244 



N'ATURE 



Sjan. 1 6, 1879 



simply one of various possible algebraic function ; that is, of 

 which energy is one of various possible quantitative measures, 

 and of winch momentum is another such measure. But although 

 the reality of force implies the reality of energy and of mo- 

 mentum, the absolute quantitative definitiveness of force does 

 not imply any corresponding quantitative definiteness of energy 

 or of momentum. Now physics is distinguished from meta- 

 physics by being essentially quantitative. It appears, then, that 

 force is a physical reality independent of relation to axes of 

 reference, and that energy and momentum become physical 

 realities only when they are referred to such axes, because when 

 not so referred, they have no quantitative definiteness. They 

 remain, however, when not referred to axes, what may be 

 called non-quantitative realities, and probably many people 

 would choose to call them on that account metaphysical 

 realities. 



In conclusion I may offer one remark not strictly bearing upon 

 the subject of this letter, which is the proper physical use of the 

 words force and energy, but which was suggested during an expla- 

 nation of the above definition of force to a friend. There are some 

 minds so constituted that they cannot get on at all without con- 

 tinually referring to metaphysical ideas. This fact should make 

 those whose minds are not so constituted unwilling to believe, as 

 they are very apt to do, that metaphysics is only an unreal, im- 

 proper, and injurious phantasy or disease of the brain. If there are 

 two such real sciences as metaphysics and physics, in the first 

 place it is clearly advantageous to avoid confusion of the two as 

 far as possible, and we may hope to be able keep them separate 

 from the top down to the base where they rest together, or one 

 upon the other. If there are certain words which it is very 

 convenient to use in both these sciences and with accuracy, it 

 is clear that they must have different definitions, i.e., different 

 meanings in the t\\'o. But it would be unfortunate if there were 

 no correspondence between the two meanings. If the two 

 sciences are realities they must consist in two different methods 

 of assimilating as part of our knowledge the same facts ; and 

 the statements of the one science ought to be capable of definite 

 translation into the language of the other. And this ought to be 

 held in view in arranging the nomenclature of the two. Now 1 

 think that the strictly physical definition of force I have given, 

 viz., the time-rate of transference of momentum, has a true 

 correspondence with the ordinarily accepted metaphysical idea 

 of force as "the cause, of the change of velocity in masses." 

 Metaphysically the cause of the acceleration of momentum of 

 the one body is the transference of momentum from the other 

 i)ody, and this transference is also the cause of the retardation 

 of momentum of the other. In the physical definition quanti- 

 tative accuracy is obtained by introducing the idea of the 

 " time-rate." In a metaphysical definition quantitative accuracy 

 is neither possible nor is it desired, the inherent difference 

 between metaphysics and physics being that the latter is quanti- 

 tative while the former is not so. The friend to whom I threw 

 out this hint objected that I was here only going one step further 

 back, and that the question became " what was the cause of the 

 transference of momentum ? " It was evidently he who had 

 made the step backwards, and of course it was a metaphysical 

 step, not objectionable in itself, but having no bearing on the 

 matter in hand. The above question is no objection to the 

 metaphysical statement or definition, that the cause of the 

 acceleration of momentum is the transference of momentum. If 

 metaphysics is fit to do anything at all it ought to be able to 

 investigate the cause of. a cause ; but even if it were not able to 

 follow the chain of causes beyond any certain point, that would 

 not constitute any objection to the statements of causative 

 sequence made in following along the chain to the possible 

 limit. The metaphysical answer to the question, " What is the 

 cause of transference of momentum ? " would probably be dif- 

 ferent according to the cb-cumstances of the transference, whether 

 it were by impact or by gravitation, or otherwise. To show, 

 however, that my physical definition of force has a true corre- 

 spondence to the metaphysical idea, it is quite imnecessary to 

 answer this question, it is unnecessary to go beyond the cause 

 which is called "force" in metaphysics. 



Robert H. Smith 



fallacy long held by many physiological botanists in antagonism 

 to the experience of plain observers of nature. 



In reference to the concluding remark on the statements of 

 Prof. Calderon, the following may perhaps be interesting. 



