In many words of one syllable k is used after c, as in crack, check, deck, being necessary to exhibit a correct pronunciation in the derivatives, cracked, checked, decked, cracking; since without it, c, before the vowels e and i, would be sounded like s. Formerly, k was added to c in certain words of Latin origin, as in musick, publick, republick; but now it is omitted.
See Guide to Pronunciation , §§ 240, 178, 179, 185.