March i6, 1876] 



NATURE 



385 



there is no question of saving life, and which is repeated a thou- 

 sand times for the privnte benefit of its performers, is omitted ! 



6. In Dr, Ferrier's research, anesthetics were so "carefully 

 and liberally given," that five animals out of the twenty-nine 

 sacrificed "died before they were touched or operated on in any 

 way" (3178). Francis Darwin 



Down, Beckenham 



The Use of the Words "Weight" and "Mass" 



In a letter with the above heading published in Nature, vol. 

 xiii. p. 325, Mr. Bottomleyhas recommended that the ambiguity 

 of the word weight shall be avoided by using the phrase "the 

 gravity of a pound " when we mean the downward force due to 

 the earth's attraction upon a pound weight. 



The ambiguity against which Mr. Bottomley wishes to guard 

 is a very real one. Not to speak of common usage, which 

 allows three meanings of the word weight to be loosely inter- 

 mingled, we have two of these meanings adopted into scientific 

 nomenclature. The universal practice in chemistry is to employ 

 the word weight to signify mass, and anyone may satisfy him- 

 self that this use of the word could not be dispensed with in 

 that science by making the attempt to substitute other forms of 

 f xpression for the convenient words weight, heavy, light, heavier, 

 lighter, as used by chemists. On the other hand, physicists 

 have generally employed the term to signify a force, and the best 

 writers on mechanics are careful to avoid using it to express 

 mass. 



But I fear Mr. Bottomley's remedy, if adopted, would intro- 

 duce quite as serious, perhaps a more serious, ambiguity. Gravity 

 is an acceleration. When we say that gravity is less in a balloon 

 or in a mine than at the surface of the earth, or greater at 

 Glasgow than at Manchester, we are speaking of alterations of ^ 

 — the acceleration due to the earth's attraction ; and it would 

 create confusion to employ this word to designate forces also. 



But a practice which I adopted in lecturing on mechanics in 

 Queen's College, Galway, many years ago seems to meet the 

 difficulty, and may perhaps recommend itself to other teachers. 

 It is to use the word gravitation in the proposed sense. 



If this were done, gravity at Glasgow would mean an acce- 

 leration ; the gravitation of a kilogramme there would be a 

 force ; and wiight would continue a word of doubtful import, to 

 be judged of by the context, sometimes used for a force, some- 

 times for a mass, and sometimes for those pieces of metal which 

 are employed as measures in weighing (as in the phrases " a set 

 of weights," "a gramme weight "). In further support of my 

 suggestion, it may be observed that the proposed use of the sub- 

 stantive gravitation follows legitimately in the English language 

 from the established meaning of its correlative, the verb gra- 

 vitate. 



I would wish to take this opportunity of also recommending a 

 prefix which I have found of the utmost service both to students 

 by assisting them to acquire clear conceptions with ease, and to 

 myself. We use the prefix h\per placed before the name of any 

 metrical weight, as hyper decigramme, hyper-gramme, hyper- 

 hektogramme. hyper-kilogramme, hyper-tonne, to indicate those 

 forces of the absolute metrical series which are slightly larger in 

 amount (about 2 per cent, more) than the gravitation at the 

 earth's surface of the decigramme, gramme, hektogramme, &c., 

 respectively. 



When a student has to use weights as forces, as he must in the 

 laboratory, he should be trained from the beginning to think of 

 them in their relation to the neighbouring absolute forces. For 

 instance, if he uses a hektogramme to exert a pressure, he should 



p 

 be encouraged to think of it rather as — ths of a hyper-hekto- 



t> lO 



gramme (which is a force) than as a hektogramme (which is 

 more properly a mass). This will also keep prominently before 

 his mind that the amount of the pressure depends on the station 

 at which the experiment is made. G. Johnstone Stoney 

 Queen's University, Ireland, March 9 



Mr. Bottomley remarks in his letter on weight and mass 

 that appeared in N.\TURe, vol. xiii. p. 325, that " During the 

 present session we have aided ourselves in Glasgow with four 

 very important helps to the teaching of the kinetic system of 

 force-measurement. . . . The third help is the construction by 

 Prof. Thomson, for the first time so far as I know, of spring 

 balances for indicating poundals and kilodjmes." 



