August 1, 1896.] 



KNOWLEDGE. 



160 



LONDON: AUGUST 1, 1896. 



CO NTEN TS. 



Hygroscopic Seeds. By Key. Alex. S. AVilson, M.A., 

 B.Sc. {Illustrated) 



The Lime. By Geo. Paxton. (Illustrated) 



English Coins. — II. By G. F. Hill, M.A. (Plate) 



Appreciation of Musical Pitch. Bv Dr. J. G. McPheesox, 

 F.R.S.K. " 



Waves. — VIM. Capillary Waves. Bv Tauohan CoBSisn, 

 M.Sc. (Illustrate'/) 



An Expedition to Discover how Coral Islands Grow. 

 By E. W. Kkhabdsox 



Science Notes * 



Notices of Books. (Illustrated) 



Letters : — H. A. Cookson ; Eev. C. Eobikson, B.A. ; A 

 Landsman ; H. P. CtTBTiS; J. Cebagu 



Note on a Photograph of the Nebula M 8 in Sagittarius. 

 Bv Prof. Kavas.iee D. Xaegamvala, M.A., F E.A.S. 

 (Plate) 



Stock-Taking of the Variable Stars. Bv Lieut. -Colonel 

 E. E. Maeku-ick, F.E.A.S. (Illu-straled) 



Our Fur Producers. — IV. Fur Seals. Bv E. Ltdeekee, 

 B.A.Cantab , F.E.S. {Illustrated) ... 



How to Observe an Earthquake. Bv Cuablbs Datison, 

 Sc.T)., F.G.S .■ 



The Face of the Sky for August. By Hebbeet Sableb, 

 F.R.A.S. 



Chess Column. By C. D. Looock, B.A.Oion 



169 

 171 

 172 



17.5 



176 



ITS 

 179 

 179 



181 



183 



183 



186 



189 



191 

 191 



HYGROSCOPIC SEEDS. 



By Rev. Alex. S. Wilsox, M.A., B.Sc. 



HYGROSCOPIC movements depend mainly on 

 changes in the humidity of the atmosphere. Air 

 of any given temperature is only capable of 

 absorbing a definite amount of aqueous vapour ; 

 when that amount is present the air is said to 

 be saturated, and evaporation ceases. As the temperature 

 rises in arithmetical progressi'jn, the quantity of vapour 

 which the air is capable of retaining is found to increase 

 in a geometrical ratio ; hence evaporation mounts up 

 rapidly as the temperature rises ; hence, also, when two 

 saturated currents of different temperatures mingle, they 

 are no longer able to retain as much vapour as they did 

 separately, and precipitation consequently occurs in the 

 liquid form. Barometric pressure also affects evaporation ; 

 the rate diminishes as the pressure increases. Since these 

 atmospheric conditions are so liable to change, the 

 rapidity with which evaporation takes place must be 

 subject to incessant Huctuation. 



As it is the superficial layer alone which furnishes vapour, 

 the rate at which any body yields up its water to the 

 atmosphere depends on the extent of surface which it 

 exposes. The force of adhesion must also be taken into 

 account. A porous body like charcoal condenses atmo- 

 spheric vapour even at ordinary temperatures ; deliquescent 



salts, too, abstract water from the air. The nature of the 

 surface exposed to the air is also of importance ; even the 

 thinnest film of oil or grease prevents, or, at least, greatly 

 reduces, evaporation. Water holding substances in solution 

 also evaporates more slowly than when perfectly pure. 

 These considerations show that the equilibrium of humidity 

 depends on a number of circumstances which must vary in 

 different bodies and at different times in the same body. 



Organic bodies containing water very commonly increase 

 or diminish in size as the proportion varies. From their 

 peculiar structure some tissues retain water tenaciously ; 

 others readily part with it to the atmosphere. There are, 

 also, great differences in their powers of reabsorption, 

 some being unable to recover their lost moisture, even when 

 it is presented to them in a liquid state, while others 

 speedily imbibe what they lost in drying. Substances such 

 as wool, hair, and feathers are extremely sensitive in this 

 respect, especially if perfectly free from oil or resin ; they 

 respond to very slight atmospheric changes and are 

 described as hygroscopic. 



Organized structures are rarely, if ever, homogeneous ; 

 it therefore frequently happens that one part of an organ 

 contracts or expands more rapidly than the rest. In this 

 way tensions arise within the 

 organ causing it to split, or 

 the organ may alter its shape 

 and exhibit motion. Hygro- 

 scopic phenomena are further 

 complicated on account of 

 the intricacies of microscopic 

 structure. A violin string is 

 rendered slacker if moistened, 

 whereas a rope tightens. In 

 the former the molecules of 

 water are apparently intro- 

 duced chiefly m the direc- 

 tion of length ; in vegetable 

 fibres they go rather to increase 

 the thickness. The effects of 



imbibition are augmented in cellular tissue by endosmose. 

 As the quantity of fluid contained in a closed cell increases 

 the elastic cell - membrane becomes distended and the 

 volume of the cell is enlarged. The range of expansion 

 arising from turgescence is, however, inconsiderable in com- 

 parison with the expansion caused by simple imb;bition ; 

 moreover, turgescence can only occur in closed and thin- 

 walled cells. The cells of wliich wood is built up have 

 open pores ; any change of dimensions must therefore be 

 brought about entirely by imbibition or desiccation taking 

 place in the perforated cell-walls. In cartilaginous or 

 coUencbymatous tissues the greatly thickened cell-walls 

 contain a large proportion of water ; they contract strongly 

 in drying and swell very much from imbibition. 



Of hygroscopic action we have a familiar example in the 

 warping of unseasoned timber, where the bending is caused 

 by the younger sap-wood on the one side contracting at a 

 different rate from the older heart-wood on the other. 

 Hairs and fibres are especially prone to bend and twist as 

 they dry. Every angler knows how catgut unbends and 

 straightens when put into water, .\dvantage is taken of 

 this property in the simple old-fashioned weather-glass 

 alluded to in Cowper's lines : — 



Fig. 1. — Spores of Equiset\iin. 

 1. In dry state. 2. With 

 Elaters ex|)anded after 

 moistening (magnified). 



" Peace to the artist whose inpeiiions tliouilit 

 Devised the weiilherhoiise, that useful toy! 

 Fearh'ss of humid air and gatluriiii! rains, 

 Forth steps the num." 



Good weather is indicated by the appearance of a small 

 female figure at one door, rain by the figure of a man 

 coming out at the other. The figures are attached to a 



