ISO 



ISO 



&c. It is in consequence of the fre- 

 quent isomerism of organic com- 

 pounds tliat changes so readily occur 

 in them, either in plants, or hy the 

 action of ferments. 



ISOMORPHISM, ISOMORPH- 

 OUS (from laoc, and f^opc^rj, form). 

 Having the same crystalline form ; 

 this is associated with the same num- 

 ber of atoms, but of unlike elements, 

 and also with similar properties. 

 Thus, alum consists of sulphuric acid, 

 alumina, and potash ; but either of 

 these bodies may be replaced by cer- 

 tain other isomorpkoiis bodies ; the 

 sulphuric acid by the selenic, chro- 

 mic, or manganic acids : the last two 

 of these produce a difference of col- 

 our in the crystal, but little else. 

 The alumina may be replaced by per- 

 oxide of iron, sesquioxide of manga- 

 nese, or sesquioxide of chromium ; 

 and the potash, by soda or hyd rated am- 

 monia. Thus, the alum may have none 

 of its primary constituents, but iso- 

 morphous atoms of the same number. 

 Isomorphism, or the study of these 

 changes, is of great importance in ag- 

 riculture, for it is found that the same 

 substitution takes place in organic 

 bodies. This is not, however, a mat- 

 ter of indifference ; for the presence 

 of soda in grapes, instead of potash, 

 deteriorates their flavour ; lime in 

 the place of potash, in tobacco, in- 

 jures its excellence ; and lime or pot- 

 ash in place of soda, in onions and 

 asparagus, is also injurious to their 

 mildness. The limit of isomorpliaus 

 substitution is a chemical inquiry : so 

 far as we know, there are groups of 

 bodies which can be substituted for 

 one another only ; these have the 

 same amount of oxygen in their com- 

 position. Such groups as are of im- 

 portance to the farmer or gardener 

 are appended. 



ISOMORPHOUS GROUPS. Pot- 

 ash, soda, hydratcd anunonia (N H4 O, 

 oxide of ammonium), hydrate of lime 

 (Ca O, H O). Of this group soda only 

 replaces potash in cerealia ; hut lime, 



amuionia (or organic alkalies), can 

 repl^ice potash or soda in plants liiat 

 do not require soluble silicates, as 

 tobacco, potatoes, &c. j 



Sulphuric acid is replaced by the 

 telluric, sclonic, chromic, and man- 

 ganic acids, none of which, however, 

 are abundant in common soils. 



Phosphoric acid is isomorphous 

 with the arsenic acid (As O5), which 

 will seldom be found in soils. 



Iron, as the peroxide (Fej O3), is 

 isomorpiious with alumina (AI2 O3), 

 oxide of chrome (Cr^ O3), and sesqui- 

 oxide of manganese. These substi- 

 tutions, although of no apparent mo- 

 ment, are continually occurring in 

 plants. 



ISOPYRE. A siliceous mineral 

 resembling obsidian, found in granite, 

 basalt, and primary rocks. 



ISOSCELES (from lao^, and oktj- 

 ^oc, a leg). A name given to the 

 angle two of whose sides are equal. 

 ISOTHERMAL (from laoc, and 

 "^ipfLT}, heat). Having the same aver- 

 age temperature. Geographical lines 

 drawn over the surface of countries, 

 and passing through places having 

 the same average heat, are termed 

 isothermal lines ; they are very irreg- 

 ular, in consequence of the occur- 

 rence of mountains and large lakes 

 or surfaces of water, which modify 

 the temperature. The terms Isothcr- 

 al and Isoch'nnenal are used also ; the 

 first, to designate a line showing the 

 places having the same summer heat, 

 and the last, those with the same 

 mean winter temperature. 



" According to Humboldt, the iso- 

 thermal line which corresponds to the 

 temperature of 32^ Fahrenheit pass- 

 es between Ulea, in Lapland, lat. 66°, 

 and Table Bay, on the coast of Lab- 

 rador, lat. 54°. The isothermal line 

 of 41° passes near Stockholm, lat. 

 59^°, and St. George's Bay, ]Vevv- 

 foundland, lat. 48^ The line of 50° 

 passes through the Netherlands, lat. 

 51°, and near Boston, in the United 

 States, lat. 42i° ; that of 59° between 

 Rome and Florence, lat. 43°, and Ra- 

 leigh, in North Carolina, lat. 36°. In 

 all these cases we see that the iso- 

 thermal lines, in passing from the 



western side of tlie continent of Eu- 

 rope to the eastern coast of America, 

 deviate very considerably towards 

 the south, the deviation, iii one case, 



443 



