5. In the derivation of English from Latin or Greek words, the changes of letters are to be governed by the rules which have generally prevailed in such cases. The Greek οι and αι, the Latin _oe_ and _ae_, are all converted into a simple _e_, as in _E_conomy, Geod_e_sy, p_e_nal, C_e_sar. Hence, according to common usage, we should write ph_e_nomena, not ph_æ_nomena, pal_e_ontology, not pal_æ_ontology, mioc_e_ne not mioc_æ_ne, p_e_kilite not {339} p_œ_kilite. But in order to keep more clearly in view the origin of our terms, it may be allowable to deviate from these rules of change, especially so long as the words are new and unfamiliar. Dr. Buckland speaks of the _poikilitic_, not _pecilitic_, group of strata: _palæontology_ is the spelling commonly adopted; and in imitation of this I have written _palætiology_. The diphthong ει was by the Latins changed into _i_, as in Arist_i_des; and hence this has been the usual form in English. Some recent authors indeed (Mr. Mitford for instance) write Arist_eid_es; but the former appears to be the more legitimate. Hence we write m_i_ocene, pl_i_ocene, not m_ei_ocene, pl_ei_ocene. The Greek υ becomes _y_, and ου becomes _u_, in English as in Latin, as cr_y_stal, col_u_re. The consonants κ and χ become _c_ and _ch_ according to common usage. Hence we write _crystal_, not _chrystal_, batra_ch_ian, not batra_c_ian, _c_ryolite, not _ch_ryolite. As, however, the letter _c_ before _e_ and _i_ differs from _k_, which is the sound we assign to the Greek κ, it may be allowable to use _k_ in order to avoid this confusion. Thus, as we have seen, poi_k_ilite has been used, as well as pe_c_ilite. Even in common language some authors write s_k_eptic, which appears to be better than s_c_eptic with our pronunciation, and is preferred by Dr. Johnson. For the same reason, namely, to avoid confusion in the pronunciation, and also, in order to keep in view the connexion with _cathode_, the elements of an electrolyte which go to the anode and cathode respectively may be termed the anion and cat_h_ion; although the Greek would suggest catïon, (κατίον).