Will you permit me to point out that about three years ago 

 Prof. Ball, when introducing the C. G. S. system of units into 

 the course of mechanics at this college, had a series of dynamo- 

 meters in absolute measure specially constructed by Messrs. 

 Salter, of West Bromwich. These dynamometers were exhi- 

 bited at the Bradford meeting of the British Association, and will 

 also be seen at the forthcoming Loan Exhibition at the South 

 Kensington. W. F. Barrett 



Royal College of Science, Dublin 



Metachromism 



Mr. Flinders Petrie in his interesting letter (vol. xiii. 

 p. 348) criticises the abstract of my paper which appeared in 

 Nature some weeks ago. Before considering his communica- 

 tion, I would remark that my argument against Schonbein's 

 theory accounting for metachromism is based upon the colour 

 relation which he mentions. I gave a small table of anhydrous 

 binary compounds which conform to the rule, and that table 

 includes the chlorides of chromium which Mr. Petrie has pushed 

 out into the cold. The relation is thus referred to : — " Those 

 compounds in a series which show the highest amount of the 

 basylous element have the most refrangible colours." So far as 

 I am aware, it is there announced for the first time. 



For the sake of clearness we will first examine Mr. Petrie's 

 proposition: — "Increase of the electro-negative element pro- 

 duces a colour- change towards the red end of the spectrum, and 

 vicez'ersd." 



Increase of the electro-negative element is accompanied \>j les? 

 refrangible colours, but to say that this increase produces the 

 change is going farther than the observations warrant, is, in 

 short, opposed to fact. For exr mple, if we take the series of 

 oxides of chromium which he gives — Cr203 green, CrO._, yellow 

 green, CrOs red, I fail to see that increase of the electro-negative 

 element, i.e., colourless oxygtn produces a change towards the red 

 end, or, on the contrary, that decrease in the positive element 

 does the same. The facts seem rather to show that colour in any 

 body is dependent upon the proximity of its molecules, since we 

 find bodies which, with like chemical composition but different 

 densities, have different hues. 



The metachromatic scale given on page 298 is not intended to 

 be absolute, and may, in fact, need a little modification with the 

 accession of more knowledge. But certainly Mr. Petrie's re- 

 marks do not affect it, because (1)1 the colour gradation he refers 

 to is attended by chemical differences, whereas in metachro- 

 matic phenomena we have purely physical alterations ; (2) white 

 does not come between yelloiv and blue, either in the "natural" 

 or in the metachromatic arrangement. For if by " natural 

 arrangement " he means that of the pure spectrum, then green 

 is what intervenes between blue and yellow, and white has its 

 nearest counterpart in the ultra-violet grey. Quite recently this 

 part of the spectrum has been termed " silvery grey ' by M. 

 Sauer. Independently of this, however, I was led to place 

 white in the ultra-violet part of the metichromatic scale by cer- 

 tain mineralogical facts which I shall not trouble your readers 

 with detailing here. 



The assertion, then, that white comes between yellow and 

 blue, would seem to rest upon the colour relation found to obtain 

 between the oxides of the alkali metals, which really. is not worth 

 much, because of the little we know about the sub-oxides ; and 

 because even the chief series he gives, that of sodium, is an ex- 

 ception to the rule, Na202 being pure white (Watt's "Dic- 

 tionary," vol. v., p. 340), and not orange, as Mr. Petrie 

 states ; and, finally, because we cannot fairly compare the order 

 of colour we see in the binary compounds with what we get in 

 metachromatic phenomena, although to a great extent there is a 

 colour parallelism which is remarkable. Wm. Ackroyd 



Royal College of Chemistry, South Kensington, 

 March 4 



The U.S. Survey Publications 



In Nature, vol. xiii. p. 314, I observed a note upon the 

 rumour that the publication of Prof. Hayden's Geological Re- 

 ports was likely to be stopped by the U.S. Government. 



Having brought the paragraph under the notice of the Museum 

 Committee of the Town Council, I am requested by them to 

 communicate with you, and to say that several of these Reports 

 have been received by the Leicester Museum, and are regarded 

 as of great value ; and that the Museum Committee will be glad 



