30 



SCIENTIFIC NEWS. 



Qan. 13, iS 



upon the lower surface of the wings, which then sustain 

 the bird aloft after the manner of a kite. 



As the resistance of the air under the wings act at a 

 point very remote from the shoulder-joint, and as the 

 pectoral muscles which depress the wings act very near 

 the joint, that is, upon a less advantageous arm of the 

 lever, it results that the effort of the muscles is much 

 greater than the resistance of the air to be overcome. 



The effort developed by the pectoral muscles of a sea- 

 gull will be 19 kilos., and as these muscles in section 

 have a surface of 11 square centimetres, the force is 1.6 

 kilo, per square centimetre. 



For the five strokes of the wing which the gull gives 

 per second at the moment when it takes flight the work 

 done is 3.668 kilogrammetres. This value is very high ; 

 it is as if a machine should raise its own weight into the 

 air with a speed of more than five metres per second. 



But the act of first taking wing is the most laborious 

 phase of flight. It has long been known that birds 

 employ all sorts of artifices to acquire a preliminary 

 speed before beating their wings. Some run along the 

 ground before rising into the air ; some take a sudden leap 

 in the direction which they wish to assume when flying. 

 Others allow themselves to drop from an elevated spot 

 with extended wings, or glide through the air at an 

 accelerated speed before moving their wings. All turn 

 their beak to the wind at the moment of taking flight. 



It has been observed that many of the larger birds are 

 unable to rise on the wing from any narrow inclosure. 



Although the mechanism of the flight of birds is only 

 a very subsidiary point in biology, it is yet worth 

 elucidating. M. Marey's researches afford an interesting 

 proof of the value of photography as a means of 

 scientific investigation. 



ANIMAL TANNIN. 



T T was formerly held that there was a wide and dis- 

 -•- tinctly marked boundary — a breach of continuity — • 

 between the vegetable and animal kingdoms. Modern 

 investigations have, however, overturned one afteranother 

 the supposed distinctions, and it now becomes a difficult 

 task to find in one of these great groups any feature 

 which is not to be traced in the other also. At one time 

 chlorophyll, the green colouring matter of leaves, was 

 thought to be entirely wanting in animals. But, not to 

 speak of other investigators, in 1879 Mr. P. Geddes pre- 

 sented to the Academy of Sciences a memoir showing 

 the presence and the activity of chlorophyll in the green 

 Planarice. These animals, on exposure to sunshine, de- 

 compose carbonic acid, just like the green leaves of 

 plants, and give ofi" bubbles of a gas containing more than 

 double the proportion of oxygen found in atmospheric air. 

 Another principle very widely diffused in the vege- 

 table world, and supposed to be entirely absent in 

 animals, is tannin. This substance has attracted the 

 attention of mankind from a very early date, on account 

 of its property of converting animal skins into leather. 

 Another of its common uses has been in the manufac- 

 ture of ink. Decoctions of gall-nuts and of other vege- 

 table matter rich in tannin give an intense blue-black, 

 or in some case a greenish or brownish black colour, if 

 brought in contact with solutions of iron, such as copperas 

 or " green vitriol " (ferrous sulphate), certain compounds 

 of chrome and other metallic salts. 



Now, M. Villon, of Lyon, has for a long time been 



engaged in searching for tannin formed in the organism 

 of an animal. He seems to have taken up this inquiry 

 from the fact that as far back as 1810, M. Penaut, a 

 pharmaceutical chemist of Bourges, found in the bodies 

 of corn-weevils gallic acid, a kindred compound. Villon, 

 having a large quantity of these insects {Calandra 

 granaria, a beetle of the Curculionide family, too well 

 known for its ravages in granaries), set to work to 

 examine them very carefully for tannin. There is no 

 occasion for us to describe the process which he followed, 

 which could be of interest only to the professional 

 chemist. It may suffice to say, that from 500 lbs. of 

 weevils, he obtained 15 lbs. of tannin, which he proved 

 to be really such, not merely by analysis, but by its 

 action upon animal skins and metallic salts, especially 

 those of iron. This form of tannin he designates by the 

 rather long-winded name of " fracticorni-tannic acid," 

 and he even thinks that it might be rendered practically 

 useful in tanning, dyeing, ink-making, and in pharmacy. 

 It would indeed be a happy thought to obtain in this 

 manner partial compensation for the damage occasioned 

 by the weevils. 



About the same time Mr. J. W. Slater, F.E.S., whilst 

 engaged in studying the generation of colouring matters 

 in the bodies of insects, was struck by the fact that a 

 very large proportion of insects, especially beetles which 

 prey on wood, bark, roots, etc., display colours closely 

 resembling those yielded by tannin and its modifications 

 in contact with animal matter. He, therefore, took the 

 elytra (wing-cases) of some cock-chafers, and having 

 freed them from fatty matters, he steeped them in solu- 

 tions of iron, chrome, and copper. The changes of colour 

 produced were exactly those which would happen if a 

 slip of tanned leather or other matter impregnated with 

 tannin were similarly treated. 



As in these experiments he used not the entire bodies 

 of the insects, but merely their outward integuments, the 

 objection is forestalled that the tannin thus detected is 

 derived from remnants of vegetable matter present in 

 their digestive organs. It must, however, be admitted 

 that with purely carnivorous insects Mr. Slater was not 

 able to obtain any similar results. 



The importance of this discovery, made simultaneously 

 by two independent observers, is that it confirms what 

 may be called the chemical continuity of the animal and 

 vegetable kingdoms. It shows also to what an extent 

 tannin ranks as a colour-former in the organic world. 

 Botanists have not failed to show to what an extent the 

 autumnal tints of leaves are due to the changes which 

 tannin undergoes on exposure to air and light. These 

 tints are the most splendid in the case of trees and shrubs 

 rich in tannin, and in such species we often notice that 

 a leaf or a part of a leaf accidentally screened from 

 light retains its green summer-colour, whilst those 

 around have turned of a rich yellow, red, or brown. 



And now, also, we see the same classes of colours 

 in insects — stone-colours, drabs, fawns, yellows, etc. — 

 traced to the same principle. 



Cleaning Brass. — It is a great mistake to clean brass 

 articles with acid, as they very soon become dull after such 

 treatment. Sweet oil and putty powder, followed by soap and 

 water, is one of the best mediums for brightening brass and 

 copper. To frost brass-work, and give it an ornamental 

 finish, boil the article in caustic potash, rinse in clean water 

 and dip in nitric acid till all oxide is removed ; then wash 

 quickly, dry in boxwood sawdust, and lacquer while warm. 



