixtures in varying proportions of oxygen and chlorine 

 monoxide are obtained. (Pelouze, Annalen der Cliem. und 

 Pharm. Bd. xlvi. 196.) 



The formation of oxygen in the second case must therefore be 

 due to the decomposition of already formed chlorine monoxide, 

 or to the occurrence of a reaction the conditions of which render 

 the existence of part of the chlorine monoxide impossible. I 

 think the majority of chemists will agree with me that the ap- 

 pearance of oxygen under conditions which insure the non- 

 existence of (or as itself a product of the decomposition of) chlorine 

 monoxide, can scarcely be admitted as in any measure explaining 

 ihc formation of the latter. 



I do not consider it a "verbal quibble " to object to the use of 

 the term " volatilized " as applied to the mechanical removal of 

 particles of a solid substance. 



As to the chemical properties of chlorine, bromine, and iodine, 

 I should indeed be open to the gravest charges of non-acquaint- 

 ance with chemical classification, had I suggested anything so 

 idiotic as that, say, potassium hypobromite and putassium 

 hypoiodite (if ihe latter exists) could be identical. 



I called the passages I quoted misleading, because some of them 

 at least were inaccurate. What amount of inaccuracy is re- 

 quired to make a statement misleading may be a matter for 

 difference of opinion. Apparently it is so. 



Prof Muir states that he will decline to take any notice of my 

 anonymous communications. This, at least, is safe ground ; but 

 I can wait for the second editions of the two books, and see if the 

 inaccuracies are eliminated. In the second edition of " Elemen- 

 tary Chemistry " I hope Messrs. Muir and Slater will also 

 describe the methods (omitted on p. 19) for removing air from 

 oxygen. Whilst these methods remain unpublished, I prefer to 

 remain Z. 



" Kinematics and Dynamics." 



May I ask a short space in your columns to refer to a few 

 points in Prof Greenhill's review of my book on " Kinematics 

 and Dynamics," published in your issue of February 16 (p. 361). 

 I shall be as brief as possible. 



(i) " In questions involving the size of the earth (pp. 74 and 

 80), it is the circumference and not the diameter which should be 

 given in metres, the circumference being 40,000,000 metres," 

 the reason being, I suppose, that in illustrative problems round 



numbers should be employed as data, with the object of facilitat- 

 ing arithmetical calculation. There are doubtless advantages in 

 this course, and in many problems I have adopted it. But should 

 it be made an invariable rule ? Problems based on exact data, 

 such as the ones referred to, on pp. 74 and 80, have for 

 many students a greater interest than those based on approxi- 

 mations. 



(2) " The expression 'knots an hour' (p. 60) is irritating to 

 a sailor." But the expression "knots" simply would be either 

 misleading or puzzling to a student unacquainted with nautical 

 abbreviations. 



(3) " The formula i^v"' = i^v^^ + as is to be preferred to that 

 on p. 34, e^"' = v^ + 2as ; in all cases the factor \ should go with 

 the v^- in the equation of energy." The formula quoted is not an 

 equation of energy, but a kinematical equation. Equations of 

 energy (see pp. 253, 256, 328) hive in all cases the form approved 

 by Prof Greenhill. 



(4) " In dealing with rotation, the author would do well to 

 study Maxwell's geometrical representation of the direction by 

 means of the screw, right-handed or left-handed." I have done 

 so ; but I find that students more readily grasp a specification of 

 the direction of a rotation when it is made by reference to the 

 face of a clock ; probably because few of them are so familiar 

 with right-handed and left-handed Fcrews as they are with clock- 

 faces. 



(5) " In a linear strain the increment of distance of two points 

 in the line of the strain is properly their elongation ; while the 

 ratio of the elongation to the original distance is called the 

 extension, not the elongation, as on p. 167." And yet Thomson 

 and Tait (" Elements of Natural Philosiphy," § 139), Clifford 

 ("Elements of Dynamic," p. 158), Minchin (" Uniplanar 

 Kinematics of Solids and Fluids," § 78), and Ibbetson (" Mathe- 

 matical Theory of Elasticity, " § 53), all define elongation exactly 

 as I have done. 



(6) " The author, disregarding the vernacular use of the word 

 ' weight,' defines the weight of a body as the force with which it 

 is attracted by the earth " [I don't (see § 290) ; but let that 

 pass], "but is at variance with his own definition in the state- 

 ment of the majority of the subsequent examples, relapsing into 

 the language of ordinary life." No references are given to these 

 instances of backsliding. I have looked pretty carefully through 

 the subsequent examples, and can find no case in which I have 

 used ihe term referred to in any other sense than that given it by 

 definition. I should be glad to have such slips pointed out to 

 me, if there are any. 



(7) "A collection of 500 different ways of spelling the name 

 of the tow n of Birmingham has been made, and a similar collec- 

 tion could be made from the present treatise of different ways of 

 expressing the simple ideas of the pound weight and the pound 

 force." It is true that these ideas are expressed by English 

 writers in various ways. And it seems to me desirable that a 

 student should be made acquainted with them. Surely in hold- 

 ing that I should choose one phrase and stick to it, your reviewer 

 is blaming me f<r not being one of the "mathematical pre- 

 cisionists" at whom he sneers. 



(8) "This terminology culminates in the solecisms that on 

 p. 477 we must suppose pressure to be measured in poundals on 

 the square foot in hydrostatical problems ; and that if the equation 

 w = mg is supposed to be used with absolute units, the weight of 

 a body is measured in poundals ; as if a mathematician asked in 

 a shop for ' half a poundal of tea, or tobacco.' " It is not quite 

 correct to say that, in the hydrostatical equations referred to, 

 pressure must be supposed to be measured in poundals per square 

 foot. In fact it may be supposed to be measured in terms of the 

 unit of pressure of any derived system, as, e.g., the dyne per 

 square centimetre, or even the pound-weight per square foot, 

 provided only the density be measured in terms of the cor- 

 responding unit. I am aware that this mode of expressing 

 hydrostatical equations is unusual, but it seems to me to have 

 great advantages, and it was adopted both for this reason and for 

 the sake of making the section on hydrostatics uniform with the 

 rest of the book. With regard to the units in which weight 

 should be measured, the practice of the tobacconist or the tea 

 merchant is surely not our best guide. 



(9) "Thus a mathematical precisionist, to express the simple 

 idea of a force of 10 pounds, to be consistent should call it ' a 

 force equal to the weight of the mass of 10 pound weights,' the 

 absurdity of which is evident." The phrase inclosed in quota- 

 tion marks is not quoted from my book. In my terminology the 



I most precise of mathematicians would express the idea referred to 



