61G 



SI'KCII H i.KAVITY 



SI'KCTRUM 



known temperature ami pressure; (3) by measur- 

 ing the weight of vapour which can occupy a known 

 volume, this lieing effected l>y putting liquid into 

 a vessel of known capacity ami netting until there 

 in, at a known temperature and the atmospheric 

 pressure, nothing but vapour in the vessel, then 

 closing and weighing when cool. The last two 

 methods are specially applicable to vapours rather 

 than to permanent gases. It is often convenient, 

 instead of taking the true specilic density of a gas 

 or va|>our e.g. that of air, the number of grammes 

 per cubic centimetre of which is 0-0012932 to state 

 it- density ax compared with air or hydrogen as a 

 standard. In this way air is said to have a den- 

 sity = 1 or = 14'47, according as air or hydrogen 

 is taken as the standard. The use of hydrogen as 

 a standard is of special convenience in chemical 

 calculations, for the densities of gases or vapours 

 so measured are, as a rule, proportional to their 

 molecular weights. The following are the specific 

 densities of some common substances : 



Air. 0-0012932 



Alcohol 0-80 



Aluminium 2'5 to 2-47 



Amber 1-08 



Ammonia gas (= 0-589 



xair) 0-000762 



Ammonia solution 0*88 



Amorphous arsenic. 4*71 



Anthracite 1-4 to 1-7 



Antimony 6-715 



Arsenic crystals 6-73 



Ash 0-84 



Bismuth 99 



Blood 1-04 



Bone 1-StoS-O 



Brown coal 1-2 to 1-4 



Butter 0-94 



Calcium 1-678 



Cannelcoal 1 18 to 1-27 



Carbonic acid gas (= 1-624 



xair) 0-00197 



Cast-Iron 7 to7H 



Chalk 2-45 



Charcoal 0-3 to 0-5 



Chlorine (2-45W x air). .0-00317 



Clay 1-8 to 2-8 



Cobalt 8-96 



Copper 8-86 to 8-94 



Cork 0-24 



Cyanogen (1-800 x alrX.0-00-234 



Diamond S'53 to 3-55 



Dry peat 0'6 



Kim. 0-87 



Flint 2-to27 



Glass 2-4 to 35 



Gold 19-26 to 19-55 



Granite 2-6 to 2-9 



Honey 1-45 



Human body alive O-H'.i 



lly.lrio.lic acid (=4-44 



thn.-s that of air or 64-11 



times that of hydrogen ) 



0-00574 

 Hydrochloric acid gas 



(=1-28 xair) 0-00163 



Hydrochloric acid solu- 

 tion at Sr f 0-908 



Hydrocyanic acid gas 



(=0-9476 xair) 0'1226 



Hydrogen (=0-06926 



xair) 000008958 



Ice. 0-91674 



Indium 28-42 



Ivory 1-8 to ID 



Lead 11-37 



Lignum-vltte. 1-8$ 



Limestone 2-6 to 2*8 



Liquefied oxygen 1-124 



Lithium 0-6936 



Loadstone. 4 -9 to 5-2 



Manganese. 7'S 



Marble -6 to 2-8 



Mercury 13 '59 



Milk 1-03 



Nitric acid 1-817 



Nitrogen (0-9713 x air).0-0012 



Oak, English OD7 



Oil of cloves 1-03 



Oil of turpentine. 0-87 



Oleflantgas(= 1-9784 



x air) 01265 



Oxygen (= 1-1066 X air)OO0143 



Platinum 2M to 21-7 



Poplar. 0-38 



Potassium 0-86 



Kuby 4-8 



Sand in bulk, dry, about.. . . 1-6 

 wet....l-to2-0 



Silver. 10-53 



So.lium 0-972 



Spanish mahogany 1-06 



Steel 7-6to7-S 



Hulphuricacid 1-854 



Sulphuric ether 072 



Sulphurous acid gas 



(=2-24 xair). 0-0029 



Tin 7-29 



Topaz. 3'4 to 3-6 



Wrought-iron 7-J8 to 7 "79 



Zinc 6-86to7-21 



Specific Gravity, the weight of any given 

 substance as compared with the weight of an cqtml 

 bull: or volume of water or other standard sub- 

 stance at the same temperature and pressure. 

 See SPECIFIC DENSITY. 