Every botanist who visits my Sewage Farm is struck with the 

 luxuriance not only of the cultivated crops, but with that of 

 weeds found growing, out of reach of the hoe, on hedge-banks 

 and places whence it is impossible for their roots to reach the 

 fertilising stream, which readily accounts for the growth of the 

 crops. 



It seems clear, therefore, that plants can absorb nitrogenous 

 organic matter which may ba wafted over their leaves by winds 

 from a sewage- irrigated field, and I welcomed Mr. Darwin's 

 account of insectivorous plants as a confirmation of my theory ; 

 but, after all, no one has ever doubted the power of absorbing 

 carbon through leaves since van Helmont's celebrated experi- 

 ment with the willow, and it can hardly be unnatural to credit 

 plant-life with the power of obtaining another element of nutri- 

 tion by the same channel. Alfred S. Jones 



Havod-y-wern Farm, Wrexham 



The Formation of Mountains 

 I HAVE deferred replying to Mr. Fisher's letter (Nature, 

 vol. xix. p. 172) till I had an opportunity of looking at 

 Maxwell's " Theory of Heat ; " but, having done so, I am no 

 wiser, for I do not find the point in dispute anywhere referred 

 to. In the "English Cyclopaedia," art. "Heat," I find, how- 

 ever, the following statement : " If we suppose the mass of the 

 earth to have been at any remote period at a very high tempera- 

 ture, the effect of the radiation of its heat through the colder 

 surrounding space would be, to cool first the superficial strata, 

 and successively, though in a less degree, the internal strata." 

 Tliis slower cooling of the internal parts of a heated mass seems 

 a necessary result of the "law of exchanges," to which the 

 supposed "more rapid cooling of the interior of the globe than 

 the crust " seems as decidedly opposed. 



Mr. Fisher's illustration certainly shows how the centre might 

 cool more rapidly than the outside, if heat were not subject to 

 laws, and could set the law of exchanges at defiance. He says : 

 "As the people disperse they move off the more quickly the 

 further they get from the dense mass." This would be true for 

 heat, and exactly corresponds to the quotation given above from 

 the "English Cyclopsedia ; " but it is inconsistent with Mr. 

 Fisher's statement a little further on, that the numbers in an 

 outer belt "may continue about the same, while those in the 

 central crowd become fewer and fewer." The two things are 

 contradictory ; and I still fail to see how the "more rapid cooling 

 of the interior of the earth," limited as it must be to that super- 

 ficial layer within which the effects of solar heat are confined, 

 can be held to furnish a vera causa for the compression and 

 contortion of deeply seated rocks and their upheaval into moun- 

 tain chains. Alfred R. Wallace 



Absorption of Water by the Leaves of Plants 

 I feel sure th»t many of your practical readers will be pleased 

 with the article in Nature, vol. xix. p. 183, on the "Absorp- 

 tion of Water by the Leaves of Plants," as a correction of a 



Musical Notes from Outflow of Water 

 Every one is familiar with the sounds produced by water 

 running out through a pipe from the bottom of a vessel, when 

 the water-level has got low. The other evening I witnessed a 

 phenomenon of this order, which has, I think, certain interesting 

 features. Desiring to empty my cistern, and the pipes being 

 frozen, I rigged up a gutta-percha tube siphonwise, and brought 

 the water through it. When the orifice of the tube in the cistern 

 got partially uncovered by the descending water-level, a series of 

 rhythmical vibrations was generated, giving a musical note. The 

 plane of the orifice was about vertical ; but notes may be had 

 when it is at any inclination with the horizontal water-surface. 

 The intensity of the notes depends, I believe, partly on the differ- 

 ence of level of the vessels ; but I cannot furnish exact data as 

 to this, or the way the pitch is affected by various influences 

 (width of pipe, &c.). Would some one proffer an explanation 

 of the "mechanism" or essential character of the phenomenon? 



M. 



Shakespeare's Colour- Names 



Mr. Brewin's assertion that Shakespeare's "word was 

 doubtless keen" (^xiot green) in the passage ("so green, so quick, | 

 so fair an eye ") in " Romeo and Juliet," iii. 5, may be put oqJ 

 a par with his " wonder that the correction was not made longj 



