Victorinus thus refers to the old custom still in use of writing C and
CN, as initials, in certain names, even where the names were pronounced
as with G.

[Mar. Vict. I. iii. 98.] C autem et nomen habuisse G et usum
praestitisse, quod nunc _Caius_ per C, et _Cneius_ per CN, quamvis
utrimque syllabae sonus G exprimat, scribuntur.

H has the same sound as in English. The grammarians never regarded it as
a consonant,--at least in more than name,--but merely as representing
the rough breathing of the Greeks.

Victorinus thus speaks of its nature:

[Keil. v. VI. p. 32.] H quoque inter litteras obviam grammatici
tradiderunt, eamque adspirationis notam cunctis vocalibus praefici; ipsi
autem consonantes tantum quattuor praeponi, quotiens graecis nominibus
latina forma est, persuaserunt, id est C, P, R, T; ut _chori_,
_Phyllis_, _rhombos_, _thymos_; quae profundo spiritu, anhelis faucibus,
exploso ore, fundetur.

By the best authorities H was looked upon as a mere mark of aspiration.
Victorinus says that Nigidius Figulus so regarded it:

[Mar. Vict. I. iv. 5.] Idem (N. F.) H non esse litteram, sed notam
adspirationis tradidit.

There appears to have been the same difference of opinion and usage
among the Romans as with us in the matter of sounding the H.