HARDWICKE'S SCIENCE-GOSSIP. 



155 



it was found to be I9"994 in loo parts of air, and on 

 April iSth I9'998. Tacchini suggests that as the 

 Sicihan scirocco is undoubtedly a wind coming from 

 the African desert — a vast area devoid of vegetation — 

 and as the ordinary Palermitan air is from a district 

 highly cultivated and with abundance of vegetation, 

 it is presumablejhat as the supply of oxygen to the 

 air is dependent upon the decomposition of carbonic 

 acid by plants, the deficiency in the air of the 

 scirocco is due to the absence of plants in the 

 Sahara. 



As to the metallic iron, Tacchini cites the well- 

 known occurrence of native iron in Greenland and 

 elsewhere, and he considers that the iron dust of the 

 Sahara may result from similar causes to those which 

 have provided iron elsewhere. Iron would rust very 

 slowly in the dry air of the Sahara ; moreover if it 

 received a superficial coating of rust it would be 

 more or less protected from the action of the air, 

 Bunsen considers that native metals have sometimes 

 been reduced from their ores in the earth by hot 

 volcanic hydrogen. This would account for the 

 presence of metallic iron in various parts of the 

 earth. 



In the concluding remarks Tacchini sums up the 

 results obtained by himself and his colleagues, as 

 follows : — 



Granules of metallic iron, of diameters varying 

 from "009 to '041 mm., were found in the scirocco 

 dust of 1879. Chemical and spectral analysis also 

 revealed the presence of nickel, cobalt, and abun- 

 dance of the alkaline metals. The similarity of the 

 composition of this dust from various localities com- 

 pels us to assign to it a common origin. It was 

 always brought to the surface of the earth by a small 

 descending cyclone, and a barometric depression 

 always preceded its fall. The wind had a very high 

 temperature, sometimes as much as 40° 4' C. 

 { 104*9° F.). Its velocity varied from 19 to 69 kilo- 

 metres per hour. Finally, without attempting to 

 decide whether the metallic iron in the dust has a 

 cosmic or telluric origin, the phenomena of the dust 

 storms of Sicily is piiramctite terrestre, and due to a 

 cyclone which transports the dust from the African 

 desert. 



G. F. RODWELL. 



Newts near London. — I shall feel greatly obliged 

 if any one can tell me of a locality not too far /rom 

 the City where I could catch specimens of some of 

 our newts. I have tried Clapham Common and 

 Blackheath. I can get plenty of water snails and 

 sticklebacks at both places, but not any newts or 

 beetles. At Southampton Common, where I have 

 been living, I could procure them by hundreds, 

 and I do not like to see my aquarium without 

 them now. I know I can buy them, but bought 

 specimens are not to me like my own collecting. — 

 S, Roberts. 



THE STRUCTURE AND LIFE-HISTORY 

 OF A SPONGE. 



By Professor W. J. Sollas, M.A., F.R.S.E., 

 F.G.S., &c. 



THE presentation of a tolerably complete account 

 of any single species of sponge is a matter of 

 considerable difficulty, since, notwithstanding the 

 existence of many hundreds of species of sponge, 

 wliich have been made known to us by excellent 

 figures and descriptions, there is not one of which 

 the complete life-history is known. That of which 

 the history makes the nearest approach to complete- 

 ness is the calcareous sponge, now known to naturalists 

 under the name of Sycaitdra raphanus. 



General Form. — This little sponge, not more than 

 three or four mm. (i.e. Jth of an inch) in height, 

 presents us with a variety of forms, being some- 

 times spindle-shaped, fig. 95, sometimes ovate, 

 at others, tumip-shaped, and occasionally almost 

 spherical. Sometimes it is supported on a short 

 stalk, and sometimes it has no stalk, or is sessile. 

 Internally it is hollow, like a sac, the walls, or sides 

 of the sac, being 2 mm. thick, and the internal 

 cavity about 2 mm. across. The sac is closed below, 

 but opens above by a circular or elliptical mouth, 

 which is surrounded by a graceful fringe of slender 

 needle-shaped spicules, composed, like the rest, of the 

 spicules of the sponge, of carbonate of lime, and an 

 organic substance known as " spiculin." 



The spicules of the fringe, or corona, are some- 

 times seen in movement, now diverging from each 

 other till they give to the corona the form of an 

 inverted cone, and again approaching one another 

 to form a cylindrical tube. The surface of the sponge 

 is covered all over by erectly projecting spicules, 

 which render it hirsute. 



General Movements. — With the exception of the 

 movements of the spicules in the corona, the sponge 

 gives very few signs of life, so that at first sight one 

 might almost regard it, as indeed the older naturalists 

 did regard it, as a plant. It is, however, in every 

 respect a true animal, lively enough in its way, and 

 of wonderfully complex structure. 



That it is not quite so inert as it seems may be easily 

 shown by putting a little finely-powdered indigo into 

 the water in which a healthy specimen is confined. 

 The particles of colouring matter will then be observed 

 to make their way towards the general surface of the 

 sponge, over which they spread themselves, and 

 then disappear below it. After being lost to sight 

 for a little while they reappear, not over the surface 

 where they went in, but streaming out of the central 

 mouth in a powerful current. From this we may 

 infer that minute currents are entering the sponge 

 through its general surface, passing through its walls 

 into the central cavity, and then outwards by way of 

 the mouth. 



