How to change startSpeed variable in particle system ?:( unity 5 2017.1

Hello.

Recently I'm trying to make a spell around a player that will slow down missiles. Spell and missiles already have, but the fireball that flies in me does not work as it should :( So far I managed to slow down the collider ^^ ,but variable startSpeed included in the animation does not want to work :( I would like to even stop this bullet as soon as it encounters a collider shell, and then when it disappears spell it "resets" to the last speed.

spell: https://zapodaj.net/b14751f6032cf.png.html fireball: https://www.assetstore.unity3d.com/en/#!/content/36825

Here: https://zapodaj.net/aa811ac6102ee.png.html You just need to modify the float variable and just do the following:

 FireProjectileScript _FireProjectileScript;
 _FireProjectileScript.ProjectileColliderSpeed -=4;

but: https://zapodaj.net/662bd949780b1.png.html The problem is complicated, and this float does not listen, even makes problems :( How can I modify it?

I noticed that I do not want to work

 void OnTriggerEnter (Collider col)
          {
              if (col.gameObject.CompareTag ("spell"))
              {
                  Debug.log ("spell");
                  _FireProjectileScript.ProjectileColliderSpeed = -20f;
                  ps.GetComponent <ParticleSystem> (). startSpeed = -20f;
              }
          }

ps is reference to particle system

Tell me how to do this