224 



NATURE 



\yan. II, 1877 



be done is to get into the young mind some notion of 

 what animal and vegetable Hfe is. You have to consider 

 in this matter practical convenience as well as other 

 things. There are difficulties in the way of a lot of 

 boys making messes with slugs and snails ; it might 

 not work in practice. But there is a very conve- 

 nient and handy animal which everybody has at hand, 

 and that is himself; and it is a very easy and simple 

 matter to obtain common plants. Hence the broader facts 

 of anatomy and physiology can be taught to young 

 people in a very real fashion by dealing with the broad 

 facts of human structure. Such viscera as they cannot 

 very well examine in themselves, such as hearts, lungs, 

 and livers, may be obtained from the nearest butcher's 

 shop. In respect to teaching something about the 

 biology of plants, there is no practical difficulty, be- 

 cause almost any of the common plants will do, 

 and plants do not make a mess — at least they do 

 not make an unpleasant mess ; so that, in my 

 judgment, the best form of Biology for teaching to 

 very young people is elementary human physiology on 

 the one hand, and the elements of botany on the 

 other ; beyond that I do not think it will be feasible to 

 advance for some time to come. But then I see no 

 reason why in secondary schools, and in the Science 

 Classes which are under the control of the Science and 

 Art Department— and which I may say, in passing, have, 

 in my judgment, done so very much for the diffusion of a 

 knowledge over the country — I think that in those cases 

 we may go further, and we may hope to see instruction in 

 the elements of Biology carried out, not perhaps to the 

 same extent, but still upon somewhat the same principle 

 as we do here. There is no difficulty, when you have to 

 deal with students of the ages of 15 or 16, in practising 

 a little dissection and getting a notion, at any rate, of the 

 four or five great modifications of the animal form, and 

 the like is true in regard to plants. 



While, lastly, to all those who are studying biological sci- 

 ence with a view to their own edification merely, or with the 

 intention of becoming zoologists or botanists ; to all those 

 who intend to pursue physiology — and especially to those 

 who propose to employ the working years of their lives 

 in the practice of medicine — I say that there is no training 

 so fitted, or which may be of such important service to 

 them, as the thorough discipline in practical biological 

 work which I have sketched out as being pursued in the 

 laboratory hard by. 



I may add that, beyond all these different classes of 

 persons who may profit by the study of Biology, there is 

 yet one other. I remember, a number of years ago, that a 

 gentleman who was a vehement opponent of Mr. Darwin's 

 views and had written some terrible articles against them, 

 apphed to me to know what was the best way m which he 

 could acquaint himself with the strongest arguments in 

 favour of evolution. I wrote back, in all good faith and 

 simplicity, recommending him to go through a course of 

 comparative anatomy and physiology, and then to study 

 development. I am sorry to say he was very much dis- 

 pleased, as people often are with good advice. Notwith- 

 standing this discouraging result, I venture, as a parting 

 word, to repeat the suggestion, and to say to all the more 

 or less acute lay and clerical " paper-philosophers " ^ who 

 venture into the regions of biological controversy — Get a 

 little sound, thorough, practical, elementary instruction in 

 biology. 



T. H. Huxley 



I Writers of this stamp are fond of talking about the Baconian method. I 

 beg them therefore to lay to heart these two weighty sayings of the herald 

 of Modern Science : — 



" Syllogisnius ex propositionibus constat, propositiones ex verbis, verba 

 notionum tesserae iunt. Itaque si notiones ipsae (id giiod basis rei est) con- 

 lusae sint et lemere a rebus ab>tracta;, nihil in iis quas supcrstruuntur est 

 firmitudinis." — " Novum Organon," ii. 14. 



" Huic autem vanitati nonnuUi ex modernis summa levitate ita indulserunt, 

 ut in primo capitulo Geneseos et in libro Job et aliis scripturis sacris, philo- 

 sophiam naturalem fundare conati sint ; inter vivos quterentes moriua. " — 

 Ibid., 65. 



EXPERIMENTS WITH THE RADIOMETER 

 I. 



ABSTRACTS of my earlier papers on "Repulsion 

 Resulting from Radiation" having appeared in 

 Nature, it has been suggested that an account of my 

 later researches, which place the subject in such a dif- 

 ferent light, may also prove of interest. 



It has already been shown that if the air is expelled 

 from a large bulb containing a suspended bar of pith, and 

 a lighted candle is placed about 2 inches from the globe, 

 the pith bar commences to oscillate to and fro, the swing 

 gradually increasing in amplitude until the dead centre is 

 passed over, when several complete revolutions are made. 

 The torsion of the suspended fibre now offers resistance 

 to the revolutions, and the bar commences to turn in the 

 opposite direction. It has been found, however, that very 

 little movement takes place until the vacuum is so good as 

 to be almost beyond the powers of an ordinary air-pump to 

 produce, and that, as the vacuum gets more nearly abso- 

 lute, so the force increases in power. The most obvious 

 explanation therefore is, that the repulsive action is due 

 to radiation ; but at a very early stage of my investiga- 

 tion I found that the best vacuum I had succeeded in 

 producing might contain enough matter to offer resistance 

 to motion, and in describing an experiment in a paper 

 sent to the Royal Society on February 5, 1876, 1 said that 

 the impression conveyed to my mind was that the torsion 

 beam was swinging in a viscous fluid, and the repulsion 

 caused by radiation was indirectly due to a difference of 

 thermometric heat between the black and white surfaces 

 of the moving body, and that it might be due to a secondary 

 action on the residual gas. 



I have recently succeeded in producing such a com- 

 plete exhaustion that I have not only reached the point 

 of maximum effect, but gone so far beyond it that repul- 

 sion nearly ceases, and the results I have thus obtained 

 seem to show conclusively that the true explanation of the 

 action of the radiometer is that given by Mr. Johnstone 

 Stoney, according to which the repulsion is due to the 

 internal movements of the molecules of the residual gas. 

 When the mean length of path between successive col- 

 lisions of the molecules is small compared with the di- 

 mensions of the vessel, the molecules, rebounding from 

 the heated surface, and therefore moving with an extra 

 velocity, help to keep back the more slowly moving mole- 

 cules which are advancing towards the heated surface ; it 

 thus happens that though the individual kicks against the 

 heated surface are increased in strength in consequence 

 of the heating, yet the number of molecules struck is di- 

 minished in the same proportion, so that there is equiH- 

 brium on the two sides of the discs, even though the 

 temperature of the faces are unequal. But when the 

 exhaustion is carried to so high a point that the mole- 

 cules are sufficiently few, and the mean length of path 

 between their successive collisions is comparable with the 

 dimensions of the vessel, the swiftly-moving, rebounding 

 molecules spend their forces in part or in whole on the 

 sides of the vessel ; and the onward crowding, more 

 slowly-moving molecules are not kept back as before, so 

 that the number which strike the warmer face approaches 

 to, and in the limit equals, the number which strike the 

 back cooler face ; and as the individual impacts are 

 stronger on the warmer than on the cooler face, pressure 

 is produced, causing the warmer face to retreat. 



Before referring at length to the experiments which led 

 to my adopting the above theory, I will describe some 

 effects of dark heat, &c., on the radiometer. In a paper 

 I sent to the Royal Society on January 5, 1876, and which 

 is now being published in the Philosophical Transactions. 

 of the Royal Society, about seventeen pages are occupied 

 with the description of my experiments with various 

 forms of this instrument. In the present paper I pro- 

 pose only to refer to a few typical experiments made 

 during the year 1875. 



