ISOCHRONOUS, ISOCHROKISM. 



OBDK. 



1000 



. 



bling lemmitcala, which arc curves defined by the 



property that the rectangle contained by two lines drawn from two 

 points called the poles to any point in the curve is constant. The 

 light coming directly from a lamp or candle may be used in the expe- 

 riment, provided a lens of abort focus be added hi front, so as to con- 

 dense the light on to the system, and thus furnish an illuminated 

 field of sufficient angular extent to comprise the rings. 



The curves receive their designation from the circumstance that 

 throughout each the tint is constant. For nitre, and other biaxal 

 crystals whose optic axes are not far apart, the tint is determined by 

 the value of a quantity varying directly, in a given kind of crystal, as 

 the rectangle above mentioned (the distance between the poles being 

 supposed constant), and inversely as the thickness of the crystalline 

 plate. In the general ease, the curves are conceived to exist on the 

 surface of a sphere of which a point in the crystal is the centre ; and 

 when the optical axes of the crystal are at a considerable distance from 

 one another, if the curves be projected on a plane, the tint in each 

 curve will depend on the product of the sines of the angles subtended 

 by two lines drawn from the poles to a point in iU periphery, and also 

 upon the length of the path described by a ray of light in passing 

 through the crystal. 



ISOCHRONOUS, ISOCHRONISM (tffos, equal, \pivot, time). 

 Vibrations or oscillations which are performed in equal times are called 

 isochronous; and isochronism is the name given to a remarkable 

 property of all systems which are in equilibrium, namely, that when 

 slight disturbance, be the same more or less, is given, the oscillations 

 which take place are all performed in the same time, or so nearly in 

 the same time, that any acceleration or retardation is totally imper- 

 ceptible. Thus, when a pendulum is allowed to vibrate till it rests, it 

 will be found that there is no perceptible difference between the vibra- 

 tions of longer and shorter extent ; of which any reader may satisfy 

 himself by attaching a weight to a string, and observing the vibrations. 

 But a still better proof may be found in a musical string : the finest 

 ear cannot detect any difference between the pitch of a note made by a 

 smart blow on the key of a pianoforte and that made by a gentle one ; 

 yet a very small difference in the number of oscillations per second 

 would be perceptible, and the amount of disturbance from the position 

 of equilibrium is twenty or thirty times greater in the first case than 

 in the second. 



When, under two different circumstances, the longer space is de- 

 scribed in the tame time as the shorter, it must be that the force acting 

 in the first case is greater than that in the second ; and it is sufficiently 

 known from experience, that the more a system at rest is disturbed, the 

 greater is the effort which it makes to return. But in order that there 

 may be isochronism, it is not sufficient that the effort to return should 

 increase with the amount of disturbance, but the increase must take 

 place according to one particular law. This law is as follows : the 

 force of restitution must be always proportional to the disturbance, so 

 that whatever force begins to act when the disturbance is a, twice as 

 much acts when the disturbance is twice a ; and so on for all propor- 

 tions. That this law does prevail when the disturbance is not great, 

 either absolutely, or so nearly that its error is extremely small, may be 

 proved both by theory and experiment. The most complete proof is 

 to be found in the ' Mecanique Analytique ' of Lagrange. Granting 

 the law, we can make it sufficiently apparent that the consequence 

 must follow, namely, that all vibrations are performed in equal times. 

 Let A and B be two material pointa which are urged towards the point 



I. K A 



BOB 



o by pressures which are proportional to o A and o B ; and further let 

 each preesure diminish as either point approaches towards o, so as 

 always to preserve between the pressures at any two points the pro- 

 portions of the distances of those points from o. Take a minute portion 

 of time, so small that the pressure may not vary sensibly during its 

 continuance: then [ACCELERATION; FALL or BODIES] the velocities 

 created and the spaces described in that time will be proportional to 

 the pressures producing them. If then, during that time, A move to 

 K and B to q, A K and B q (and therefore o K and o o.) will be in the pro- 

 portion of p A to o B, and the points will be at K and q, with pressures 

 and velocities proportional to o K and o q. In a second such instant 

 let the points move to I, and R : then K L and q n, partly d.ue to velo- 

 cities which are as o K to o q, and partly to accelerations which are in 

 the same proportion, will still be in the proportion of <i K to o o., or <>f 

 O A to o B. Consequently the whole A L is to the whole B R in that 

 proportion : and reasoning in this way for successive small accelera- 

 tion*, we show that the whole space moved over by A in any time is to 

 that moved over by a in the same time in the proportion uf O A to OB. 

 Conaequrntly A describes AO in the same time in which B describes 

 B o ; or the half of a vibration of A is made in the same time as half a 

 vibration of B. 



To make this process perfectly strict, recourse must be bad to the 

 oomidrrations in IXTXORAL CALCULI**. 



