CLA 



CLA 



lar downward progression, in orders, or 

 sections, as they are called by Tourne- 

 fort, genera, and species, with occasional 

 intermediate subdivisions, all subordinate 

 to the division which stands immediately 

 above them. So that the classes have 

 been compared to the first layer of a 

 truncated pyramid, which increases gra- 

 dually as it receives the orders, genera, 

 and occasional intermediate subdivisions, 

 till at length it terminates in an immense 

 base, consisting entirely of species. Ac- 

 cording to the definition of Linnaeus, a 

 class is founded on the agreement of the 

 several genera with each other in the 

 parts of fructification, according to the 

 principles of nature and art. It is ob- 

 served, that, in the formation of classes, 

 they should not be very numerous, and 

 thai their boundaries should be strongly 

 and distinctly marked. 



CLATHRUS, in botany, a genus of 

 Fungi. Essential character : roundish, 

 consisting of a reticular, windowed, hol- 

 low body ; the ramifications connected 

 on every side. Linnaus reckons only 

 four species, other botanists seven and 

 eight. 



CLAVA, in natural history, a genus of 

 Vernies Mollusca. Body fleshy, grega- 

 rious, clavate, and fixed by a round pe- 

 duncle ; aperture single and vertical. 

 There is but one species, viz. C. parasiti- 

 ca, covered with pellucid conic erect 

 spines. It inhabits the Baltic, on sea- 

 weeds, shell fish, and floating timber. 

 Like the Hydra it possesses the power 

 of dilating and contracting the mouth. 

 See HYDRA. 



CLAVARIA, in botany, a genus of 

 Fungi ; one of the lowest order in the 

 scale of vegetation, differing sometimes 

 very little in substance from the rotten 

 wood whence it issues. It is a smooth 

 oblong body, of one wniform substance. 



CLAVICLES, in anatomy, are two 

 bones situated transversely, and a little 

 obliquely opposite to each other, at the 

 superior and anterior part of the thorax, 

 between <he scapula and sternum. 



CLAUSE, signifies an article or parti- 

 cular stipulation in a contract, a charge 

 or condition in a testament. &c. 



Thus we say, a derogatory clause, a 

 'penal clause, saving clause, codicillary 

 clause, &c. 



CLAY. Any natural earthy mixture, 

 which possesses plasticity and ductility 

 when kneaded up with water, is in com- 

 mon lang .age called a clay. All mine- 

 ralogists, however, have comprehended 

 within the appellation, not only clays, 

 properly so called) but a few other mine - 



ral substances nearly allied to some of 

 the clays, and wh'.ch become plastic by 

 decomposition. Clay, however, is by no 

 means strictly a mineral species, being 

 in most cases the result ofthe decompo- 

 sition of other minerals. It seems ad- 

 visable, therefore, to consider the pro- 

 perty of plasticity as an essential cha- 

 racter, and to exclude from the class of 

 clays all earthy bodies that are destitute 

 of it. 



Mineralogists have generally arranged 

 all the plastic clays under two species, 

 rather from the economical uses to which 

 they are applied, than according to their 

 external characters, composition, or geo- 

 logical situation. The first species is 

 the white infusible porcelain clay, and 

 the second contains all the rest com- 

 pounded together, under the general 

 appellation potter's clay. We have, 

 however, a different arrangement in 

 Aikin's dictionary, which we shall lay be- 

 fore the reader. 



Essential character: plastic by inti- 

 mate mixture with water. 



1. Porcelain clay. Its colour is gene- 

 rally reddish white, also greyish and yel- 

 lowish white; it has no lustre, no trans- 

 parency. It occurs either friable or com- 

 pact; stains the fingers ; adheres to the 

 tongue; is soft but meagre to the feel; is 

 easily broken. Specific gravity about 

 2.3. It falls to pieces in water, and by 

 kneading becomes ductile, though not in 

 a very great degree. The Cornish por- 

 celain clay'certainly originates from the 

 decomposition of felspar, and contains 

 particles of quartz, mica, and talc, from 

 which it is separated by eleutriation. The 

 Chinese kaolin also contains mica, and is 

 probably of the same origin as the Cor- 

 nish. The same remark may be applied 

 to the French, &c. It is, however, by no 

 means certain, that all porcelain clay is 

 derived from felspar, as it varies consi- 

 derably in its composition and fusibility; 

 all the kinds indeed are infusible at any 

 temperature less than a white heat ; but 

 some, especially the Japanese, are re- 

 fractory in the most powerful furnaces. 

 The Cornish clay, according to Wedge- 

 wood, consists of 60 per cent, alumina, 



nd 40 si lex 



2. Steatitic clay. Its colour is a light 

 flesh red, passing into cream colour; its 

 texture is minutely foliated ; it has a 

 slight somewhat greasy lustre, and takes 

 a polish from the nail. It stains the fin- 

 gers, is very friable, and has a smooth 

 unctuous feel. When laid on the tongue, 

 it dissolve^ into a smooth pulp, without 

 any gritty particles. It is very plastic, 



