CHEVRON

The substances in the last division have been bu't Imperfectly examined, and silicium should perhaps rather be regarded as a metalloid than a metal. In one or other, then, of these cla es each elementary body will find a place, but the arrangement only relates to the simple suOstances; the cla ification of their combinations, which are almost indefinite, is a much more intricate and ditficult subject, and it would be impo ible here to give even an outline of the different plans which have been pursued or suggested. As far, however, as the teacher of chemistry is concerned, none of the systematic arrangements of compounds can be conveniently adopted; his best plan, therefore, is to develope their history arid properties as he proceeds. Having, for instance, discu ed the abstract properties oi oxygen and hydrogen, he proceeds to the compound which they form, and which is water; then he proceeds to the third element, nitrogen, and its combinations with those previously described; thus, nitrogen and oxygen form five distinct compounds, namely, -protoxider^vid i binoxide of nitrogen, and hyp on it rotis, nitrous, and nitric acids. Nitrogen also combines with hydrogen to form ammxmia, and ammonia combines with nitric acid to form the XX csiX^A nitrate of avmnonia. In a state of mixture, nitrogen and oxygen constitute tlie great bulk of the atmosphere. From this brief notice of the mode of dealing with the three elementary bodies of the first cla , the reader will readily understand the method of applying the same principle to the entire list; the student will thus be conducted, step by step, from the simplest to the most complicated chemical combinations. It is always convenient, however, in adopting this plan, to exclude the products of organization, and to consider them apart, under a separate head, entitled organic chemistry, which is of course subdivided into the chemistry of vegetable and of animal p roducts. If, having studied chemistry upon this plan, we look back upon the path which has been traversed, we shall immediately see that the compounds maybe grouped into cla es, related to each other by certain analogies both of properties and composition. Thus we have the cla of oxides, or combinations of oxygen which are not acid; of chlorides, iodides, ., and of acids; and these again subdivided into oxyacids, and hydracids; and lastly, the numerous cla of salts, or compounds of the acids with salifiable bases. When compound bodies are susceptible of electro-chemical decomposition (or electrolytes) the elements always tend to one or other pole; in other words, they are either separated at the point or surface at which the (presumed) electrical current enters, or at the surface at which it leaves the electrolyte; hence the arrangement of the elements into electro-negative bodies, or those which tend to the anode, and electro-positive bodies, or those which tend to the cathode (using the terms anode and cathode in reference to the ingre and egre of the electric influence); oxygpn, the halogens, and sulphur are anions, or electro-negatives; hydrogen, and probably all the metals, are cathians, or electro-positives. A good arrangement of the elementary bodies is founded upon such properties; and although tljere are several whose elec trical relations have not hitHerto" been accurately determined, the electro-chemical arrangement is that which will probably be most generally adopted. CHENOPOD I ACE^E. (Che no podium, one of the g:enera.) A natural order of herbaceous Exogens, distinguished with difficulty from Amarantacetz by their herbaceous calyx; from Phytolaccacece by their solitary carpel, and the stamens never exceeding the number of the segments of the calyx, to which they are opposite. They consist of weeds inhabiting most parts of the world, abounding least within the tropics; and they po e few sensible properties. Che no podium anthelminticitm produces the wormseed oil of the shops; but in consequence of their insipidity the species are often used for food, as in the case of spinach, &-c. Chevalier has re. marked the curious. fact of Che no podium vulvar i a exhaling pure ammonia during its whole existence. CHEQUY, CHECKY, or [s. 235]