How to tell 'orthokinetic' apart from 'perikinetic' based on their prefixes?

During a test in colloid and surface chemistry, students were asked to distinguish between two terms: orthokinetic and perikinetic coagulation.



> 
> The rate of aggregation is in general determined by the frequency of
>  collisions and the probability of cohesion during collision.
> 
> 
> * If the collisions are caused by Brownian motion, the process is called *perikinetic aggregation*;
> * [If the collisions are caused] by hydrodynamic motions (e.g. convection or sedimentation), one may speak of *orthokinetic aggregation*.
> 
> 
> from Everett, D. H. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry'. *Pure and Applied Chemistry*, **2009**, 31 (4), 577–638, p 610. doi: [10.1351/pac197231040577](https://doi.org/10.1351/pac197231040577)
> 
> 
> 


Perhaps unsurprisigly, I got the terms mixed up, and used definitions the wrong way round. The problem was that nothing too obvious popped out when considering the prefixes and the respective processes.


* ***Question***: To which meaning or association are the prefixes *peri-* and *ortho-* referring? The definitions are mostly clear. It is just that I would rather avoid memorisation, and would prefer to be able to always recall which is which based on the prefixes alone.


This means other meanings and etymology of *peri-* and *ortho-* are welcome, but will be considered off-topic on their own, unless a clear connection to the definitions of orthokinetic and perikinetic is provided. A short answer with an authoritative reference is very acceptable.


