Mat 29, 1SS5.] 



♦ KNOWLEDGE ♦ 



451 



of hair, feathers, aaJ other solids, or, as ho calls it, " the 

 attractiou which subsists between air and the bodies above- 

 mentioued, and otlier like substances coustitutiug natural 

 and artificial clothing." He describes this as a necessary 

 supplement to the mei-e mechanical entanglement of otlier- 

 wise freely moving air. 



That such an attractiou, and a very strong one, subsists 

 between air and the clothing material appejii's, as Rumford 

 says, '■ by the obstiuacy with which these sul)stances retain 

 the air which adheres to them, even when immersed in 

 water, and put under the receiver of an air-pumj) ; and 

 that this attraction is essential to the warmth of these 

 bodies, I think, is very easy to be demonstrated." 



His demonstration is that the attractiou between the 

 particles of air and the surface of the hairs, feathers, &c., 

 " being greater than the increased elasticity, or repulsion 

 of those particles with regard to each other, arising from 

 the heat communicated to them by the animal body, the 

 air in the fur, though heated, is not easily displaced ; and 

 this coat of confined air is the real barrier which defends 

 the animal from external cold. This air cannot carri/ off 

 the heat of the animal, because it is itself confined by its 

 attraction to the hair or fur ; and it transmits it with great 

 difficulty, if it transmits it at all, as has been abundantly 

 shown by the foregoing experiments." 



The running of water off a duck's back has become pro- 

 verbial, and everybody knows that these and other aquatic 

 birds are not wetted by the water in which they swim. It 

 is commonly believed that these animals oil their feathers, 

 and I have read accounts of special oil-glands with which 

 the animal is said to be supplied, and by means of which it 

 oils its feathers in order to keep the water from wetting 

 them ; but the writers do not provide any paint brush for 

 the j)urpose, with a handle sufficiently long and curved to 

 reach all over the back, neck, ifec, every part of which in 

 this and all other aquatic birds being equally waterproof. 

 As this idea of feather-oiling has considerable popular ex- 

 tension, and even some amount of learned pretension, and 

 as the subject has all the importance which Ilumford 

 claimed for it in reference to the philosophy of clothing I 

 will explain it further. 



Let the reader take a few iron filings and sprinkle them 

 on the surface of some water in a tumbler. If he has not 

 already made the experiment, he will be rather surprised 

 to find that, instead of sinking, as iron should, according to 

 its specific gravity, they will float. Let him persevere, 

 sprinkling gradually more and more. Ultimately the lower 

 stratum will be borne down by those above, and they will 

 all sink, forming a curious agglomeration at the bottom of 

 the tumbler. Leave them for a while, and then pour off 

 the water as completely as possible and examine the filings. 

 They will be found quite dry, in spite of their immersion. 

 Repeat the experiment, and observe carefully all that 

 occurs. It will be seen that the sinking agglomeration 

 consists of iron filings in a sort of air-balloon, the bulk of 

 the air far exceeding that of the filings. 



Now take a little of the calcined magnesia that is sold 

 by the druggists, selecting for this experiment the lighter 

 form, which is a powder so fine that when blown as dust 

 into the aii' it falls very slowly. Sprinkle this on the 

 water, and the paradox of the light powder sinking while 

 the heavy metal floats will be demonstrated. If a suitable 

 sample of magnesia is used, it will sink more rapidly in 

 water than it falls through air, and you may ask your 

 mathematical friends to reconcile this with their formulated 

 theories of " viscosity " and "shearing force." 



Another experiment, one which I devised many years 

 ago for illustrating this subject, may now be tried. Take 

 a piece of white card, and hold one side over the upper 



I'iirt of the llumo of a candle or lamp until it becomes 

 coated with a tliin sooty deposit. When this has cooled 

 dip the card edgewise in a tumbku-, or other glass v('ssel, of 

 water, and observe the result. First look at both sides of 

 the card perpendicularly to its faces. Thus seen, one side 

 appears black the other white, as they appeared before 

 immersion. Now look obliquely, and a wonderful ditter- 

 ence will be observed. The white side remains as before, 

 but the black surface appears like a plate of burnished 

 silver. Remove the card from the water, and its black 

 side will be found quite dry, the white side freely wetted. 



Sprinkle some water lightly on both sides of the card. 

 That which falls on the white side will adhere, th:vt on the 

 black .'-ide will roll .about in little spheres like drops of 

 quicksilver, or like corresponding sprinklings of water on 

 the duck's back. f 



These results depend upon the fact that the iron and the 

 carlion condense upon their sufaces a film of air which 

 adheres with a force exceeding that which tends to float it. 

 In the case of the floating iron, the quantity of air re- 

 quired to effect the buoyancy must exceed six and a half 

 times the bulk of the iron, the specific gravity of iron being 

 seven and ;i half times that of water. The silver-like 

 brilliancy of the film of air upon the carbon deposit is due 

 to that total reflection which occurs whenever light cannot 

 be transmitted without perpetrating a mathematical incon- 

 sistency. When a ray of light passes from a dense to a 

 rarer medium, or vice versd, it is bent or refracted in such a 

 manner that for given media the sine of the angle of its 

 emergence bears a fixed ratio to that of incidence. When 

 the obliquity of incidence exceeds a certain amount, the 

 observance of this ratio becomes impossible, and the light, 

 rather than mathematically stultify itself, refuses to pass 

 tlirough at all, turns back altogether. Students of optics 

 understand this, and I must not be refracted from my 

 present path by further explanation of it, though I may be 

 allowed to add that it aflbrds a most elegant physical 

 demonstration of the perfect inflexibility of a natural law. 

 All natural laws, ichcic trulij expounded, are equally free 

 from exception. 



Many aquatic animals are so ellectively clothed with 

 adhering air that they carry with them under the water a 

 little atmosphere of their own, and this atmosphere is 

 rendered beautifully visible to the student by means of the 

 total reflection above described. The water-beetles may be 

 named as examples. Some of them may be seen under 

 water reflecting the sunlight by means of their serial enve- 

 lope, which appears like an animated globule of quick- 

 silver. The water-spider (one of the most interesting 

 inhabitants of a small fresh water aquarium) displays this 

 very beautifully. He is covered with hairs to which the 

 air adheres so firmly that he lives under water in the midst 

 of a reservoir of air of such dimensions as to supply him a 

 good store of respiratory gas. When he has exhausted the 

 oxygen of this cargo of air he comes to the surface, where 

 the bubble instantly bursts, its nitrogen and carbonic acid 

 are difl'used away, and a new^supply of breathable air takes 

 their place, with which supply its proprietor descends 

 again. 



In some of our public aquaria there are diving birds 

 which afford an interesting opportunity of studying this 

 atmospheric envelope. They may be seen under water 

 enveloped in a .shining gaseous coat of mail, and the abso- 

 lute opacity of air when viewed obliquely through water is 

 further demonstrated by the appearance of their immer.sed 

 legs and bellies while swimming, and the utter invisibility 

 of all that is above the surface of the water to the spectator 

 who is looking upwards obliquely through the glass and 

 the water. In the absence of an aquarium, take a tumbler 