Spectacles, for the purpose of aiding the sight 

 when impaired by age or otherwise (see KVK), 

 are commonly said to have been invented during 

 the l3th century. The merit is variously attributed 

 to Alessandro di Spina, a monk who ilied at I'isa 

 in 1313, and to Salvino degli Amati, who died at 

 Florence in 1317 ; but spectacles seem to be re- 

 ferred to by the Arab writer Alhazen (Hth cen- 

 tury) and by Roger Bacon (c. 1214-94). In 14X2 

 there were spectacle-makers at Nuremberg. At 

 first spectacles were exceedingly clumsy, both in 

 the lenses themselves and also in their frames ; 

 and very little improvement took place in them 

 until the beginning of the 19th century, when light 

 metal frames were introduced instead of the cum- 

 brous horn or tortoiseshell mountings, which are 



still occasionally seen, and have obtained the 

 name of goggles. So skilful are the workmen of 

 Wolverhampton, where they are chiefly miule, in 

 the manufacture of steel frames that Mime of 

 exquisite workmanship are now turned out, which, 

 with their lenses complete, are under a quarter of 

 an ounce in weight. They have consequently dis- 

 placed gold, silver, and all other material-., when 

 comfort and effectiveness lire de-ired. The h-nse* 

 themselves are nearly always m.-i.le of the best 

 optical glass, and by the best milkers are ground 

 with extreme care. Many profess to be made of 

 'pebbles 'or rock-crystal; but lenses really made 

 of that material are exceeding! v rare and have 

 no real advantage over good glass. The spec- 

 tacle-frame ought to be so fitted that the optic 

 axes of the lenses shall coincide with those of the 

 eyes ; otherwise there is a strain on the e\ 



It is most important that the glasses worn should 

 be properly selected, otherwise they may do much 

 harm. In cases of Astigmatism (q.v. ), and those 

 where the two eyes are different, competent medi- 

 cal advice should always be sought. In simple 

 myopia (short -sight) and hypermetropia (long- 

 sijj'ht; see EYE, Vol. IV. p. 515) the general 

 principles of selection are less complex, though 

 their proper application is ofien a difficult 

 matter. In short-sight the glassc- (concave) used 

 should be the weakest with which distant ob- 



Jects are clearly seen, or somewhat weaker; in 

 ong-sight the glasses (convex) should be the 

 strongest with which distant objects are clearly 

 seen ; for reading and near work still stronger 

 glasses are often required. In presbyopia or old- 

 sight (Vol. IV. p. 512) the glasses "should be of 

 such a strength as to enable prim to lie comfort- 

 ably read at about 10 inches from the eyes. 



Spectrum. As explained under the article 

 CoLOVl!. light emanating from any ordinary 

 source is rarely if ever homogeneous. It is 

 composed of rays of different wave lengths, each 

 of which if viewed singly would appear to have 

 an appropriate colour. The general colour-sen- 

 sation produced by such a heterogeneous ray can 

 teach us very little concerning its composition. 

 Not until we have formed its spectrum by appro- 

 priate means are we able to analyse it. A spec- 

 trum is in fact an image in which the component 

 parts of a given ray of light are separated from 

 one another so that each may be viewed singly. 



Newton was the first who scientifically produced 

 and studied the spectrum of sunlight. This he did 

 by interposing a glass prism in t lie path of a ray 

 which was allowed to enter a dark room through a 

 small hole in the shutter. The arrangement is 

 shown diagrammatical!}' in fig. I. Here the rays 

 are bent out of their original course, SA, as they 

 pass through the prism P; and on the screen, H, the 

 sped i ii in of colours is formed instead of the image 

 A. Newton regarded the spectrum as l>eing divis- 

 ible into seven differently coloured spaces, which he 

 called in order red, orange, yellow, green, blue, 

 indigo, and violet. It is impossible, nowever, to 

 settle precisely the exact boundary between any 

 two of these fancied species of colour, which pass 

 by insensible gradations one into another. As 

 Newton clearly demonstrated, the spectrum is pro- 

 duced because the differently coloured constituent* 

 of sunlight have different refrangihiliticn, the red 

 l>eing refracted least of all and the violet greatest 

 of all (see REFRACTION). If light of a particular 

 refrangibility were absent or of less intensity than 

 the other constituents gaps would appear in the 

 spectrum. As a matter of fact such gaps do exist 

 in the solar spectrum, and were first observed by 

 Wollaston in 1802. In 1817 Fraunhofer, with 

 much more perfect optical apparatus, measured the 

 relative positions of a great number of these dark 



