SPHERICAL ABERRATION 



SPHINX 



629 



Spherical Aberration. 



MIRROR. 



See LENSES, and 



Spheroidal Condition of liquids is the 

 name usually given to a series of very singular 

 phenomena discovered by Leidenfrost (1715-94), 

 but first carefully investigated by Boutigny (1858). 

 Indeed one, at least, of those phenomena has been 

 popularly known for a very long time, being the 

 foundation of the rough practical method of deter- 

 mining whether or not a flat- iron is so hot as to be 

 likely to singe the linen to which it is to be applied. 

 The test consists simply in letting a drop of water 

 fall npon the iron. The drop will either glance off 

 the surface without wetting it or will spread over 

 the surface and evaporate, according as the iron is 

 or is not too hot. 



The common experimental method of exhibiting 

 the spheroidal condition is easily performed thus : 

 A metallic disc, slightly concave, like a watch- 



flass, is heated by a lamp, and water is cautiously 

 ropped on it from a pipette. If this be done 

 before the disc is sufficiently heated the water boils 

 almost explosively, and is dispersed at once in 

 vapour. But when the disc is 

 >^^ ^^ ^^ hot enough the water remains 

 ^ "^ suspended, as shown in the cut, 

 above the surface ; and the drop, 

 when small, takes nearly the form of an oblate 

 spheroid. The most cunous fact connected with 

 the experiment is that the water does not boil. In 

 fact it evaporates so freely that the heat carried 

 off from it, as latent heat, by the vapour which is 

 constantly formed keeps its temperature some- 

 where about 206" F. only. Boiling water, dropped 

 on a red-hot plate of metal, instantly assumes the 

 spheroidal state, and is cooled six degrees below 

 boiling. Other liquids, and even some bodies 

 which are solid at ordinary temperatures, can be 

 easily brought into the spheroidal state the lowest 

 requisite temperature of the disc being dependent 

 on the boiling-point of the substance. A good 

 example of a solid entering this state is furnished 

 by dropping crystals of iodine on a hot platinum 

 disc. It is not necessary that a metal plate be 

 used. For example, a watch-glass will suffice for 

 the experiment ; but hot water must be dropped on 

 it, else the glass will crack. Also we may obtain 

 ether, and even water, in the spheroidal state over 

 the surface of hot oil. The explanation of the 

 phenomenon is to lie sought for in the existence of 

 a cushion of vapour between the hot plate and the 

 drop. The high temperature of the plate causes a 

 rapid evaporation of the lower surface of the drop. 

 The vapour particles coming into contact with the 

 hot plate are raised in teni|>erature, that Ls, have 

 tlicir kinetic energy increased. They' tend to 

 rebound with increased energy back upon the drop, 

 whose weight they are thus able to support. 



An interesting illustration of the spheroidal state 

 is the fact that the hand may be dipped for a short 

 time with impunity into melted lead, and even 

 into melted copper. The vapour instantly raised 

 from the moisture of the skin prevents, so long as 

 that moisture lasts, more than an endurable amount 

 of radiant heat from reaching the hand, and also 

 prevent* direct contact. 



Spheral! tic Structure. See PETROGRAPHY. 



Sphex, a genus of hymenopterons insects of 

 the family Sphegichp, closely allied to the true 

 Wasps (Vespidse). The Sphex wasps are solitary 

 in habit, and there are no workers as in the social 

 forms. The female hollows out, at the end of a 

 long passage, three or four chambers, in each of 

 which she deposits an egg and a store of food for 

 the larva she will never see. The food consists of 

 grasshoppers or other insects, and Fabre gives a 

 minute account of the way in which the Sphex 



attacks her victim, and, after a long and violent 

 struggle, throws it on its back and stings it in the 

 neck and between the thorax and abdomen, each 

 time piercing a ganglion. The insect, completely 

 paralysed, but alive, and therefore not liable to 

 putrefaction, is then dragged to the mouth of the 

 nest, where it is relinquished for a short time, while 

 the wasp enters alone to see that all is right. So 

 automatic is this habit of reconnoitring that if 

 the grasshopper be removed to a little distance the 

 wasp drags it back to the same spot and again 

 enters alone. This was tested by the observer 

 forty times in succession, and each time the wasp 

 paid her preliminary visit of inspection. But, in 

 proof that she is not wholly the slave of habit, it 

 should lie noted that when Fabre substituted a 

 fresh grasshopper for the paralysed one she at once 

 perceived the difference, and proceeded to attack 

 and sting her recalcitrant victim. Four paralysed 

 insects are placed in each chamber, which is se'aled 

 up as it is finished. When all are full the mouth 

 of the passage is also closed, and the nest is aban- 

 doned. See WASP. 



Sphincter Muscles (Gr. sphinkter, 'that 

 which binds tight ' ) surround an opening or canal 

 which, by their action, they are able to close or 

 diminish in size. They are found round the mouth, 

 the opening between the eyelids, round the pupil 

 of the eye, the pylorus of the stomach, the outlet 

 of the urinary bladder, and the anus. It is to the 

 presence of these muscles that the higher animals 

 owe the power of retaining for a considerable period 

 the excrementitious matters collected in the bladder 

 and rectum, and of discharging them at intervals, 

 the sphincter muscles being, like those engaged in 

 the process of respiration, mainly, but not entirely, 

 under the control of the will. 



Sphinx, a Greek word signifying the 'strangler,' 

 applied to certain symbolical forms of Egyptian 

 origin, having the body of a lion, a human or an 

 animal head, and two wings. Various other com- 

 binations of animal forms have been called by this 

 name, although they are rather griffins or chimaeras. 

 Human-headed sphinxes have been called andro- 

 sphinxes ; that with the head of a rain, a criosphinx ; 

 and that with a hawk's head, a hieracosphinx. 

 The form when complete had the wings added at 

 the sides ; but these are of a later period, and seem 

 to have originated with the Babylonians or Assyr- 

 ians. In the Egyptian hieroglyphics the wingless 

 Sphinx bears the name of Neb, or Lord, and Akar, 

 or Intelligence, corresponding to the account of 

 Clement tYiat this emblematic figure depicted intel- 

 lect and force. Others see in it the idea of resur- 

 rection, symbolised by the triumph of the dawn 

 over the darkness of night. The idea that it alle- 

 gorised the overflow of the Nile when the sun was 

 in the constellations Leo and Virgo appears to be 

 unfounded. In Egypt the Sphinx also occurs as 

 the symbolical form of the monarch considered as a 

 conqueror, the head of the reigning king being 

 placed upon a lion's body, the face oearded, and 

 the usual head-dress. Thus used, the Sphinx was 

 generally male ; but in the case of female nilers 

 the figure has a female head and the body of a 

 ioness. 



The most remarkable Sphinx is the Great Sphinx 

 at Gizeh (Gfza), a colossal form hewn out of the 

 natural rock, and lying about a quarter of a mile 

 south-east of the Great Pyramid. It is sculptured 

 >ut of a spur of the rock itself, to which masonry 

 las been added in certain places to complete the 

 shape, and it measures 172 feet 6 inches long by 56 

 'eet high ( Vyse, Pyramids, iii. 107). Immediately 

 in front of the breast Caviglia found in 1816 a 

 small naos or chapel, formed of three hieroglyphic 

 tablets, dedicated by Thothmes III. and Rameses 