ISOCLINAL. A term applied to those lines on the surface of the 

 globe which connect place* where magnetic needle has the same 

 inclination or dip. The term means t^xal inrlnwti'in, just as IBODT- 

 XAMIC means tqual font ; isodynamic lines connecting those place* 



where the total magnetic power of the earth is the same. [MAGNETISM, 

 TERRESTRIAL] 



ISOCYANURIC ACID. [FULJIINURIC ACID.] 



ISOMKKISM. There are numerous substances, especially amongst 

 organic compounds, which contain the same elements in exactly the 

 same proportions, but which nevertheless differ more or less widely 

 from each other in their properties. Such substances are said to be 

 itomeric, and this property or attribute is termed itomtritn. A 

 striking instance of this relation between a number of compounds is 

 seen in cyanic acid and its itoimri. Thus the following compound*, 

 though widely different from each other in properties, all possess the 

 same percentage composition : 



Cyanic acid ( II NO 



Cramclide c'.IINO, 



Fulminic acid C.H,N,O 4 



Fulminurlc acid C,H,N,O, 



Cyanuric acid C,I1,N,O, 



Cyanilic acid C,H 3 N,O, 



When two or more isomeric compounds have the same equivalent 

 number, as cyanic acid and cyamelide in the above list, they are said 

 to be metamcric. In some cases where the equivalent numbers of 

 isomeric compounds differ, and where it may be supposed that the 

 difference in properties results simply from the different state of con- 

 densation of their elements, as in the defiant gas family, the bodies are 

 said to be polymeric. 



ISOMETRICAL PERSPECTIVE. [PERSPECTIVE.] 

 ISOMORPHISM. This term, employed in chemistry and crystal- 

 lography, strictly signifies similarity of form, but chemists restrict the 

 term to such bodies as are not only similar in crystalline form but 

 also in chemical composition ; thus diamond, magnetic oxide of iron, 

 and alum, all possess the same crystalline form, that of the octahedron 

 but they are not regarded as isomorphous, because they possess no 

 analogy in chemical composition. On the other hand spinelle-ruby 

 (MgO,Al,O,), magnetic oxide of iron (FeO,Fe,0,), and chrome iron ore 

 (FeO,Cr,0,) are termed isomorphous because they not only crystallise 

 in the same form, but are analogous in their composition. The 

 following are some of the most important isomorphous groups : 



I. 



Alumina Al,Oj 



Sesquioxide of Iron F e,O, 



Oxide of chromium . , . . Cr t O s 

 lime-lite FcTiOj 



II. 



Sulphuric acid . 

 Belenic acid . 

 Chromic acid . 

 Manganic acid 



III. 



Chloride of potassium 

 Bromide of potassium 

 Iodide of potaolum . 

 Fluoride of potassium 



SO, 



no. 



CrO, 



MnO, 



, KC1 

 KBr 

 KI 

 KF 



For further details regarding isomorphism, and also for a description 

 of the doctrine of dimorphitm, see Miller's ' Chemical Physics.' 



ISOI'KUIMETRICAL. [VARIATION. CALCULUS OF.] 



ISO'SCELES (Iffor, equal, oWAos, leg, the c usually pronounced 

 hard, like I:), a term applied to a triangle of which two sides (or legs) 

 are equal. 



I si iTARTARIC ACID. [TAHTARIC ACID.] 



ISi iTKHKBENTHIN. [TURPENTINE.] 



IsnTHERMAL LINES. [MKTEROHOLO(IY ; TEMPERA-ITU*:, Ten- 

 KKSTRIAL, DISTRIBUTION or.] 



ISSUE. (Law.) [PLEADING.] 



ISSUE is an ulcer artificially formed for the purpose of maintaining 

 a constant purulent discharge from some part of the body. It is 

 usually made by placing one or more beads or peas on an incision 

 through the integuments in one of the limbs, or in the neighbourhood 

 of a diseased part, and there retaining them by adhesive plaster, so as 

 to prevent the wound from healing, and keep it in a constant state of 

 suppuration. Other issues are made by rubbing caustic potash, or 

 potash and quicklime, on a part of the nkin till it is destroyed and 

 sloughs, and by keeping open the ulcer thus formed, either with peas, 

 or very stimulant dressings. Setons are another form of issue, made 

 by Tm-nring a broad flat needle beneath a portion of the skin, and 

 retaining in the passage thus formed either a skein of silk or a flat 

 band of caoutchouc. The moxa and the actual cautery are also some- 

 times employed with the same \ i< v. . 



The iwe of issues for the cure of constitutional diseases, under the 

 idea that they remove noxious principle* from the blood, is now 

 entirely abandoned ; but they are sometimes had recourse to in order 

 to restore an habitual discharge which has been checked by the cure 

 of any chronic local particularly skin disease, and the cessation of 

 vlii<-)i has seemed to give rise to congestion of the head or of any 

 internal organ. But the principal value of issues is as counter- 



