266 



KNOWLEDGE & SCIENTIFIC NEWS. 



[November, 1905. 



But the peculiar instance of the twin leaves illus- 

 trrited can scarcely be explained on the above lines, 

 and, although it is often impossible to penetrate 

 Nature's mysterious influences determining changes of 

 form, nevertheless, the following explanation is sug- 

 gested. 



Lack of moisture, followed by excessive supply, 

 causes, on the one hand, an arrest of development, and, 

 on the other, rapid growth. Retarded growth favours 

 fusion, and (in the case of a floral member, at least) a 

 symmetrical condition is brought about by an abundant 

 supply of nutriment. Pressure on the growing point 

 would, probably, play an important part also in causing 

 fusion, owiniT to the delicate nature of the primary 

 tissue. 



t^lZ- 3.- Normal Syctmore Seedling: (two-thirds natural .si/e). 



Now, when the circumstances are considered in 

 which the present specimen grew, it is found that all 

 the aljove conditions were fulfilled. It is, therefore, 

 reasonable to suppose that the above explanation is 

 the correct one. 



It will be observed that both cotyledons are present, 

 otherwise the suppression of one might have 

 been a determining factor in producing the irregular 

 seedling. The specimen was found, by the author, 

 growing on a gravel path near a stone wall, on the 

 south side, during a dry spring, followed by a wet 

 summer. Thus the fact of its adverse environment and 

 irregular nourishment resulted in the inability of the 

 young tissue to work out its hcrcdilnry inclination, or 

 to follow that mysterious ancestral impression which 

 has determined ;'ll form', tlirdnflioni ilu rminilf^^ 

 aons of the pas' 



Whott is Scent? 



By Dr. J. G. McPherson, F.R.S.E. 



A DICTIONARY will Say that " scent is that which, 

 issuing from a body, affects the olfactory nerves of 

 animals." But that is not a complete definition. For 

 the issuing source of the scent may be solid or gaseous. 

 Until very recently it was assumed that there were 

 solid particles of the perfuming body to produce the 

 effect. 



And certainly that theory has unduly startled think- 

 ing f)eople as to the extraordinary divisiiiility of 

 matter. We are faced bv very startling facts ; but it 

 is a stretch of the imagination to account for them by 

 the ordinary matter-divisible theory. The tenth part 

 of a grain of musk will continue for years to fill a 

 room with its odour, and at the end of that time will 

 not be appreciably diminished in weight by the finest 

 balance. So acute is the sense of smell in some 

 trained men that one part of prussic acid can be de- 

 tected in about two million parts of water; and it has 

 no decided smell, only a strange fustiness. 



The faculty of scent is very acute in certain insects. 

 If a virgin female of the moth known as Sattirnia 

 Carpini is shut up in a box, males of the same species 

 will trace her out for a mile through the parti-odoured 

 air of a wood. The infinitesimal emanation from the 

 female is powerful enough to direct the male all that 

 distance. All are familiar with the remarkable scent 

 of the condor for carrion in Eastern countries, but the 

 scent of dogs seems to eclipse all in its marvellous 

 effects. 



For ages it was considered certain that the musk 

 and other scents exist in the air after they leave their 

 visible form in solid particles. Now, Dr. John 

 .'\ilken, F.R.S., has, by experiments, pro\cd that these 

 pass off as a gas or vapour. He has principally em- 

 ployed his w^ell-known cloudy-condensation test. If 

 the musk is in solid particles, these particles will be- 

 come nuclei of cloudy condensation in super-saturated 

 air, and thus make their presence visible. But this 

 is not the case. 



.^s to cloudy formation in general ; ocular demon- 

 stration can easily testify to it. If two closed glass 

 receivers be placed beside each other, the one con- 

 taining ordinary air, and the other filtered air (that is 

 air deprived of its dust, by being driven through cotton- 

 wool), and if jets of steam be successively introduced 

 into these, a strange effect is noticed. In the vessel 

 containing common air, the steam will be seen rising 

 in a dense cloud, but in the vessel containing the 

 filtered air, the steam is not seen at all. Dust-particles 

 are necessary in the air to allow v.-ipour-condensation 

 on the free surfaces, so as to form cloud-p:irticles. 



Dr. Aitkcn has given us a simple method of showing 

 that the solid particles in the air seem to have a lower 

 limit to their siz->, but that they are never so small as 

 to be capable of diffusing or not being separated but 

 by the action of gravitation. If a glass flask contain- 

 ing common air be provided with an india-rubber 

 stopper, which has two apertures in it, in which are 

 fitted two tubes, one of these tubes is connected with 

 an air pump, and the other with a cotton-wool filter, 

 a stop-cock being introduced in the latter. .\ little 

 water is put in the flask to moisten the air. If the 

 stop-cock is closed and the pinnp-handlc is pulled out 

 a very short length, cloudy cf)ndensation at once takes 

 place, the very smallest expansion being sullicient to 



