Is this what we mean?
All the devices that can be introduced into a fugue depend upon the knowledge, the skill, and the judgment of the composer, and, at the same time, upon the nature of the SUBJECT and of the COUNTER-SUBJECT which may offer more of less scope for these devices. These said devices consist, firstly, in the employment of imitations formed by detaching portions either of the SUBJECT or of the COUNTER-SUBJECT; secondly, in the transposition of the subject into different keys, and in the advantage which may be derived with respect to this from double counterpoints; thirdly, in the inversion of the SUBJECT by contrary movement; fourthly, in a new SUBJECT that may be introduced for combination with the first SUBJECT and the first COUNTER-SUBJECT; fifthly in the manner of combining the STRETTO in several ways, each time more and more closely approaching the RESPONSE to the SUBJECT; sixthly, in the means that may be employed to let the SUBJECT be heard simultaneously with its inversion by contrary movement; seventhly and lastly, in the method of combining the SUBJECT, the COUNTER-SUBJECT, and the STRETTO upon the pedal, and in the skill and taste with which these devices are brought in and worked throughout the extent of a fugue.
That's seven things, and not really essential, just possible, devices -- perhaps the eighth thing is the 'skill and taste'? Don't you just love old counterpoint treatises? Here's how to write a fugue: "Exhibit skill and taste".
There's also this:
The indispensable conditions of a fugue are the subject, the response, the counter-subject; and the stretto. There may be added to these the pedal, which is almost always employed in a fugue of any extent.
Stretto is indispensable? What would Bach say? Certainly Cherubini was talking about the older fugues eg Battiferri, Frescobaldi, ...
Will have to dig further.
The treatise itself is quite amusing, so strange how terminology has changed since Cherubini, and how far his own terminology is from that of yet older theorists. But he also seems to contradict himself on more than one occasion.